Store an owned `Score` in `ScorerAccountingForInFlightHtlcs`

[rust-lightning] / lightning / src / routing / router.rs

// This file is Copyright its original authors, visible in version control
// history.
//
// This file is licensed under the Apache License, Version 2.0 <LICENSE-APACHE
// or http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your option.
// You may not use this file except in accordance with one or both of these
// licenses.

//! The top-level routing/network map tracking logic lives here.
//!
//! You probably want to create a P2PGossipSync and use that as your RoutingMessageHandler and then
//! interrogate it to get routes for your own payments.

use bitcoin::secp256k1::PublicKey;
use bitcoin::hashes::Hash;
use bitcoin::hashes::sha256::Hash as Sha256;

use crate::ln::PaymentHash;
use crate::ln::channelmanager::{ChannelDetails, PaymentId};
use crate::ln::features::{ChannelFeatures, InvoiceFeatures, NodeFeatures};
use crate::ln::msgs::{DecodeError, ErrorAction, LightningError, MAX_VALUE_MSAT};
use crate::routing::gossip::{DirectedChannelInfo, EffectiveCapacity, ReadOnlyNetworkGraph, NetworkGraph, NodeId, RoutingFees};
use crate::routing::scoring::{ChannelUsage, LockableScore, Score};
use crate::util::ser::{Writeable, Readable, Writer};
use crate::util::logger::{Level, Logger};
use crate::util::chacha20::ChaCha20;

use crate::io;
use crate::prelude::*;
use crate::sync::Mutex;
use alloc::collections::BinaryHeap;
use core::cmp;
use core::ops::Deref;

/// A [`Router`] implemented using [`find_route`].
pub struct DefaultRouter<G: Deref<Target = NetworkGraph<L>>, L: Deref, S: Deref> where
        L::Target: Logger,
        S::Target: for <'a> LockableScore<'a>,
{
        network_graph: G,
        logger: L,
        random_seed_bytes: Mutex<[u8; 32]>,
        scorer: S
}

impl<G: Deref<Target = NetworkGraph<L>>, L: Deref, S: Deref> DefaultRouter<G, L, S> where
        L::Target: Logger,
        S::Target: for <'a> LockableScore<'a>,
{
        /// Creates a new router.
        pub fn new(network_graph: G, logger: L, random_seed_bytes: [u8; 32], scorer: S) -> Self {
                let random_seed_bytes = Mutex::new(random_seed_bytes);
                Self { network_graph, logger, random_seed_bytes, scorer }
        }
}

impl<G: Deref<Target = NetworkGraph<L>>, L: Deref, S: Deref> Router for DefaultRouter<G, L, S> where
        L::Target: Logger,
        S::Target: for <'a> LockableScore<'a>,
{
        fn find_route(
                &self, payer: &PublicKey, params: &RouteParameters, first_hops: Option<&[&ChannelDetails]>,
                inflight_htlcs: InFlightHtlcs
        ) -> Result<Route, LightningError> {
                let random_seed_bytes = {
                        let mut locked_random_seed_bytes = self.random_seed_bytes.lock().unwrap();
                        *locked_random_seed_bytes = Sha256::hash(&*locked_random_seed_bytes).into_inner();
                        *locked_random_seed_bytes
                };

                find_route(
                        payer, params, &self.network_graph, first_hops, &*self.logger,
                        &ScorerAccountingForInFlightHtlcs::new(self.scorer.lock(), inflight_htlcs),
                        &random_seed_bytes
                )
        }

        fn notify_payment_path_failed(&self, path: &[&RouteHop], short_channel_id: u64) {
                self.scorer.lock().payment_path_failed(path, short_channel_id);
        }

        fn notify_payment_path_successful(&self, path: &[&RouteHop]) {
                self.scorer.lock().payment_path_successful(path);
        }

        fn notify_payment_probe_successful(&self, path: &[&RouteHop]) {
                self.scorer.lock().probe_successful(path);
        }

        fn notify_payment_probe_failed(&self, path: &[&RouteHop], short_channel_id: u64) {
                self.scorer.lock().probe_failed(path, short_channel_id);
        }
}

/// A trait defining behavior for routing a payment.
pub trait Router {
        /// Finds a [`Route`] between `payer` and `payee` for a payment with the given values.
        fn find_route(
                &self, payer: &PublicKey, route_params: &RouteParameters,
                first_hops: Option<&[&ChannelDetails]>, inflight_htlcs: InFlightHtlcs
        ) -> Result<Route, LightningError>;
        /// Finds a [`Route`] between `payer` and `payee` for a payment with the given values. Includes
        /// `PaymentHash` and `PaymentId` to be able to correlate the request with a specific payment.
        fn find_route_with_id(
                &self, payer: &PublicKey, route_params: &RouteParameters,
                first_hops: Option<&[&ChannelDetails]>, inflight_htlcs: InFlightHtlcs,
                _payment_hash: PaymentHash, _payment_id: PaymentId
        ) -> Result<Route, LightningError> {
                self.find_route(payer, route_params, first_hops, inflight_htlcs)
        }
        /// Lets the router know that payment through a specific path has failed.
        fn notify_payment_path_failed(&self, path: &[&RouteHop], short_channel_id: u64);
        /// Lets the router know that payment through a specific path was successful.
        fn notify_payment_path_successful(&self, path: &[&RouteHop]);
        /// Lets the router know that a payment probe was successful.
        fn notify_payment_probe_successful(&self, path: &[&RouteHop]);
        /// Lets the router know that a payment probe failed.
        fn notify_payment_probe_failed(&self, path: &[&RouteHop], short_channel_id: u64);
}

/// [`Score`] implementation that factors in in-flight HTLC liquidity.
///
/// Useful for custom [`Router`] implementations to wrap their [`Score`] on-the-fly when calling
/// [`find_route`].
///
/// [`Score`]: crate::routing::scoring::Score
pub struct ScorerAccountingForInFlightHtlcs<S: Score> {
        scorer: S,
        // Maps a channel's short channel id and its direction to the liquidity used up.
        inflight_htlcs: InFlightHtlcs,
}

impl<S: Score> ScorerAccountingForInFlightHtlcs<S> {
        /// Initialize a new `ScorerAccountingForInFlightHtlcs`.
        pub fn new(scorer: S, inflight_htlcs: InFlightHtlcs) -> Self {
                ScorerAccountingForInFlightHtlcs {
                        scorer,
                        inflight_htlcs
                }
        }
}

#[cfg(c_bindings)]
impl<S: Score> Writeable for ScorerAccountingForInFlightHtlcs<S> {
        fn write<W: Writer>(&self, writer: &mut W) -> Result<(), io::Error> { self.scorer.write(writer) }
}

impl<S: Score> Score for ScorerAccountingForInFlightHtlcs<S> {
        fn channel_penalty_msat(&self, short_channel_id: u64, source: &NodeId, target: &NodeId, usage: ChannelUsage) -> u64 {
                if let Some(used_liquidity) = self.inflight_htlcs.used_liquidity_msat(
                        source, target, short_channel_id
                ) {
                        let usage = ChannelUsage {
                                inflight_htlc_msat: usage.inflight_htlc_msat + used_liquidity,
                                ..usage
                        };

                        self.scorer.channel_penalty_msat(short_channel_id, source, target, usage)
                } else {
                        self.scorer.channel_penalty_msat(short_channel_id, source, target, usage)
                }
        }

        fn payment_path_failed(&mut self, path: &[&RouteHop], short_channel_id: u64) {
                self.scorer.payment_path_failed(path, short_channel_id)
        }

        fn payment_path_successful(&mut self, path: &[&RouteHop]) {
                self.scorer.payment_path_successful(path)
        }

        fn probe_failed(&mut self, path: &[&RouteHop], short_channel_id: u64) {
                self.scorer.probe_failed(path, short_channel_id)
        }

        fn probe_successful(&mut self, path: &[&RouteHop]) {
                self.scorer.probe_successful(path)
        }
}

/// A data structure for tracking in-flight HTLCs. May be used during pathfinding to account for
/// in-use channel liquidity.
pub struct InFlightHtlcs(
        // A map with liquidity value (in msat) keyed by a short channel id and the direction the HTLC
        // is traveling in. The direction boolean is determined by checking if the HTLC source's public
        // key is less than its destination. See `InFlightHtlcs::used_liquidity_msat` for more
        // details.
        HashMap<(u64, bool), u64>
);

impl InFlightHtlcs {
        /// Constructs an empty `InFlightHtlcs`.
        pub fn new() -> Self { InFlightHtlcs(HashMap::new()) }

        /// Takes in a path with payer's node id and adds the path's details to `InFlightHtlcs`.
        pub fn process_path(&mut self, path: &[RouteHop], payer_node_id: PublicKey) {
                if path.is_empty() { return };
                // total_inflight_map needs to be direction-sensitive when keeping track of the HTLC value
                // that is held up. However, the `hops` array, which is a path returned by `find_route` in
                // the router excludes the payer node. In the following lines, the payer's information is
                // hardcoded with an inflight value of 0 so that we can correctly represent the first hop
                // in our sliding window of two.
                let reversed_hops_with_payer = path.iter().rev().skip(1)
                        .map(|hop| hop.pubkey)
                        .chain(core::iter::once(payer_node_id));
                let mut cumulative_msat = 0;

                // Taking the reversed vector from above, we zip it with just the reversed hops list to
                // work "backwards" of the given path, since the last hop's `fee_msat` actually represents
                // the total amount sent.
                for (next_hop, prev_hop) in path.iter().rev().zip(reversed_hops_with_payer) {
                        cumulative_msat += next_hop.fee_msat;
                        self.0
                                .entry((next_hop.short_channel_id, NodeId::from_pubkey(&prev_hop) < NodeId::from_pubkey(&next_hop.pubkey)))
                                .and_modify(|used_liquidity_msat| *used_liquidity_msat += cumulative_msat)
                                .or_insert(cumulative_msat);
                }
        }

        /// Returns liquidity in msat given the public key of the HTLC source, target, and short channel
        /// id.
        pub fn used_liquidity_msat(&self, source: &NodeId, target: &NodeId, channel_scid: u64) -> Option<u64> {
                self.0.get(&(channel_scid, source < target)).map(|v| *v)
        }
}

impl Writeable for InFlightHtlcs {
        fn write<W: Writer>(&self, writer: &mut W) -> Result<(), io::Error> { self.0.write(writer) }
}

impl Readable for InFlightHtlcs {
        fn read<R: io::Read>(reader: &mut R) -> Result<Self, DecodeError> {
                let infight_map: HashMap<(u64, bool), u64> = Readable::read(reader)?;
                Ok(Self(infight_map))
        }
}

/// A hop in a route
#[derive(Clone, Debug, Hash, PartialEq, Eq)]
pub struct RouteHop {
        /// The node_id of the node at this hop.
        pub pubkey: PublicKey,
        /// The node_announcement features of the node at this hop. For the last hop, these may be
        /// amended to match the features present in the invoice this node generated.
        pub node_features: NodeFeatures,
        /// The channel that should be used from the previous hop to reach this node.
        pub short_channel_id: u64,
        /// The channel_announcement features of the channel that should be used from the previous hop
        /// to reach this node.
        pub channel_features: ChannelFeatures,
        /// The fee taken on this hop (for paying for the use of the *next* channel in the path).
        /// For the last hop, this should be the full value of the payment (might be more than
        /// requested if we had to match htlc_minimum_msat).
        pub fee_msat: u64,
        /// The CLTV delta added for this hop. For the last hop, this should be the full CLTV value
        /// expected at the destination, in excess of the current block height.
        pub cltv_expiry_delta: u32,
}

impl_writeable_tlv_based!(RouteHop, {
        (0, pubkey, required),
        (2, node_features, required),
        (4, short_channel_id, required),
        (6, channel_features, required),
        (8, fee_msat, required),
        (10, cltv_expiry_delta, required),
});

/// A route directs a payment from the sender (us) to the recipient. If the recipient supports MPP,
/// it can take multiple paths. Each path is composed of one or more hops through the network.
#[derive(Clone, Hash, PartialEq, Eq)]
pub struct Route {
        /// The list of routes taken for a single (potentially-)multi-part payment. The pubkey of the
        /// last RouteHop in each path must be the same. Each entry represents a list of hops, NOT
        /// INCLUDING our own, where the last hop is the destination. Thus, this must always be at
        /// least length one. While the maximum length of any given path is variable, keeping the length
        /// of any path less or equal to 19 should currently ensure it is viable.
        pub paths: Vec<Vec<RouteHop>>,
        /// The `payment_params` parameter passed to [`find_route`].
        /// This is used by `ChannelManager` to track information which may be required for retries,
        /// provided back to you via [`Event::PaymentPathFailed`].
        ///
        /// [`Event::PaymentPathFailed`]: crate::util::events::Event::PaymentPathFailed
        pub payment_params: Option<PaymentParameters>,
}

pub(crate) trait RoutePath {
        /// Gets the fees for a given path, excluding any excess paid to the recipient.
        fn get_path_fees(&self) -> u64;
}
impl RoutePath for Vec<RouteHop> {
        fn get_path_fees(&self) -> u64 {
                // Do not count last hop of each path since that's the full value of the payment
                self.split_last().map(|(_, path_prefix)| path_prefix).unwrap_or(&[])
                        .iter().map(|hop| &hop.fee_msat)
                        .sum()
        }
}

impl Route {
        /// Returns the total amount of fees paid on this [`Route`].
        ///
        /// This doesn't include any extra payment made to the recipient, which can happen in excess of
        /// the amount passed to [`find_route`]'s `params.final_value_msat`.
        pub fn get_total_fees(&self) -> u64 {
                self.paths.iter().map(|path| path.get_path_fees()).sum()
        }

        /// Returns the total amount paid on this [`Route`], excluding the fees.
        pub fn get_total_amount(&self) -> u64 {
                return self.paths.iter()
                        .map(|path| path.split_last().map(|(hop, _)| hop.fee_msat).unwrap_or(0))
                        .sum();
        }
}

const SERIALIZATION_VERSION: u8 = 1;
const MIN_SERIALIZATION_VERSION: u8 = 1;

impl Writeable for Route {
        fn write<W: crate::util::ser::Writer>(&self, writer: &mut W) -> Result<(), io::Error> {
                write_ver_prefix!(writer, SERIALIZATION_VERSION, MIN_SERIALIZATION_VERSION);
                (self.paths.len() as u64).write(writer)?;
                for hops in self.paths.iter() {
                        (hops.len() as u8).write(writer)?;
                        for hop in hops.iter() {
                                hop.write(writer)?;
                        }
                }
                write_tlv_fields!(writer, {
                        (1, self.payment_params, option),
                });
                Ok(())
        }
}

impl Readable for Route {
        fn read<R: io::Read>(reader: &mut R) -> Result<Route, DecodeError> {
                let _ver = read_ver_prefix!(reader, SERIALIZATION_VERSION);
                let path_count: u64 = Readable::read(reader)?;
                let mut paths = Vec::with_capacity(cmp::min(path_count, 128) as usize);
                for _ in 0..path_count {
                        let hop_count: u8 = Readable::read(reader)?;
                        let mut hops = Vec::with_capacity(hop_count as usize);
                        for _ in 0..hop_count {
                                hops.push(Readable::read(reader)?);
                        }
                        paths.push(hops);
                }
                let mut payment_params = None;
                read_tlv_fields!(reader, {
                        (1, payment_params, option),
                });
                Ok(Route { paths, payment_params })
        }
}

/// Parameters needed to find a [`Route`].
///
/// Passed to [`find_route`] and [`build_route_from_hops`], but also provided in
/// [`Event::PaymentPathFailed`] for retrying a failed payment path.
///
/// [`Event::PaymentPathFailed`]: crate::util::events::Event::PaymentPathFailed
#[derive(Clone, Debug)]
pub struct RouteParameters {
        /// The parameters of the failed payment path.
        pub payment_params: PaymentParameters,

        /// The amount in msats sent on the failed payment path.
        pub final_value_msat: u64,

        /// The CLTV on the final hop of the failed payment path.
        pub final_cltv_expiry_delta: u32,
}

impl_writeable_tlv_based!(RouteParameters, {
        (0, payment_params, required),
        (2, final_value_msat, required),
        (4, final_cltv_expiry_delta, required),
});

/// Maximum total CTLV difference we allow for a full payment path.
pub const DEFAULT_MAX_TOTAL_CLTV_EXPIRY_DELTA: u32 = 1008;

/// Maximum number of paths we allow an (MPP) payment to have.
// The default limit is currently set rather arbitrary - there aren't any real fundamental path-count
// limits, but for now more than 10 paths likely carries too much one-path failure.
pub const DEFAULT_MAX_PATH_COUNT: u8 = 10;

// The median hop CLTV expiry delta currently seen in the network.
const MEDIAN_HOP_CLTV_EXPIRY_DELTA: u32 = 40;

// During routing, we only consider paths shorter than our maximum length estimate.
// In the TLV onion format, there is no fixed maximum length, but the `hop_payloads`
// field is always 1300 bytes. As the `tlv_payload` for each hop may vary in length, we have to
// estimate how many hops the route may have so that it actually fits the `hop_payloads` field.
//
// We estimate 3+32 (payload length and HMAC) + 2+8 (amt_to_forward) + 2+4 (outgoing_cltv_value) +
// 2+8 (short_channel_id) = 61 bytes for each intermediate hop and 3+32
// (payload length and HMAC) + 2+8 (amt_to_forward) + 2+4 (outgoing_cltv_value) + 2+32+8
// (payment_secret and total_msat) = 93 bytes for the final hop.
// Since the length of the potentially included `payment_metadata` is unknown to us, we round
// down from (1300-93) / 61 = 19.78... to arrive at a conservative estimate of 19.
const MAX_PATH_LENGTH_ESTIMATE: u8 = 19;

/// The recipient of a payment.
#[derive(Clone, Debug, Hash, PartialEq, Eq)]
pub struct PaymentParameters {
        /// The node id of the payee.
        pub payee_pubkey: PublicKey,

        /// Features supported by the payee.
        ///
        /// May be set from the payee's invoice or via [`for_keysend`]. May be `None` if the invoice
        /// does not contain any features.
        ///
        /// [`for_keysend`]: Self::for_keysend
        pub features: Option<InvoiceFeatures>,

        /// Hints for routing to the payee, containing channels connecting the payee to public nodes.
        pub route_hints: Vec<RouteHint>,

        /// Expiration of a payment to the payee, in seconds relative to the UNIX epoch.
        pub expiry_time: Option<u64>,

        /// The maximum total CLTV delta we accept for the route.
        /// Defaults to [`DEFAULT_MAX_TOTAL_CLTV_EXPIRY_DELTA`].
        pub max_total_cltv_expiry_delta: u32,

        /// The maximum number of paths that may be used by (MPP) payments.
        /// Defaults to [`DEFAULT_MAX_PATH_COUNT`].
        pub max_path_count: u8,

        /// Selects the maximum share of a channel's total capacity which will be sent over a channel,
        /// as a power of 1/2. A higher value prefers to send the payment using more MPP parts whereas
        /// a lower value prefers to send larger MPP parts, potentially saturating channels and
        /// increasing failure probability for those paths.
        ///
        /// Note that this restriction will be relaxed during pathfinding after paths which meet this
        /// restriction have been found. While paths which meet this criteria will be searched for, it
        /// is ultimately up to the scorer to select them over other paths.
        ///
        /// A value of 0 will allow payments up to and including a channel's total announced usable
        /// capacity, a value of one will only use up to half its capacity, two 1/4, etc.
        ///
        /// Default value: 2
        pub max_channel_saturation_power_of_half: u8,

        /// A list of SCIDs which this payment was previously attempted over and which caused the
        /// payment to fail. Future attempts for the same payment shouldn't be relayed through any of
        /// these SCIDs.
        pub previously_failed_channels: Vec<u64>,
}

impl_writeable_tlv_based!(PaymentParameters, {
        (0, payee_pubkey, required),
        (1, max_total_cltv_expiry_delta, (default_value, DEFAULT_MAX_TOTAL_CLTV_EXPIRY_DELTA)),
        (2, features, option),
        (3, max_path_count, (default_value, DEFAULT_MAX_PATH_COUNT)),
        (4, route_hints, vec_type),
        (5, max_channel_saturation_power_of_half, (default_value, 2)),
        (6, expiry_time, option),
        (7, previously_failed_channels, vec_type),
});

impl PaymentParameters {
        /// Creates a payee with the node id of the given `pubkey`.
        pub fn from_node_id(payee_pubkey: PublicKey) -> Self {
                Self {
                        payee_pubkey,
                        features: None,
                        route_hints: vec![],
                        expiry_time: None,
                        max_total_cltv_expiry_delta: DEFAULT_MAX_TOTAL_CLTV_EXPIRY_DELTA,
                        max_path_count: DEFAULT_MAX_PATH_COUNT,
                        max_channel_saturation_power_of_half: 2,
                        previously_failed_channels: Vec::new(),
                }
        }

        /// Creates a payee with the node id of the given `pubkey` to use for keysend payments.
        pub fn for_keysend(payee_pubkey: PublicKey) -> Self {
                Self::from_node_id(payee_pubkey).with_features(InvoiceFeatures::for_keysend())
        }

        /// Includes the payee's features.
        ///
        /// (C-not exported) since bindings don't support move semantics
        pub fn with_features(self, features: InvoiceFeatures) -> Self {
                Self { features: Some(features), ..self }
        }

        /// Includes hints for routing to the payee.
        ///
        /// (C-not exported) since bindings don't support move semantics
        pub fn with_route_hints(self, route_hints: Vec<RouteHint>) -> Self {
                Self { route_hints, ..self }
        }

        /// Includes a payment expiration in seconds relative to the UNIX epoch.
        ///
        /// (C-not exported) since bindings don't support move semantics
        pub fn with_expiry_time(self, expiry_time: u64) -> Self {
                Self { expiry_time: Some(expiry_time), ..self }
        }

        /// Includes a limit for the total CLTV expiry delta which is considered during routing
        ///
        /// (C-not exported) since bindings don't support move semantics
        pub fn with_max_total_cltv_expiry_delta(self, max_total_cltv_expiry_delta: u32) -> Self {
                Self { max_total_cltv_expiry_delta, ..self }
        }

        /// Includes a limit for the maximum number of payment paths that may be used.
        ///
        /// (C-not exported) since bindings don't support move semantics
        pub fn with_max_path_count(self, max_path_count: u8) -> Self {
                Self { max_path_count, ..self }
        }

        /// Includes a limit for the maximum number of payment paths that may be used.
        ///
        /// (C-not exported) since bindings don't support move semantics
        pub fn with_max_channel_saturation_power_of_half(self, max_channel_saturation_power_of_half: u8) -> Self {
                Self { max_channel_saturation_power_of_half, ..self }
        }
}

/// A list of hops along a payment path terminating with a channel to the recipient.
#[derive(Clone, Debug, Hash, Eq, PartialEq)]
pub struct RouteHint(pub Vec<RouteHintHop>);

impl Writeable for RouteHint {
        fn write<W: crate::util::ser::Writer>(&self, writer: &mut W) -> Result<(), io::Error> {
                (self.0.len() as u64).write(writer)?;
                for hop in self.0.iter() {
                        hop.write(writer)?;
                }
                Ok(())
        }
}

impl Readable for RouteHint {
        fn read<R: io::Read>(reader: &mut R) -> Result<Self, DecodeError> {
                let hop_count: u64 = Readable::read(reader)?;
                let mut hops = Vec::with_capacity(cmp::min(hop_count, 16) as usize);
                for _ in 0..hop_count {
                        hops.push(Readable::read(reader)?);
                }
                Ok(Self(hops))
        }
}

/// A channel descriptor for a hop along a payment path.
#[derive(Clone, Debug, Hash, Eq, PartialEq)]
pub struct RouteHintHop {
        /// The node_id of the non-target end of the route
        pub src_node_id: PublicKey,
        /// The short_channel_id of this channel
        pub short_channel_id: u64,
        /// The fees which must be paid to use this channel
        pub fees: RoutingFees,
        /// The difference in CLTV values between this node and the next node.
        pub cltv_expiry_delta: u16,
        /// The minimum value, in msat, which must be relayed to the next hop.
        pub htlc_minimum_msat: Option<u64>,
        /// The maximum value in msat available for routing with a single HTLC.
        pub htlc_maximum_msat: Option<u64>,
}

impl_writeable_tlv_based!(RouteHintHop, {
        (0, src_node_id, required),
        (1, htlc_minimum_msat, option),
        (2, short_channel_id, required),
        (3, htlc_maximum_msat, option),
        (4, fees, required),
        (6, cltv_expiry_delta, required),
});

#[derive(Eq, PartialEq)]
struct RouteGraphNode {
        node_id: NodeId,
        lowest_fee_to_peer_through_node: u64,
        lowest_fee_to_node: u64,
        total_cltv_delta: u32,
        // The maximum value a yet-to-be-constructed payment path might flow through this node.
        // This value is upper-bounded by us by:
        // - how much is needed for a path being constructed
        // - how much value can channels following this node (up to the destination) can contribute,
        //   considering their capacity and fees
        value_contribution_msat: u64,
        /// The effective htlc_minimum_msat at this hop. If a later hop on the path had a higher HTLC
        /// minimum, we use it, plus the fees required at each earlier hop to meet it.
        path_htlc_minimum_msat: u64,
        /// All penalties incurred from this hop on the way to the destination, as calculated using
        /// channel scoring.
        path_penalty_msat: u64,
        /// The number of hops walked up to this node.
        path_length_to_node: u8,
}

impl cmp::Ord for RouteGraphNode {
        fn cmp(&self, other: &RouteGraphNode) -> cmp::Ordering {
                let other_score = cmp::max(other.lowest_fee_to_peer_through_node, other.path_htlc_minimum_msat)
                        .saturating_add(other.path_penalty_msat);
                let self_score = cmp::max(self.lowest_fee_to_peer_through_node, self.path_htlc_minimum_msat)
                        .saturating_add(self.path_penalty_msat);
                other_score.cmp(&self_score).then_with(|| other.node_id.cmp(&self.node_id))
        }
}

impl cmp::PartialOrd for RouteGraphNode {
        fn partial_cmp(&self, other: &RouteGraphNode) -> Option<cmp::Ordering> {
                Some(self.cmp(other))
        }
}

/// A wrapper around the various hop representations.
///
/// Used to construct a [`PathBuildingHop`] and to estimate [`EffectiveCapacity`].
#[derive(Clone, Debug)]
enum CandidateRouteHop<'a> {
        /// A hop from the payer, where the outbound liquidity is known.
        FirstHop {
                details: &'a ChannelDetails,
        },
        /// A hop found in the [`ReadOnlyNetworkGraph`], where the channel capacity may be unknown.
        PublicHop {
                info: DirectedChannelInfo<'a>,
                short_channel_id: u64,
        },
        /// A hop to the payee found in the payment invoice, though not necessarily a direct channel.
        PrivateHop {
                hint: &'a RouteHintHop,
        }
}

impl<'a> CandidateRouteHop<'a> {
        fn short_channel_id(&self) -> u64 {
                match self {
                        CandidateRouteHop::FirstHop { details } => details.get_outbound_payment_scid().unwrap(),
                        CandidateRouteHop::PublicHop { short_channel_id, .. } => *short_channel_id,
                        CandidateRouteHop::PrivateHop { hint } => hint.short_channel_id,
                }
        }

        // NOTE: This may alloc memory so avoid calling it in a hot code path.
        fn features(&self) -> ChannelFeatures {
                match self {
                        CandidateRouteHop::FirstHop { details } => details.counterparty.features.to_context(),
                        CandidateRouteHop::PublicHop { info, .. } => info.channel().features.clone(),
                        CandidateRouteHop::PrivateHop { .. } => ChannelFeatures::empty(),
                }
        }

        fn cltv_expiry_delta(&self) -> u32 {
                match self {
                        CandidateRouteHop::FirstHop { .. } => 0,
                        CandidateRouteHop::PublicHop { info, .. } => info.direction().cltv_expiry_delta as u32,
                        CandidateRouteHop::PrivateHop { hint } => hint.cltv_expiry_delta as u32,
                }
        }

        fn htlc_minimum_msat(&self) -> u64 {
                match self {
                        CandidateRouteHop::FirstHop { .. } => 0,
                        CandidateRouteHop::PublicHop { info, .. } => info.direction().htlc_minimum_msat,
                        CandidateRouteHop::PrivateHop { hint } => hint.htlc_minimum_msat.unwrap_or(0),
                }
        }

        fn fees(&self) -> RoutingFees {
                match self {
                        CandidateRouteHop::FirstHop { .. } => RoutingFees {
                                base_msat: 0, proportional_millionths: 0,
                        },
                        CandidateRouteHop::PublicHop { info, .. } => info.direction().fees,
                        CandidateRouteHop::PrivateHop { hint } => hint.fees,
                }
        }

        fn effective_capacity(&self) -> EffectiveCapacity {
                match self {
                        CandidateRouteHop::FirstHop { details } => EffectiveCapacity::ExactLiquidity {
                                liquidity_msat: details.next_outbound_htlc_limit_msat,
                        },
                        CandidateRouteHop::PublicHop { info, .. } => info.effective_capacity(),
                        CandidateRouteHop::PrivateHop { .. } => EffectiveCapacity::Infinite,
                }
        }
}

#[inline]
fn max_htlc_from_capacity(capacity: EffectiveCapacity, max_channel_saturation_power_of_half: u8) -> u64 {
        let saturation_shift: u32 = max_channel_saturation_power_of_half as u32;
        match capacity {
                EffectiveCapacity::ExactLiquidity { liquidity_msat } => liquidity_msat,
                EffectiveCapacity::Infinite => u64::max_value(),
                EffectiveCapacity::Unknown => EffectiveCapacity::Unknown.as_msat(),
                EffectiveCapacity::MaximumHTLC { amount_msat } =>
                        amount_msat.checked_shr(saturation_shift).unwrap_or(0),
                EffectiveCapacity::Total { capacity_msat, htlc_maximum_msat } =>
                        cmp::min(capacity_msat.checked_shr(saturation_shift).unwrap_or(0), htlc_maximum_msat),
        }
}

fn iter_equal<I1: Iterator, I2: Iterator>(mut iter_a: I1, mut iter_b: I2)
-> bool where I1::Item: PartialEq<I2::Item> {
        loop {
                let a = iter_a.next();
                let b = iter_b.next();
                if a.is_none() && b.is_none() { return true; }
                if a.is_none() || b.is_none() { return false; }
                if a.unwrap().ne(&b.unwrap()) { return false; }
        }
}

/// It's useful to keep track of the hops associated with the fees required to use them,
/// so that we can choose cheaper paths (as per Dijkstra's algorithm).
/// Fee values should be updated only in the context of the whole path, see update_value_and_recompute_fees.
/// These fee values are useful to choose hops as we traverse the graph "payee-to-payer".
#[derive(Clone)]
struct PathBuildingHop<'a> {
        // Note that this should be dropped in favor of loading it from CandidateRouteHop, but doing so
        // is a larger refactor and will require careful performance analysis.
        node_id: NodeId,
        candidate: CandidateRouteHop<'a>,
        fee_msat: u64,

        /// Minimal fees required to route to the source node of the current hop via any of its inbound channels.
        src_lowest_inbound_fees: RoutingFees,
        /// All the fees paid *after* this channel on the way to the destination
        next_hops_fee_msat: u64,
        /// Fee paid for the use of the current channel (see candidate.fees()).
        /// The value will be actually deducted from the counterparty balance on the previous link.
        hop_use_fee_msat: u64,
        /// Used to compare channels when choosing the for routing.
        /// Includes paying for the use of a hop and the following hops, as well as
        /// an estimated cost of reaching this hop.
        /// Might get stale when fees are recomputed. Primarily for internal use.
        total_fee_msat: u64,
        /// A mirror of the same field in RouteGraphNode. Note that this is only used during the graph
        /// walk and may be invalid thereafter.
        path_htlc_minimum_msat: u64,
        /// All penalties incurred from this channel on the way to the destination, as calculated using
        /// channel scoring.
        path_penalty_msat: u64,
        /// If we've already processed a node as the best node, we shouldn't process it again. Normally
        /// we'd just ignore it if we did as all channels would have a higher new fee, but because we
        /// may decrease the amounts in use as we walk the graph, the actual calculated fee may
        /// decrease as well. Thus, we have to explicitly track which nodes have been processed and
        /// avoid processing them again.
        was_processed: bool,
        #[cfg(all(not(feature = "_bench_unstable"), any(test, fuzzing)))]
        // In tests, we apply further sanity checks on cases where we skip nodes we already processed
        // to ensure it is specifically in cases where the fee has gone down because of a decrease in
        // value_contribution_msat, which requires tracking it here. See comments below where it is
        // used for more info.
        value_contribution_msat: u64,
}

impl<'a> core::fmt::Debug for PathBuildingHop<'a> {
        fn fmt(&self, f: &mut core::fmt::Formatter) -> Result<(), core::fmt::Error> {
                let mut debug_struct = f.debug_struct("PathBuildingHop");
                debug_struct
                        .field("node_id", &self.node_id)
                        .field("short_channel_id", &self.candidate.short_channel_id())
                        .field("total_fee_msat", &self.total_fee_msat)
                        .field("next_hops_fee_msat", &self.next_hops_fee_msat)
                        .field("hop_use_fee_msat", &self.hop_use_fee_msat)
                        .field("total_fee_msat - (next_hops_fee_msat + hop_use_fee_msat)", &(&self.total_fee_msat - (&self.next_hops_fee_msat + &self.hop_use_fee_msat)))
                        .field("path_penalty_msat", &self.path_penalty_msat)
                        .field("path_htlc_minimum_msat", &self.path_htlc_minimum_msat)
                        .field("cltv_expiry_delta", &self.candidate.cltv_expiry_delta());
                #[cfg(all(not(feature = "_bench_unstable"), any(test, fuzzing)))]
                let debug_struct = debug_struct
                        .field("value_contribution_msat", &self.value_contribution_msat);
                debug_struct.finish()
        }
}

// Instantiated with a list of hops with correct data in them collected during path finding,
// an instance of this struct should be further modified only via given methods.
#[derive(Clone)]
struct PaymentPath<'a> {
        hops: Vec<(PathBuildingHop<'a>, NodeFeatures)>,
}

impl<'a> PaymentPath<'a> {
        // TODO: Add a value_msat field to PaymentPath and use it instead of this function.
        fn get_value_msat(&self) -> u64 {
                self.hops.last().unwrap().0.fee_msat
        }

        fn get_path_penalty_msat(&self) -> u64 {
                self.hops.first().map(|h| h.0.path_penalty_msat).unwrap_or(u64::max_value())
        }

        fn get_total_fee_paid_msat(&self) -> u64 {
                if self.hops.len() < 1 {
                        return 0;
                }
                let mut result = 0;
                // Can't use next_hops_fee_msat because it gets outdated.
                for (i, (hop, _)) in self.hops.iter().enumerate() {
                        if i != self.hops.len() - 1 {
                                result += hop.fee_msat;
                        }
                }
                return result;
        }

        fn get_cost_msat(&self) -> u64 {
                self.get_total_fee_paid_msat().saturating_add(self.get_path_penalty_msat())
        }

        // If the amount transferred by the path is updated, the fees should be adjusted. Any other way
        // to change fees may result in an inconsistency.
        //
        // Sometimes we call this function right after constructing a path which is inconsistent in
        // that it the value being transferred has decreased while we were doing path finding, leading
        // to the fees being paid not lining up with the actual limits.
        //
        // Note that this function is not aware of the available_liquidity limit, and thus does not
        // support increasing the value being transferred beyond what was selected during the initial
        // routing passes.
        fn update_value_and_recompute_fees(&mut self, value_msat: u64) {
                let mut total_fee_paid_msat = 0 as u64;
                for i in (0..self.hops.len()).rev() {
                        let last_hop = i == self.hops.len() - 1;

                        // For non-last-hop, this value will represent the fees paid on the current hop. It
                        // will consist of the fees for the use of the next hop, and extra fees to match
                        // htlc_minimum_msat of the current channel. Last hop is handled separately.
                        let mut cur_hop_fees_msat = 0;
                        if !last_hop {
                                cur_hop_fees_msat = self.hops.get(i + 1).unwrap().0.hop_use_fee_msat;
                        }

                        let mut cur_hop = &mut self.hops.get_mut(i).unwrap().0;
                        cur_hop.next_hops_fee_msat = total_fee_paid_msat;
                        // Overpay in fees if we can't save these funds due to htlc_minimum_msat.
                        // We try to account for htlc_minimum_msat in scoring (add_entry!), so that nodes don't
                        // set it too high just to maliciously take more fees by exploiting this
                        // match htlc_minimum_msat logic.
                        let mut cur_hop_transferred_amount_msat = total_fee_paid_msat + value_msat;
                        if let Some(extra_fees_msat) = cur_hop.candidate.htlc_minimum_msat().checked_sub(cur_hop_transferred_amount_msat) {
                                // Note that there is a risk that *previous hops* (those closer to us, as we go
                                // payee->our_node here) would exceed their htlc_maximum_msat or available balance.
                                //
                                // This might make us end up with a broken route, although this should be super-rare
                                // in practice, both because of how healthy channels look like, and how we pick
                                // channels in add_entry.
                                // Also, this can't be exploited more heavily than *announce a free path and fail
                                // all payments*.
                                cur_hop_transferred_amount_msat += extra_fees_msat;
                                total_fee_paid_msat += extra_fees_msat;
                                cur_hop_fees_msat += extra_fees_msat;
                        }

                        if last_hop {
                                // Final hop is a special case: it usually has just value_msat (by design), but also
                                // it still could overpay for the htlc_minimum_msat.
                                cur_hop.fee_msat = cur_hop_transferred_amount_msat;
                        } else {
                                // Propagate updated fees for the use of the channels to one hop back, where they
                                // will be actually paid (fee_msat). The last hop is handled above separately.
                                cur_hop.fee_msat = cur_hop_fees_msat;
                        }

                        // Fee for the use of the current hop which will be deducted on the previous hop.
                        // Irrelevant for the first hop, as it doesn't have the previous hop, and the use of
                        // this channel is free for us.
                        if i != 0 {
                                if let Some(new_fee) = compute_fees(cur_hop_transferred_amount_msat, cur_hop.candidate.fees()) {
                                        cur_hop.hop_use_fee_msat = new_fee;
                                        total_fee_paid_msat += new_fee;
                                } else {
                                        // It should not be possible because this function is called only to reduce the
                                        // value. In that case, compute_fee was already called with the same fees for
                                        // larger amount and there was no overflow.
                                        unreachable!();
                                }
                        }
                }
        }
}

fn compute_fees(amount_msat: u64, channel_fees: RoutingFees) -> Option<u64> {
        let proportional_fee_millions =
                amount_msat.checked_mul(channel_fees.proportional_millionths as u64);
        if let Some(new_fee) = proportional_fee_millions.and_then(|part| {
                        (channel_fees.base_msat as u64).checked_add(part / 1_000_000) }) {

                Some(new_fee)
        } else {
                // This function may be (indirectly) called without any verification,
                // with channel_fees provided by a caller. We should handle it gracefully.
                None
        }
}

/// The default `features` we assume for a node in a route, when no `features` are known about that
/// specific node.
///
/// Default features are:
/// * variable_length_onion_optional
fn default_node_features() -> NodeFeatures {
        let mut features = NodeFeatures::empty();
        features.set_variable_length_onion_optional();
        features
}

/// Finds a route from us (payer) to the given target node (payee).
///
/// If the payee provided features in their invoice, they should be provided via `params.payee`.
/// Without this, MPP will only be used if the payee's features are available in the network graph.
///
/// Private routing paths between a public node and the target may be included in `params.payee`.
///
/// If some channels aren't announced, it may be useful to fill in `first_hops` with the results
/// from [`ChannelManager::list_usable_channels`]. If it is filled in, the view of these channels
/// from `network_graph` will be ignored, and only those in `first_hops` will be used.
///
/// The fees on channels from us to the next hop are ignored as they are assumed to all be equal.
/// However, the enabled/disabled bit on such channels as well as the `htlc_minimum_msat` /
/// `htlc_maximum_msat` *are* checked as they may change based on the receiving node.
///
/// # Note
///
/// May be used to re-compute a [`Route`] when handling a [`Event::PaymentPathFailed`]. Any
/// adjustments to the [`NetworkGraph`] and channel scores should be made prior to calling this
/// function.
///
/// # Panics
///
/// Panics if first_hops contains channels without short_channel_ids;
/// [`ChannelManager::list_usable_channels`] will never include such channels.
///
/// [`ChannelManager::list_usable_channels`]: crate::ln::channelmanager::ChannelManager::list_usable_channels
/// [`Event::PaymentPathFailed`]: crate::util::events::Event::PaymentPathFailed
/// [`NetworkGraph`]: crate::routing::gossip::NetworkGraph
pub fn find_route<L: Deref, GL: Deref, S: Score>(
        our_node_pubkey: &PublicKey, route_params: &RouteParameters,
        network_graph: &NetworkGraph<GL>, first_hops: Option<&[&ChannelDetails]>, logger: L,
        scorer: &S, random_seed_bytes: &[u8; 32]
) -> Result<Route, LightningError>
where L::Target: Logger, GL::Target: Logger {
        let graph_lock = network_graph.read_only();
        let mut route = get_route(our_node_pubkey, &route_params.payment_params, &graph_lock, first_hops,
                route_params.final_value_msat, route_params.final_cltv_expiry_delta, logger, scorer,
                random_seed_bytes)?;
        add_random_cltv_offset(&mut route, &route_params.payment_params, &graph_lock, random_seed_bytes);
        Ok(route)
}

pub(crate) fn get_route<L: Deref, S: Score>(
        our_node_pubkey: &PublicKey, payment_params: &PaymentParameters, network_graph: &ReadOnlyNetworkGraph,
        first_hops: Option<&[&ChannelDetails]>, final_value_msat: u64, final_cltv_expiry_delta: u32,
        logger: L, scorer: &S, _random_seed_bytes: &[u8; 32]
) -> Result<Route, LightningError>
where L::Target: Logger {
        let payee_node_id = NodeId::from_pubkey(&payment_params.payee_pubkey);
        let our_node_id = NodeId::from_pubkey(&our_node_pubkey);

        if payee_node_id == our_node_id {
                return Err(LightningError{err: "Cannot generate a route to ourselves".to_owned(), action: ErrorAction::IgnoreError});
        }

        if final_value_msat > MAX_VALUE_MSAT {
                return Err(LightningError{err: "Cannot generate a route of more value than all existing satoshis".to_owned(), action: ErrorAction::IgnoreError});
        }

        if final_value_msat == 0 {
                return Err(LightningError{err: "Cannot send a payment of 0 msat".to_owned(), action: ErrorAction::IgnoreError});
        }

        for route in payment_params.route_hints.iter() {
                for hop in &route.0 {
                        if hop.src_node_id == payment_params.payee_pubkey {
                                return Err(LightningError{err: "Route hint cannot have the payee as the source.".to_owned(), action: ErrorAction::IgnoreError});
                        }
                }
        }
        if payment_params.max_total_cltv_expiry_delta <= final_cltv_expiry_delta {
                return Err(LightningError{err: "Can't find a route where the maximum total CLTV expiry delta is below the final CLTV expiry.".to_owned(), action: ErrorAction::IgnoreError});
        }

        // The general routing idea is the following:
        // 1. Fill first/last hops communicated by the caller.
        // 2. Attempt to construct a path from payer to payee for transferring
        //    any ~sufficient (described later) value.
        //    If succeed, remember which channels were used and how much liquidity they have available,
        //    so that future paths don't rely on the same liquidity.
        // 3. Proceed to the next step if:
        //    - we hit the recommended target value;
        //    - OR if we could not construct a new path. Any next attempt will fail too.
        //    Otherwise, repeat step 2.
        // 4. See if we managed to collect paths which aggregately are able to transfer target value
        //    (not recommended value).
        // 5. If yes, proceed. If not, fail routing.
        // 6. Select the paths which have the lowest cost (fee plus scorer penalty) per amount
        //    transferred up to the transfer target value.
        // 7. Reduce the value of the last path until we are sending only the target value.
        // 8. If our maximum channel saturation limit caused us to pick two identical paths, combine
        //    them so that we're not sending two HTLCs along the same path.

        // As for the actual search algorithm,
        // we do a payee-to-payer pseudo-Dijkstra's sorting by each node's distance from the payee
        // plus the minimum per-HTLC fee to get from it to another node (aka "shitty pseudo-A*").
        //
        // We are not a faithful Dijkstra's implementation because we can change values which impact
        // earlier nodes while processing later nodes. Specifically, if we reach a channel with a lower
        // liquidity limit (via htlc_maximum_msat, on-chain capacity or assumed liquidity limits) than
        // the value we are currently attempting to send over a path, we simply reduce the value being
        // sent along the path for any hops after that channel. This may imply that later fees (which
        // we've already tabulated) are lower because a smaller value is passing through the channels
        // (and the proportional fee is thus lower). There isn't a trivial way to recalculate the
        // channels which were selected earlier (and which may still be used for other paths without a
        // lower liquidity limit), so we simply accept that some liquidity-limited paths may be
        // de-preferenced.
        //
        // One potentially problematic case for this algorithm would be if there are many
        // liquidity-limited paths which are liquidity-limited near the destination (ie early in our
        // graph walking), we may never find a path which is not liquidity-limited and has lower
        // proportional fee (and only lower absolute fee when considering the ultimate value sent).
        // Because we only consider paths with at least 5% of the total value being sent, the damage
        // from such a case should be limited, however this could be further reduced in the future by
        // calculating fees on the amount we wish to route over a path, ie ignoring the liquidity
        // limits for the purposes of fee calculation.
        //
        // Alternatively, we could store more detailed path information in the heap (targets, below)
        // and index the best-path map (dist, below) by node *and* HTLC limits, however that would blow
        // up the runtime significantly both algorithmically (as we'd traverse nodes multiple times)
        // and practically (as we would need to store dynamically-allocated path information in heap
        // objects, increasing malloc traffic and indirect memory access significantly). Further, the
        // results of such an algorithm would likely be biased towards lower-value paths.
        //
        // Further, we could return to a faithful Dijkstra's algorithm by rejecting paths with limits
        // outside of our current search value, running a path search more times to gather candidate
        // paths at different values. While this may be acceptable, further path searches may increase
        // runtime for little gain. Specifically, the current algorithm rather efficiently explores the
        // graph for candidate paths, calculating the maximum value which can realistically be sent at
        // the same time, remaining generic across different payment values.
        //
        // TODO: There are a few tweaks we could do, including possibly pre-calculating more stuff
        // to use as the A* heuristic beyond just the cost to get one node further than the current
        // one.

        let network_channels = network_graph.channels();
        let network_nodes = network_graph.nodes();

        if payment_params.max_path_count == 0 {
                return Err(LightningError{err: "Can't find a route with no paths allowed.".to_owned(), action: ErrorAction::IgnoreError});
        }

        // Allow MPP only if we have a features set from somewhere that indicates the payee supports
        // it. If the payee supports it they're supposed to include it in the invoice, so that should
        // work reliably.
        let allow_mpp = if payment_params.max_path_count == 1 {
                false
        } else if let Some(features) = &payment_params.features {
                features.supports_basic_mpp()
        } else if let Some(node) = network_nodes.get(&payee_node_id) {
                if let Some(node_info) = node.announcement_info.as_ref() {
                        node_info.features.supports_basic_mpp()
                } else { false }
        } else { false };

        log_trace!(logger, "Searching for a route from payer {} to payee {} {} MPP and {} first hops {}overriding the network graph", our_node_pubkey,
                payment_params.payee_pubkey, if allow_mpp { "with" } else { "without" },
                first_hops.map(|hops| hops.len()).unwrap_or(0), if first_hops.is_some() { "" } else { "not " });

        // Step (1).
        // Prepare the data we'll use for payee-to-payer search by
        // inserting first hops suggested by the caller as targets.
        // Our search will then attempt to reach them while traversing from the payee node.
        let mut first_hop_targets: HashMap<_, Vec<&ChannelDetails>> =
                HashMap::with_capacity(if first_hops.is_some() { first_hops.as_ref().unwrap().len() } else { 0 });
        if let Some(hops) = first_hops {
                for chan in hops {
                        if chan.get_outbound_payment_scid().is_none() {
                                panic!("first_hops should be filled in with usable channels, not pending ones");
                        }
                        if chan.counterparty.node_id == *our_node_pubkey {
                                return Err(LightningError{err: "First hop cannot have our_node_pubkey as a destination.".to_owned(), action: ErrorAction::IgnoreError});
                        }
                        first_hop_targets
                                .entry(NodeId::from_pubkey(&chan.counterparty.node_id))
                                .or_insert(Vec::new())
                                .push(chan);
                }
                if first_hop_targets.is_empty() {
                        return Err(LightningError{err: "Cannot route when there are no outbound routes away from us".to_owned(), action: ErrorAction::IgnoreError});
                }
        }

        // The main heap containing all candidate next-hops sorted by their score (max(A* fee,
        // htlc_minimum)). Ideally this would be a heap which allowed cheap score reduction instead of
        // adding duplicate entries when we find a better path to a given node.
        let mut targets: BinaryHeap<RouteGraphNode> = BinaryHeap::new();

        // Map from node_id to information about the best current path to that node, including feerate
        // information.
        let mut dist: HashMap<NodeId, PathBuildingHop> = HashMap::with_capacity(network_nodes.len());

        // During routing, if we ignore a path due to an htlc_minimum_msat limit, we set this,
        // indicating that we may wish to try again with a higher value, potentially paying to meet an
        // htlc_minimum with extra fees while still finding a cheaper path.
        let mut hit_minimum_limit;

        // When arranging a route, we select multiple paths so that we can make a multi-path payment.
        // We start with a path_value of the exact amount we want, and if that generates a route we may
        // return it immediately. Otherwise, we don't stop searching for paths until we have 3x the
        // amount we want in total across paths, selecting the best subset at the end.
        const ROUTE_CAPACITY_PROVISION_FACTOR: u64 = 3;
        let recommended_value_msat = final_value_msat * ROUTE_CAPACITY_PROVISION_FACTOR as u64;
        let mut path_value_msat = final_value_msat;

        // Routing Fragmentation Mitigation heuristic:
        //
        // Routing fragmentation across many payment paths increases the overall routing
        // fees as you have irreducible routing fees per-link used (`fee_base_msat`).
        // Taking too many smaller paths also increases the chance of payment failure.
        // Thus to avoid this effect, we require from our collected links to provide
        // at least a minimal contribution to the recommended value yet-to-be-fulfilled.
        // This requirement is currently set to be 1/max_path_count of the payment
        // value to ensure we only ever return routes that do not violate this limit.
        let minimal_value_contribution_msat: u64 = if allow_mpp {
                (final_value_msat + (payment_params.max_path_count as u64 - 1)) / payment_params.max_path_count as u64
        } else {
                final_value_msat
        };

        // When we start collecting routes we enforce the max_channel_saturation_power_of_half
        // requirement strictly. After we've collected enough (or if we fail to find new routes) we
        // drop the requirement by setting this to 0.
        let mut channel_saturation_pow_half = payment_params.max_channel_saturation_power_of_half;

        // Keep track of how much liquidity has been used in selected channels. Used to determine
        // if the channel can be used by additional MPP paths or to inform path finding decisions. It is
        // aware of direction *only* to ensure that the correct htlc_maximum_msat value is used. Hence,
        // liquidity used in one direction will not offset any used in the opposite direction.
        let mut used_channel_liquidities: HashMap<(u64, bool), u64> =
                HashMap::with_capacity(network_nodes.len());

        // Keeping track of how much value we already collected across other paths. Helps to decide
        // when we want to stop looking for new paths.
        let mut already_collected_value_msat = 0;

        for (_, channels) in first_hop_targets.iter_mut() {
                // Sort the first_hops channels to the same node(s) in priority order of which channel we'd
                // most like to use.
                //
                // First, if channels are below `recommended_value_msat`, sort them in descending order,
                // preferring larger channels to avoid splitting the payment into more MPP parts than is
                // required.
                //
                // Second, because simply always sorting in descending order would always use our largest
                // available outbound capacity, needlessly fragmenting our available channel capacities,
                // sort channels above `recommended_value_msat` in ascending order, preferring channels
                // which have enough, but not too much, capacity for the payment.
                channels.sort_unstable_by(|chan_a, chan_b| {
                        if chan_b.next_outbound_htlc_limit_msat < recommended_value_msat || chan_a.next_outbound_htlc_limit_msat < recommended_value_msat {
                                // Sort in descending order
                                chan_b.next_outbound_htlc_limit_msat.cmp(&chan_a.next_outbound_htlc_limit_msat)
                        } else {
                                // Sort in ascending order
                                chan_a.next_outbound_htlc_limit_msat.cmp(&chan_b.next_outbound_htlc_limit_msat)
                        }
                });
        }

        log_trace!(logger, "Building path from {} (payee) to {} (us/payer) for value {} msat.", payment_params.payee_pubkey, our_node_pubkey, final_value_msat);

        macro_rules! add_entry {
                // Adds entry which goes from $src_node_id to $dest_node_id over the $candidate hop.
                // $next_hops_fee_msat represents the fees paid for using all the channels *after* this one,
                // since that value has to be transferred over this channel.
                // Returns whether this channel caused an update to `targets`.
                ( $candidate: expr, $src_node_id: expr, $dest_node_id: expr, $next_hops_fee_msat: expr,
                        $next_hops_value_contribution: expr, $next_hops_path_htlc_minimum_msat: expr,
                        $next_hops_path_penalty_msat: expr, $next_hops_cltv_delta: expr, $next_hops_path_length: expr ) => { {
                        // We "return" whether we updated the path at the end, via this:
                        let mut did_add_update_path_to_src_node = false;
                        // Channels to self should not be used. This is more of belt-and-suspenders, because in
                        // practice these cases should be caught earlier:
                        // - for regular channels at channel announcement (TODO)
                        // - for first and last hops early in get_route
                        if $src_node_id != $dest_node_id {
                                let short_channel_id = $candidate.short_channel_id();
                                let effective_capacity = $candidate.effective_capacity();
                                let htlc_maximum_msat = max_htlc_from_capacity(effective_capacity, channel_saturation_pow_half);

                                // It is tricky to subtract $next_hops_fee_msat from available liquidity here.
                                // It may be misleading because we might later choose to reduce the value transferred
                                // over these channels, and the channel which was insufficient might become sufficient.
                                // Worst case: we drop a good channel here because it can't cover the high following
                                // fees caused by one expensive channel, but then this channel could have been used
                                // if the amount being transferred over this path is lower.
                                // We do this for now, but this is a subject for removal.
                                if let Some(mut available_value_contribution_msat) = htlc_maximum_msat.checked_sub($next_hops_fee_msat) {
                                        let used_liquidity_msat = used_channel_liquidities
                                                .get(&(short_channel_id, $src_node_id < $dest_node_id))
                                                .map_or(0, |used_liquidity_msat| {
                                                        available_value_contribution_msat = available_value_contribution_msat
                                                                .saturating_sub(*used_liquidity_msat);
                                                        *used_liquidity_msat
                                                });

                                        // Verify the liquidity offered by this channel complies to the minimal contribution.
                                        let contributes_sufficient_value = available_value_contribution_msat >= minimal_value_contribution_msat;
                                        // Do not consider candidate hops that would exceed the maximum path length.
                                        let path_length_to_node = $next_hops_path_length + 1;
                                        let exceeds_max_path_length = path_length_to_node > MAX_PATH_LENGTH_ESTIMATE;

                                        // Do not consider candidates that exceed the maximum total cltv expiry limit.
                                        // In order to already account for some of the privacy enhancing random CLTV
                                        // expiry delta offset we add on top later, we subtract a rough estimate
                                        // (2*MEDIAN_HOP_CLTV_EXPIRY_DELTA) here.
                                        let max_total_cltv_expiry_delta = (payment_params.max_total_cltv_expiry_delta - final_cltv_expiry_delta)
                                                .checked_sub(2*MEDIAN_HOP_CLTV_EXPIRY_DELTA)
                                                .unwrap_or(payment_params.max_total_cltv_expiry_delta - final_cltv_expiry_delta);
                                        let hop_total_cltv_delta = ($next_hops_cltv_delta as u32)
                                                .saturating_add($candidate.cltv_expiry_delta());
                                        let exceeds_cltv_delta_limit = hop_total_cltv_delta > max_total_cltv_expiry_delta;

                                        let value_contribution_msat = cmp::min(available_value_contribution_msat, $next_hops_value_contribution);
                                        // Includes paying fees for the use of the following channels.
                                        let amount_to_transfer_over_msat: u64 = match value_contribution_msat.checked_add($next_hops_fee_msat) {
                                                Some(result) => result,
                                                // Can't overflow due to how the values were computed right above.
                                                None => unreachable!(),
                                        };
                                        #[allow(unused_comparisons)] // $next_hops_path_htlc_minimum_msat is 0 in some calls so rustc complains
                                        let over_path_minimum_msat = amount_to_transfer_over_msat >= $candidate.htlc_minimum_msat() &&
                                                amount_to_transfer_over_msat >= $next_hops_path_htlc_minimum_msat;

                                        #[allow(unused_comparisons)] // $next_hops_path_htlc_minimum_msat is 0 in some calls so rustc complains
                                        let may_overpay_to_meet_path_minimum_msat =
                                                ((amount_to_transfer_over_msat < $candidate.htlc_minimum_msat() &&
                                                  recommended_value_msat > $candidate.htlc_minimum_msat()) ||
                                                 (amount_to_transfer_over_msat < $next_hops_path_htlc_minimum_msat &&
                                                  recommended_value_msat > $next_hops_path_htlc_minimum_msat));

                                        let payment_failed_on_this_channel =
                                                payment_params.previously_failed_channels.contains(&short_channel_id);

                                        // If HTLC minimum is larger than the amount we're going to transfer, we shouldn't
                                        // bother considering this channel. If retrying with recommended_value_msat may
                                        // allow us to hit the HTLC minimum limit, set htlc_minimum_limit so that we go
                                        // around again with a higher amount.
                                        if !contributes_sufficient_value || exceeds_max_path_length ||
                                                exceeds_cltv_delta_limit || payment_failed_on_this_channel {
                                                // Path isn't useful, ignore it and move on.
                                        } else if may_overpay_to_meet_path_minimum_msat {
                                                hit_minimum_limit = true;
                                        } else if over_path_minimum_msat {
                                                // Note that low contribution here (limited by available_liquidity_msat)
                                                // might violate htlc_minimum_msat on the hops which are next along the
                                                // payment path (upstream to the payee). To avoid that, we recompute
                                                // path fees knowing the final path contribution after constructing it.
                                                let path_htlc_minimum_msat = compute_fees($next_hops_path_htlc_minimum_msat, $candidate.fees())
                                                        .and_then(|fee_msat| fee_msat.checked_add($next_hops_path_htlc_minimum_msat))
                                                        .map(|fee_msat| cmp::max(fee_msat, $candidate.htlc_minimum_msat()))
                                                        .unwrap_or_else(|| u64::max_value());
                                                let hm_entry = dist.entry($src_node_id);
                                                let old_entry = hm_entry.or_insert_with(|| {
                                                        // If there was previously no known way to access the source node
                                                        // (recall it goes payee-to-payer) of short_channel_id, first add a
                                                        // semi-dummy record just to compute the fees to reach the source node.
                                                        // This will affect our decision on selecting short_channel_id
                                                        // as a way to reach the $dest_node_id.
                                                        let mut fee_base_msat = 0;
                                                        let mut fee_proportional_millionths = 0;
                                                        if let Some(Some(fees)) = network_nodes.get(&$src_node_id).map(|node| node.lowest_inbound_channel_fees) {
                                                                fee_base_msat = fees.base_msat;
                                                                fee_proportional_millionths = fees.proportional_millionths;
                                                        }
                                                        PathBuildingHop {
                                                                node_id: $dest_node_id.clone(),
                                                                candidate: $candidate.clone(),
                                                                fee_msat: 0,
                                                                src_lowest_inbound_fees: RoutingFees {
                                                                        base_msat: fee_base_msat,
                                                                        proportional_millionths: fee_proportional_millionths,
                                                                },
                                                                next_hops_fee_msat: u64::max_value(),
                                                                hop_use_fee_msat: u64::max_value(),
                                                                total_fee_msat: u64::max_value(),
                                                                path_htlc_minimum_msat,
                                                                path_penalty_msat: u64::max_value(),
                                                                was_processed: false,
                                                                #[cfg(all(not(feature = "_bench_unstable"), any(test, fuzzing)))]
                                                                value_contribution_msat,
                                                        }
                                                });

                                                #[allow(unused_mut)] // We only use the mut in cfg(test)
                                                let mut should_process = !old_entry.was_processed;
                                                #[cfg(all(not(feature = "_bench_unstable"), any(test, fuzzing)))]
                                                {
                                                        // In test/fuzzing builds, we do extra checks to make sure the skipping
                                                        // of already-seen nodes only happens in cases we expect (see below).
                                                        if !should_process { should_process = true; }
                                                }

                                                if should_process {
                                                        let mut hop_use_fee_msat = 0;
                                                        let mut total_fee_msat = $next_hops_fee_msat;

                                                        // Ignore hop_use_fee_msat for channel-from-us as we assume all channels-from-us
                                                        // will have the same effective-fee
                                                        if $src_node_id != our_node_id {
                                                                match compute_fees(amount_to_transfer_over_msat, $candidate.fees()) {
                                                                        // max_value means we'll always fail
                                                                        // the old_entry.total_fee_msat > total_fee_msat check
                                                                        None => total_fee_msat = u64::max_value(),
                                                                        Some(fee_msat) => {
                                                                                hop_use_fee_msat = fee_msat;
                                                                                total_fee_msat += hop_use_fee_msat;
                                                                                // When calculating the lowest inbound fees to a node, we
                                                                                // calculate fees here not based on the actual value we think
                                                                                // will flow over this channel, but on the minimum value that
                                                                                // we'll accept flowing over it. The minimum accepted value
                                                                                // is a constant through each path collection run, ensuring
                                                                                // consistent basis. Otherwise we may later find a
                                                                                // different path to the source node that is more expensive,
                                                                                // but which we consider to be cheaper because we are capacity
                                                                                // constrained and the relative fee becomes lower.
                                                                                match compute_fees(minimal_value_contribution_msat, old_entry.src_lowest_inbound_fees)
                                                                                                .map(|a| a.checked_add(total_fee_msat)) {
                                                                                        Some(Some(v)) => {
                                                                                                total_fee_msat = v;
                                                                                        },
                                                                                        _ => {
                                                                                                total_fee_msat = u64::max_value();
                                                                                        }
                                                                                };
                                                                        }
                                                                }
                                                        }

                                                        let channel_usage = ChannelUsage {
                                                                amount_msat: amount_to_transfer_over_msat,
                                                                inflight_htlc_msat: used_liquidity_msat,
                                                                effective_capacity,
                                                        };
                                                        let channel_penalty_msat = scorer.channel_penalty_msat(
                                                                short_channel_id, &$src_node_id, &$dest_node_id, channel_usage
                                                        );
                                                        let path_penalty_msat = $next_hops_path_penalty_msat
                                                                .saturating_add(channel_penalty_msat);
                                                        let new_graph_node = RouteGraphNode {
                                                                node_id: $src_node_id,
                                                                lowest_fee_to_peer_through_node: total_fee_msat,
                                                                lowest_fee_to_node: $next_hops_fee_msat as u64 + hop_use_fee_msat,
                                                                total_cltv_delta: hop_total_cltv_delta,
                                                                value_contribution_msat,
                                                                path_htlc_minimum_msat,
                                                                path_penalty_msat,
                                                                path_length_to_node,
                                                        };

                                                        // Update the way of reaching $src_node_id with the given short_channel_id (from $dest_node_id),
                                                        // if this way is cheaper than the already known
                                                        // (considering the cost to "reach" this channel from the route destination,
                                                        // the cost of using this channel,
                                                        // and the cost of routing to the source node of this channel).
                                                        // Also, consider that htlc_minimum_msat_difference, because we might end up
                                                        // paying it. Consider the following exploit:
                                                        // we use 2 paths to transfer 1.5 BTC. One of them is 0-fee normal 1 BTC path,
                                                        // and for the other one we picked a 1sat-fee path with htlc_minimum_msat of
                                                        // 1 BTC. Now, since the latter is more expensive, we gonna try to cut it
                                                        // by 0.5 BTC, but then match htlc_minimum_msat by paying a fee of 0.5 BTC
                                                        // to this channel.
                                                        // Ideally the scoring could be smarter (e.g. 0.5*htlc_minimum_msat here),
                                                        // but it may require additional tracking - we don't want to double-count
                                                        // the fees included in $next_hops_path_htlc_minimum_msat, but also
                                                        // can't use something that may decrease on future hops.
                                                        let old_cost = cmp::max(old_entry.total_fee_msat, old_entry.path_htlc_minimum_msat)
                                                                .saturating_add(old_entry.path_penalty_msat);
                                                        let new_cost = cmp::max(total_fee_msat, path_htlc_minimum_msat)
                                                                .saturating_add(path_penalty_msat);

                                                        if !old_entry.was_processed && new_cost < old_cost {
                                                                targets.push(new_graph_node);
                                                                old_entry.next_hops_fee_msat = $next_hops_fee_msat;
                                                                old_entry.hop_use_fee_msat = hop_use_fee_msat;
                                                                old_entry.total_fee_msat = total_fee_msat;
                                                                old_entry.node_id = $dest_node_id.clone();
                                                                old_entry.candidate = $candidate.clone();
                                                                old_entry.fee_msat = 0; // This value will be later filled with hop_use_fee_msat of the following channel
                                                                old_entry.path_htlc_minimum_msat = path_htlc_minimum_msat;
                                                                old_entry.path_penalty_msat = path_penalty_msat;
                                                                #[cfg(all(not(feature = "_bench_unstable"), any(test, fuzzing)))]
                                                                {
                                                                        old_entry.value_contribution_msat = value_contribution_msat;
                                                                }
                                                                did_add_update_path_to_src_node = true;
                                                        } else if old_entry.was_processed && new_cost < old_cost {
                                                                #[cfg(all(not(feature = "_bench_unstable"), any(test, fuzzing)))]
                                                                {
                                                                        // If we're skipping processing a node which was previously
                                                                        // processed even though we found another path to it with a
                                                                        // cheaper fee, check that it was because the second path we
                                                                        // found (which we are processing now) has a lower value
                                                                        // contribution due to an HTLC minimum limit.
                                                                        //
                                                                        // e.g. take a graph with two paths from node 1 to node 2, one
                                                                        // through channel A, and one through channel B. Channel A and
                                                                        // B are both in the to-process heap, with their scores set by
                                                                        // a higher htlc_minimum than fee.
                                                                        // Channel A is processed first, and the channels onwards from
                                                                        // node 1 are added to the to-process heap. Thereafter, we pop
                                                                        // Channel B off of the heap, note that it has a much more
                                                                        // restrictive htlc_maximum_msat, and recalculate the fees for
                                                                        // all of node 1's channels using the new, reduced, amount.
                                                                        //
                                                                        // This would be bogus - we'd be selecting a higher-fee path
                                                                        // with a lower htlc_maximum_msat instead of the one we'd
                                                                        // already decided to use.
                                                                        debug_assert!(path_htlc_minimum_msat < old_entry.path_htlc_minimum_msat);
                                                                        debug_assert!(
                                                                                value_contribution_msat + path_penalty_msat <
                                                                                old_entry.value_contribution_msat + old_entry.path_penalty_msat
                                                                        );
                                                                }
                                                        }
                                                }
                                        }
                                }
                        }
                        did_add_update_path_to_src_node
                } }
        }

        let default_node_features = default_node_features();

        // Find ways (channels with destination) to reach a given node and store them
        // in the corresponding data structures (routing graph etc).
        // $fee_to_target_msat represents how much it costs to reach to this node from the payee,
        // meaning how much will be paid in fees after this node (to the best of our knowledge).
        // This data can later be helpful to optimize routing (pay lower fees).
        macro_rules! add_entries_to_cheapest_to_target_node {
                ( $node: expr, $node_id: expr, $fee_to_target_msat: expr, $next_hops_value_contribution: expr,
                  $next_hops_path_htlc_minimum_msat: expr, $next_hops_path_penalty_msat: expr,
                  $next_hops_cltv_delta: expr, $next_hops_path_length: expr ) => {
                        let skip_node = if let Some(elem) = dist.get_mut(&$node_id) {
                                let was_processed = elem.was_processed;
                                elem.was_processed = true;
                                was_processed
                        } else {
                                // Entries are added to dist in add_entry!() when there is a channel from a node.
                                // Because there are no channels from payee, it will not have a dist entry at this point.
                                // If we're processing any other node, it is always be the result of a channel from it.
                                assert_eq!($node_id, payee_node_id);
                                false
                        };

                        if !skip_node {
                                if let Some(first_channels) = first_hop_targets.get(&$node_id) {
                                        for details in first_channels {
                                                let candidate = CandidateRouteHop::FirstHop { details };
                                                add_entry!(candidate, our_node_id, $node_id, $fee_to_target_msat,
                                                        $next_hops_value_contribution,
                                                        $next_hops_path_htlc_minimum_msat, $next_hops_path_penalty_msat,
                                                        $next_hops_cltv_delta, $next_hops_path_length);
                                        }
                                }

                                let features = if let Some(node_info) = $node.announcement_info.as_ref() {
                                        &node_info.features
                                } else {
                                        &default_node_features
                                };

                                if !features.requires_unknown_bits() {
                                        for chan_id in $node.channels.iter() {
                                                let chan = network_channels.get(chan_id).unwrap();
                                                if !chan.features.requires_unknown_bits() {
                                                        if let Some((directed_channel, source)) = chan.as_directed_to(&$node_id) {
                                                                if first_hops.is_none() || *source != our_node_id {
                                                                        if directed_channel.direction().enabled {
                                                                                let candidate = CandidateRouteHop::PublicHop {
                                                                                        info: directed_channel,
                                                                                        short_channel_id: *chan_id,
                                                                                };
                                                                                add_entry!(candidate, *source, $node_id,
                                                                                        $fee_to_target_msat,
                                                                                        $next_hops_value_contribution,
                                                                                        $next_hops_path_htlc_minimum_msat,
                                                                                        $next_hops_path_penalty_msat,
                                                                                        $next_hops_cltv_delta, $next_hops_path_length);
                                                                        }
                                                                }
                                                        }
                                                }
                                        }
                                }
                        }
                };
        }

        let mut payment_paths = Vec::<PaymentPath>::new();

        // TODO: diversify by nodes (so that all paths aren't doomed if one node is offline).
        'paths_collection: loop {
                // For every new path, start from scratch, except for used_channel_liquidities, which
                // helps to avoid reusing previously selected paths in future iterations.
                targets.clear();
                dist.clear();
                hit_minimum_limit = false;

                // If first hop is a private channel and the only way to reach the payee, this is the only
                // place where it could be added.
                if let Some(first_channels) = first_hop_targets.get(&payee_node_id) {
                        for details in first_channels {
                                let candidate = CandidateRouteHop::FirstHop { details };
                                let added = add_entry!(candidate, our_node_id, payee_node_id, 0, path_value_msat,
                                                                        0, 0u64, 0, 0);
                                log_trace!(logger, "{} direct route to payee via SCID {}",
                                                if added { "Added" } else { "Skipped" }, candidate.short_channel_id());
                        }
                }

                // Add the payee as a target, so that the payee-to-payer
                // search algorithm knows what to start with.
                match network_nodes.get(&payee_node_id) {
                        // The payee is not in our network graph, so nothing to add here.
                        // There is still a chance of reaching them via last_hops though,
                        // so don't yet fail the payment here.
                        // If not, targets.pop() will not even let us enter the loop in step 2.
                        None => {},
                        Some(node) => {
                                add_entries_to_cheapest_to_target_node!(node, payee_node_id, 0, path_value_msat, 0, 0u64, 0, 0);
                        },
                }

                // Step (2).
                // If a caller provided us with last hops, add them to routing targets. Since this happens
                // earlier than general path finding, they will be somewhat prioritized, although currently
                // it matters only if the fees are exactly the same.
                for route in payment_params.route_hints.iter().filter(|route| !route.0.is_empty()) {
                        let first_hop_in_route = &(route.0)[0];
                        let have_hop_src_in_graph =
                                // Only add the hops in this route to our candidate set if either
                                // we have a direct channel to the first hop or the first hop is
                                // in the regular network graph.
                                first_hop_targets.get(&NodeId::from_pubkey(&first_hop_in_route.src_node_id)).is_some() ||
                                network_nodes.get(&NodeId::from_pubkey(&first_hop_in_route.src_node_id)).is_some();
                        if have_hop_src_in_graph {
                                // We start building the path from reverse, i.e., from payee
                                // to the first RouteHintHop in the path.
                                let hop_iter = route.0.iter().rev();
                                let prev_hop_iter = core::iter::once(&payment_params.payee_pubkey).chain(
                                        route.0.iter().skip(1).rev().map(|hop| &hop.src_node_id));
                                let mut hop_used = true;
                                let mut aggregate_next_hops_fee_msat: u64 = 0;
                                let mut aggregate_next_hops_path_htlc_minimum_msat: u64 = 0;
                                let mut aggregate_next_hops_path_penalty_msat: u64 = 0;
                                let mut aggregate_next_hops_cltv_delta: u32 = 0;
                                let mut aggregate_next_hops_path_length: u8 = 0;

                                for (idx, (hop, prev_hop_id)) in hop_iter.zip(prev_hop_iter).enumerate() {
                                        let source = NodeId::from_pubkey(&hop.src_node_id);
                                        let target = NodeId::from_pubkey(&prev_hop_id);
                                        let candidate = network_channels
                                                .get(&hop.short_channel_id)
                                                .and_then(|channel| channel.as_directed_to(&target))
                                                .map(|(info, _)| CandidateRouteHop::PublicHop {
                                                        info,
                                                        short_channel_id: hop.short_channel_id,
                                                })
                                                .unwrap_or_else(|| CandidateRouteHop::PrivateHop { hint: hop });

                                        if !add_entry!(candidate, source, target, aggregate_next_hops_fee_msat,
                                                                path_value_msat, aggregate_next_hops_path_htlc_minimum_msat,
                                                                aggregate_next_hops_path_penalty_msat,
                                                                aggregate_next_hops_cltv_delta, aggregate_next_hops_path_length) {
                                                // If this hop was not used then there is no use checking the preceding
                                                // hops in the RouteHint. We can break by just searching for a direct
                                                // channel between last checked hop and first_hop_targets.
                                                hop_used = false;
                                        }

                                        let used_liquidity_msat = used_channel_liquidities
                                                .get(&(hop.short_channel_id, source < target)).copied().unwrap_or(0);
                                        let channel_usage = ChannelUsage {
                                                amount_msat: final_value_msat + aggregate_next_hops_fee_msat,
                                                inflight_htlc_msat: used_liquidity_msat,
                                                effective_capacity: candidate.effective_capacity(),
                                        };
                                        let channel_penalty_msat = scorer.channel_penalty_msat(
                                                hop.short_channel_id, &source, &target, channel_usage
                                        );
                                        aggregate_next_hops_path_penalty_msat = aggregate_next_hops_path_penalty_msat
                                                .saturating_add(channel_penalty_msat);

                                        aggregate_next_hops_cltv_delta = aggregate_next_hops_cltv_delta
                                                .saturating_add(hop.cltv_expiry_delta as u32);

                                        aggregate_next_hops_path_length = aggregate_next_hops_path_length
                                                .saturating_add(1);

                                        // Searching for a direct channel between last checked hop and first_hop_targets
                                        if let Some(first_channels) = first_hop_targets.get(&NodeId::from_pubkey(&prev_hop_id)) {
                                                for details in first_channels {
                                                        let candidate = CandidateRouteHop::FirstHop { details };
                                                        add_entry!(candidate, our_node_id, NodeId::from_pubkey(&prev_hop_id),
                                                                aggregate_next_hops_fee_msat, path_value_msat,
                                                                aggregate_next_hops_path_htlc_minimum_msat,
                                                                aggregate_next_hops_path_penalty_msat, aggregate_next_hops_cltv_delta,
                                                                aggregate_next_hops_path_length);
                                                }
                                        }

                                        if !hop_used {
                                                break;
                                        }

                                        // In the next values of the iterator, the aggregate fees already reflects
                                        // the sum of value sent from payer (final_value_msat) and routing fees
                                        // for the last node in the RouteHint. We need to just add the fees to
                                        // route through the current node so that the preceding node (next iteration)
                                        // can use it.
                                        let hops_fee = compute_fees(aggregate_next_hops_fee_msat + final_value_msat, hop.fees)
                                                .map_or(None, |inc| inc.checked_add(aggregate_next_hops_fee_msat));
                                        aggregate_next_hops_fee_msat = if let Some(val) = hops_fee { val } else { break; };

                                        let hop_htlc_minimum_msat = candidate.htlc_minimum_msat();
                                        let hop_htlc_minimum_msat_inc = if let Some(val) = compute_fees(aggregate_next_hops_path_htlc_minimum_msat, hop.fees) { val } else { break; };
                                        let hops_path_htlc_minimum = aggregate_next_hops_path_htlc_minimum_msat
                                                .checked_add(hop_htlc_minimum_msat_inc);
                                        aggregate_next_hops_path_htlc_minimum_msat = if let Some(val) = hops_path_htlc_minimum { cmp::max(hop_htlc_minimum_msat, val) } else { break; };

                                        if idx == route.0.len() - 1 {
                                                // The last hop in this iterator is the first hop in
                                                // overall RouteHint.
                                                // If this hop connects to a node with which we have a direct channel,
                                                // ignore the network graph and, if the last hop was added, add our
                                                // direct channel to the candidate set.
                                                //
                                                // Note that we *must* check if the last hop was added as `add_entry`
                                                // always assumes that the third argument is a node to which we have a
                                                // path.
                                                if let Some(first_channels) = first_hop_targets.get(&NodeId::from_pubkey(&hop.src_node_id)) {
                                                        for details in first_channels {
                                                                let candidate = CandidateRouteHop::FirstHop { details };
                                                                add_entry!(candidate, our_node_id,
                                                                        NodeId::from_pubkey(&hop.src_node_id),
                                                                        aggregate_next_hops_fee_msat, path_value_msat,
                                                                        aggregate_next_hops_path_htlc_minimum_msat,
                                                                        aggregate_next_hops_path_penalty_msat,
                                                                        aggregate_next_hops_cltv_delta,
                                                                        aggregate_next_hops_path_length);
                                                        }
                                                }
                                        }
                                }
                        }
                }

                log_trace!(logger, "Starting main path collection loop with {} nodes pre-filled from first/last hops.", targets.len());

                // At this point, targets are filled with the data from first and
                // last hops communicated by the caller, and the payment receiver.
                let mut found_new_path = false;

                // Step (3).
                // If this loop terminates due the exhaustion of targets, two situations are possible:
                // - not enough outgoing liquidity:
                //   0 < already_collected_value_msat < final_value_msat
                // - enough outgoing liquidity:
                //   final_value_msat <= already_collected_value_msat < recommended_value_msat
                // Both these cases (and other cases except reaching recommended_value_msat) mean that
                // paths_collection will be stopped because found_new_path==false.
                // This is not necessarily a routing failure.
                'path_construction: while let Some(RouteGraphNode { node_id, lowest_fee_to_node, total_cltv_delta, mut value_contribution_msat, path_htlc_minimum_msat, path_penalty_msat, path_length_to_node, .. }) = targets.pop() {

                        // Since we're going payee-to-payer, hitting our node as a target means we should stop
                        // traversing the graph and arrange the path out of what we found.
                        if node_id == our_node_id {
                                let mut new_entry = dist.remove(&our_node_id).unwrap();
                                let mut ordered_hops: Vec<(PathBuildingHop, NodeFeatures)> = vec!((new_entry.clone(), default_node_features.clone()));

                                'path_walk: loop {
                                        let mut features_set = false;
                                        if let Some(first_channels) = first_hop_targets.get(&ordered_hops.last().unwrap().0.node_id) {
                                                for details in first_channels {
                                                        if details.get_outbound_payment_scid().unwrap() == ordered_hops.last().unwrap().0.candidate.short_channel_id() {
                                                                ordered_hops.last_mut().unwrap().1 = details.counterparty.features.to_context();
                                                                features_set = true;
                                                                break;
                                                        }
                                                }
                                        }
                                        if !features_set {
                                                if let Some(node) = network_nodes.get(&ordered_hops.last().unwrap().0.node_id) {
                                                        if let Some(node_info) = node.announcement_info.as_ref() {
                                                                ordered_hops.last_mut().unwrap().1 = node_info.features.clone();
                                                        } else {
                                                                ordered_hops.last_mut().unwrap().1 = default_node_features.clone();
                                                        }
                                                } else {
                                                        // We can fill in features for everything except hops which were
                                                        // provided via the invoice we're paying. We could guess based on the
                                                        // recipient's features but for now we simply avoid guessing at all.
                                                }
                                        }

                                        // Means we succesfully traversed from the payer to the payee, now
                                        // save this path for the payment route. Also, update the liquidity
                                        // remaining on the used hops, so that we take them into account
                                        // while looking for more paths.
                                        if ordered_hops.last().unwrap().0.node_id == payee_node_id {
                                                break 'path_walk;
                                        }

                                        new_entry = match dist.remove(&ordered_hops.last().unwrap().0.node_id) {
                                                Some(payment_hop) => payment_hop,
                                                // We can't arrive at None because, if we ever add an entry to targets,
                                                // we also fill in the entry in dist (see add_entry!).
                                                None => unreachable!(),
                                        };
                                        // We "propagate" the fees one hop backward (topologically) here,
                                        // so that fees paid for a HTLC forwarding on the current channel are
                                        // associated with the previous channel (where they will be subtracted).
                                        ordered_hops.last_mut().unwrap().0.fee_msat = new_entry.hop_use_fee_msat;
                                        ordered_hops.push((new_entry.clone(), default_node_features.clone()));
                                }
                                ordered_hops.last_mut().unwrap().0.fee_msat = value_contribution_msat;
                                ordered_hops.last_mut().unwrap().0.hop_use_fee_msat = 0;

                                log_trace!(logger, "Found a path back to us from the target with {} hops contributing up to {} msat: \n {:#?}",
                                        ordered_hops.len(), value_contribution_msat, ordered_hops.iter().map(|h| &(h.0)).collect::<Vec<&PathBuildingHop>>());

                                let mut payment_path = PaymentPath {hops: ordered_hops};

                                // We could have possibly constructed a slightly inconsistent path: since we reduce
                                // value being transferred along the way, we could have violated htlc_minimum_msat
                                // on some channels we already passed (assuming dest->source direction). Here, we
                                // recompute the fees again, so that if that's the case, we match the currently
                                // underpaid htlc_minimum_msat with fees.
                                debug_assert_eq!(payment_path.get_value_msat(), value_contribution_msat);
                                value_contribution_msat = cmp::min(value_contribution_msat, final_value_msat);
                                payment_path.update_value_and_recompute_fees(value_contribution_msat);

                                // Since a path allows to transfer as much value as
                                // the smallest channel it has ("bottleneck"), we should recompute
                                // the fees so sender HTLC don't overpay fees when traversing
                                // larger channels than the bottleneck. This may happen because
                                // when we were selecting those channels we were not aware how much value
                                // this path will transfer, and the relative fee for them
                                // might have been computed considering a larger value.
                                // Remember that we used these channels so that we don't rely
                                // on the same liquidity in future paths.
                                let mut prevented_redundant_path_selection = false;
                                let prev_hop_iter = core::iter::once(&our_node_id)
                                        .chain(payment_path.hops.iter().map(|(hop, _)| &hop.node_id));
                                for (prev_hop, (hop, _)) in prev_hop_iter.zip(payment_path.hops.iter()) {
                                        let spent_on_hop_msat = value_contribution_msat + hop.next_hops_fee_msat;
                                        let used_liquidity_msat = used_channel_liquidities
                                                .entry((hop.candidate.short_channel_id(), *prev_hop < hop.node_id))
                                                .and_modify(|used_liquidity_msat| *used_liquidity_msat += spent_on_hop_msat)
                                                .or_insert(spent_on_hop_msat);
                                        let hop_capacity = hop.candidate.effective_capacity();
                                        let hop_max_msat = max_htlc_from_capacity(hop_capacity, channel_saturation_pow_half);
                                        if *used_liquidity_msat == hop_max_msat {
                                                // If this path used all of this channel's available liquidity, we know
                                                // this path will not be selected again in the next loop iteration.
                                                prevented_redundant_path_selection = true;
                                        }
                                        debug_assert!(*used_liquidity_msat <= hop_max_msat);
                                }
                                if !prevented_redundant_path_selection {
                                        // If we weren't capped by hitting a liquidity limit on a channel in the path,
                                        // we'll probably end up picking the same path again on the next iteration.
                                        // Decrease the available liquidity of a hop in the middle of the path.
                                        let victim_scid = payment_path.hops[(payment_path.hops.len()) / 2].0.candidate.short_channel_id();
                                        let exhausted = u64::max_value();
                                        log_trace!(logger, "Disabling channel {} for future path building iterations to avoid duplicates.", victim_scid);
                                        *used_channel_liquidities.entry((victim_scid, false)).or_default() = exhausted;
                                        *used_channel_liquidities.entry((victim_scid, true)).or_default() = exhausted;
                                }

                                // Track the total amount all our collected paths allow to send so that we know
                                // when to stop looking for more paths
                                already_collected_value_msat += value_contribution_msat;

                                payment_paths.push(payment_path);
                                found_new_path = true;
                                break 'path_construction;
                        }

                        // If we found a path back to the payee, we shouldn't try to process it again. This is
                        // the equivalent of the `elem.was_processed` check in
                        // add_entries_to_cheapest_to_target_node!() (see comment there for more info).
                        if node_id == payee_node_id { continue 'path_construction; }

                        // Otherwise, since the current target node is not us,
                        // keep "unrolling" the payment graph from payee to payer by
                        // finding a way to reach the current target from the payer side.
                        match network_nodes.get(&node_id) {
                                None => {},
                                Some(node) => {
                                        add_entries_to_cheapest_to_target_node!(node, node_id, lowest_fee_to_node,
                                                value_contribution_msat, path_htlc_minimum_msat, path_penalty_msat,
                                                total_cltv_delta, path_length_to_node);
                                },
                        }
                }

                if !allow_mpp {
                        if !found_new_path && channel_saturation_pow_half != 0 {
                                channel_saturation_pow_half = 0;
                                continue 'paths_collection;
                        }
                        // If we don't support MPP, no use trying to gather more value ever.
                        break 'paths_collection;
                }

                // Step (4).
                // Stop either when the recommended value is reached or if no new path was found in this
                // iteration.
                // In the latter case, making another path finding attempt won't help,
                // because we deterministically terminated the search due to low liquidity.
                if !found_new_path && channel_saturation_pow_half != 0 {
                        channel_saturation_pow_half = 0;
                } else if already_collected_value_msat >= recommended_value_msat || !found_new_path {
                        log_trace!(logger, "Have now collected {} msat (seeking {} msat) in paths. Last path loop {} a new path.",
                                already_collected_value_msat, recommended_value_msat, if found_new_path { "found" } else { "did not find" });
                        break 'paths_collection;
                } else if found_new_path && already_collected_value_msat == final_value_msat && payment_paths.len() == 1 {
                        // Further, if this was our first walk of the graph, and we weren't limited by an
                        // htlc_minimum_msat, return immediately because this path should suffice. If we were
                        // limited by an htlc_minimum_msat value, find another path with a higher value,
                        // potentially allowing us to pay fees to meet the htlc_minimum on the new path while
                        // still keeping a lower total fee than this path.
                        if !hit_minimum_limit {
                                log_trace!(logger, "Collected exactly our payment amount on the first pass, without hitting an htlc_minimum_msat limit, exiting.");
                                break 'paths_collection;
                        }
                        log_trace!(logger, "Collected our payment amount on the first pass, but running again to collect extra paths with a potentially higher limit.");
                        path_value_msat = recommended_value_msat;
                }
        }

        // Step (5).
        if payment_paths.len() == 0 {
                return Err(LightningError{err: "Failed to find a path to the given destination".to_owned(), action: ErrorAction::IgnoreError});
        }

        if already_collected_value_msat < final_value_msat {
                return Err(LightningError{err: "Failed to find a sufficient route to the given destination".to_owned(), action: ErrorAction::IgnoreError});
        }

        // Step (6).
        let mut selected_route = payment_paths;

        debug_assert_eq!(selected_route.iter().map(|p| p.get_value_msat()).sum::<u64>(), already_collected_value_msat);
        let mut overpaid_value_msat = already_collected_value_msat - final_value_msat;

        // First, sort by the cost-per-value of the path, dropping the paths that cost the most for
        // the value they contribute towards the payment amount.
        // We sort in descending order as we will remove from the front in `retain`, next.
        selected_route.sort_unstable_by(|a, b|
                (((b.get_cost_msat() as u128) << 64) / (b.get_value_msat() as u128))
                        .cmp(&(((a.get_cost_msat() as u128) << 64) / (a.get_value_msat() as u128)))
        );

        // We should make sure that at least 1 path left.
        let mut paths_left = selected_route.len();
        selected_route.retain(|path| {
                if paths_left == 1 {
                        return true
                }
                let path_value_msat = path.get_value_msat();
                if path_value_msat <= overpaid_value_msat {
                        overpaid_value_msat -= path_value_msat;
                        paths_left -= 1;
                        return false;
                }
                true
        });
        debug_assert!(selected_route.len() > 0);

        if overpaid_value_msat != 0 {
                // Step (7).
                // Now, subtract the remaining overpaid value from the most-expensive path.
                // TODO: this could also be optimized by also sorting by feerate_per_sat_routed,
                // so that the sender pays less fees overall. And also htlc_minimum_msat.
                selected_route.sort_unstable_by(|a, b| {
                        let a_f = a.hops.iter().map(|hop| hop.0.candidate.fees().proportional_millionths as u64).sum::<u64>();
                        let b_f = b.hops.iter().map(|hop| hop.0.candidate.fees().proportional_millionths as u64).sum::<u64>();
                        a_f.cmp(&b_f).then_with(|| b.get_cost_msat().cmp(&a.get_cost_msat()))
                });
                let expensive_payment_path = selected_route.first_mut().unwrap();

                // We already dropped all the paths with value below `overpaid_value_msat` above, thus this
                // can't go negative.
                let expensive_path_new_value_msat = expensive_payment_path.get_value_msat() - overpaid_value_msat;
                expensive_payment_path.update_value_and_recompute_fees(expensive_path_new_value_msat);
        }

        // Step (8).
        // Sort by the path itself and combine redundant paths.
        // Note that we sort by SCIDs alone as its simpler but when combining we have to ensure we
        // compare both SCIDs and NodeIds as individual nodes may use random aliases causing collisions
        // across nodes.
        selected_route.sort_unstable_by_key(|path| {
                let mut key = [0u64; MAX_PATH_LENGTH_ESTIMATE as usize];
                debug_assert!(path.hops.len() <= key.len());
                for (scid, key) in path.hops.iter().map(|h| h.0.candidate.short_channel_id()).zip(key.iter_mut()) {
                        *key = scid;
                }
                key
        });
        for idx in 0..(selected_route.len() - 1) {
                if idx + 1 >= selected_route.len() { break; }
                if iter_equal(selected_route[idx    ].hops.iter().map(|h| (h.0.candidate.short_channel_id(), h.0.node_id)),
                              selected_route[idx + 1].hops.iter().map(|h| (h.0.candidate.short_channel_id(), h.0.node_id))) {
                        let new_value = selected_route[idx].get_value_msat() + selected_route[idx + 1].get_value_msat();
                        selected_route[idx].update_value_and_recompute_fees(new_value);
                        selected_route.remove(idx + 1);
                }
        }

        let mut selected_paths = Vec::<Vec<Result<RouteHop, LightningError>>>::new();
        for payment_path in selected_route {
                let mut path = payment_path.hops.iter().map(|(payment_hop, node_features)| {
                        Ok(RouteHop {
                                pubkey: PublicKey::from_slice(payment_hop.node_id.as_slice()).map_err(|_| LightningError{err: format!("Public key {:?} is invalid", &payment_hop.node_id), action: ErrorAction::IgnoreAndLog(Level::Trace)})?,
                                node_features: node_features.clone(),
                                short_channel_id: payment_hop.candidate.short_channel_id(),
                                channel_features: payment_hop.candidate.features(),
                                fee_msat: payment_hop.fee_msat,
                                cltv_expiry_delta: payment_hop.candidate.cltv_expiry_delta(),
                        })
                }).collect::<Vec<_>>();
                // Propagate the cltv_expiry_delta one hop backwards since the delta from the current hop is
                // applicable for the previous hop.
                path.iter_mut().rev().fold(final_cltv_expiry_delta, |prev_cltv_expiry_delta, hop| {
                        core::mem::replace(&mut hop.as_mut().unwrap().cltv_expiry_delta, prev_cltv_expiry_delta)
                });
                selected_paths.push(path);
        }
        // Make sure we would never create a route with more paths than we allow.
        debug_assert!(selected_paths.len() <= payment_params.max_path_count.into());

        if let Some(features) = &payment_params.features {
                for path in selected_paths.iter_mut() {
                        if let Ok(route_hop) = path.last_mut().unwrap() {
                                route_hop.node_features = features.to_context();
                        }
                }
        }

        let route = Route {
                paths: selected_paths.into_iter().map(|path| path.into_iter().collect()).collect::<Result<Vec<_>, _>>()?,
                payment_params: Some(payment_params.clone()),
        };
        log_info!(logger, "Got route to {}: {}", payment_params.payee_pubkey, log_route!(route));
        Ok(route)
}

// When an adversarial intermediary node observes a payment, it may be able to infer its
// destination, if the remaining CLTV expiry delta exactly matches a feasible path in the network
// graph. In order to improve privacy, this method obfuscates the CLTV expiry deltas along the
// payment path by adding a randomized 'shadow route' offset to the final hop.
fn add_random_cltv_offset(route: &mut Route, payment_params: &PaymentParameters,
        network_graph: &ReadOnlyNetworkGraph, random_seed_bytes: &[u8; 32]
) {
        let network_channels = network_graph.channels();
        let network_nodes = network_graph.nodes();

        for path in route.paths.iter_mut() {
                let mut shadow_ctlv_expiry_delta_offset: u32 = 0;

                // Remember the last three nodes of the random walk and avoid looping back on them.
                // Init with the last three nodes from the actual path, if possible.
                let mut nodes_to_avoid: [NodeId; 3] = [NodeId::from_pubkey(&path.last().unwrap().pubkey),
                        NodeId::from_pubkey(&path.get(path.len().saturating_sub(2)).unwrap().pubkey),
                        NodeId::from_pubkey(&path.get(path.len().saturating_sub(3)).unwrap().pubkey)];

                // Choose the last publicly known node as the starting point for the random walk.
                let mut cur_hop: Option<NodeId> = None;
                let mut path_nonce = [0u8; 12];
                if let Some(starting_hop) = path.iter().rev()
                        .find(|h| network_nodes.contains_key(&NodeId::from_pubkey(&h.pubkey))) {
                                cur_hop = Some(NodeId::from_pubkey(&starting_hop.pubkey));
                                path_nonce.copy_from_slice(&cur_hop.unwrap().as_slice()[..12]);
                }

                // Init PRNG with the path-dependant nonce, which is static for private paths.
                let mut prng = ChaCha20::new(random_seed_bytes, &path_nonce);
                let mut random_path_bytes = [0u8; ::core::mem::size_of::<usize>()];

                // Pick a random path length in [1 .. 3]
                prng.process_in_place(&mut random_path_bytes);
                let random_walk_length = usize::from_be_bytes(random_path_bytes).wrapping_rem(3).wrapping_add(1);

                for random_hop in 0..random_walk_length {
                        // If we don't find a suitable offset in the public network graph, we default to
                        // MEDIAN_HOP_CLTV_EXPIRY_DELTA.
                        let mut random_hop_offset = MEDIAN_HOP_CLTV_EXPIRY_DELTA;

                        if let Some(cur_node_id) = cur_hop {
                                if let Some(cur_node) = network_nodes.get(&cur_node_id) {
                                        // Randomly choose the next unvisited hop.
                                        prng.process_in_place(&mut random_path_bytes);
                                        if let Some(random_channel) = usize::from_be_bytes(random_path_bytes)
                                                .checked_rem(cur_node.channels.len())
                                                .and_then(|index| cur_node.channels.get(index))
                                                .and_then(|id| network_channels.get(id)) {
                                                        random_channel.as_directed_from(&cur_node_id).map(|(dir_info, next_id)| {
                                                                if !nodes_to_avoid.iter().any(|x| x == next_id) {
                                                                        nodes_to_avoid[random_hop] = *next_id;
                                                                        random_hop_offset = dir_info.direction().cltv_expiry_delta.into();
                                                                        cur_hop = Some(*next_id);
                                                                }
                                                        });
                                                }
                                }
                        }

                        shadow_ctlv_expiry_delta_offset = shadow_ctlv_expiry_delta_offset
                                .checked_add(random_hop_offset)
                                .unwrap_or(shadow_ctlv_expiry_delta_offset);
                }

                // Limit the total offset to reduce the worst-case locked liquidity timevalue
                const MAX_SHADOW_CLTV_EXPIRY_DELTA_OFFSET: u32 = 3*144;
                shadow_ctlv_expiry_delta_offset = cmp::min(shadow_ctlv_expiry_delta_offset, MAX_SHADOW_CLTV_EXPIRY_DELTA_OFFSET);

                // Limit the offset so we never exceed the max_total_cltv_expiry_delta. To improve plausibility,
                // we choose the limit to be the largest possible multiple of MEDIAN_HOP_CLTV_EXPIRY_DELTA.
                let path_total_cltv_expiry_delta: u32 = path.iter().map(|h| h.cltv_expiry_delta).sum();
                let mut max_path_offset = payment_params.max_total_cltv_expiry_delta - path_total_cltv_expiry_delta;
                max_path_offset = cmp::max(
                        max_path_offset - (max_path_offset % MEDIAN_HOP_CLTV_EXPIRY_DELTA),
                        max_path_offset % MEDIAN_HOP_CLTV_EXPIRY_DELTA);
                shadow_ctlv_expiry_delta_offset = cmp::min(shadow_ctlv_expiry_delta_offset, max_path_offset);

                // Add 'shadow' CLTV offset to the final hop
                if let Some(last_hop) = path.last_mut() {
                        last_hop.cltv_expiry_delta = last_hop.cltv_expiry_delta
                                .checked_add(shadow_ctlv_expiry_delta_offset).unwrap_or(last_hop.cltv_expiry_delta);
                }
        }
}

/// Construct a route from us (payer) to the target node (payee) via the given hops (which should
/// exclude the payer, but include the payee). This may be useful, e.g., for probing the chosen path.
///
/// Re-uses logic from `find_route`, so the restrictions described there also apply here.
pub fn build_route_from_hops<L: Deref, GL: Deref>(
        our_node_pubkey: &PublicKey, hops: &[PublicKey], route_params: &RouteParameters,
        network_graph: &NetworkGraph<GL>, logger: L, random_seed_bytes: &[u8; 32]
) -> Result<Route, LightningError>
where L::Target: Logger, GL::Target: Logger {
        let graph_lock = network_graph.read_only();
        let mut route = build_route_from_hops_internal(
                our_node_pubkey, hops, &route_params.payment_params, &graph_lock,
                route_params.final_value_msat, route_params.final_cltv_expiry_delta, logger, random_seed_bytes)?;
        add_random_cltv_offset(&mut route, &route_params.payment_params, &graph_lock, random_seed_bytes);
        Ok(route)
}

fn build_route_from_hops_internal<L: Deref>(
        our_node_pubkey: &PublicKey, hops: &[PublicKey], payment_params: &PaymentParameters,
        network_graph: &ReadOnlyNetworkGraph, final_value_msat: u64, final_cltv_expiry_delta: u32,
        logger: L, random_seed_bytes: &[u8; 32]
) -> Result<Route, LightningError> where L::Target: Logger {

        struct HopScorer {
                our_node_id: NodeId,
                hop_ids: [Option<NodeId>; MAX_PATH_LENGTH_ESTIMATE as usize],
        }

        impl Score for HopScorer {
                fn channel_penalty_msat(&self, _short_channel_id: u64, source: &NodeId, target: &NodeId,
                        _usage: ChannelUsage) -> u64
                {
                        let mut cur_id = self.our_node_id;
                        for i in 0..self.hop_ids.len() {
                                if let Some(next_id) = self.hop_ids[i] {
                                        if cur_id == *source && next_id == *target {
                                                return 0;
                                        }
                                        cur_id = next_id;
                                } else {
                                        break;
                                }
                        }
                        u64::max_value()
                }

                fn payment_path_failed(&mut self, _path: &[&RouteHop], _short_channel_id: u64) {}

                fn payment_path_successful(&mut self, _path: &[&RouteHop]) {}

                fn probe_failed(&mut self, _path: &[&RouteHop], _short_channel_id: u64) {}

                fn probe_successful(&mut self, _path: &[&RouteHop]) {}
        }

        impl<'a> Writeable for HopScorer {
                #[inline]
                fn write<W: Writer>(&self, _w: &mut W) -> Result<(), io::Error> {
                        unreachable!();
                }
        }

        if hops.len() > MAX_PATH_LENGTH_ESTIMATE.into() {
                return Err(LightningError{err: "Cannot build a route exceeding the maximum path length.".to_owned(), action: ErrorAction::IgnoreError});
        }

        let our_node_id = NodeId::from_pubkey(our_node_pubkey);
        let mut hop_ids = [None; MAX_PATH_LENGTH_ESTIMATE as usize];
        for i in 0..hops.len() {
                hop_ids[i] = Some(NodeId::from_pubkey(&hops[i]));
        }

        let scorer = HopScorer { our_node_id, hop_ids };

        get_route(our_node_pubkey, payment_params, network_graph, None, final_value_msat,
                final_cltv_expiry_delta, logger, &scorer, random_seed_bytes)
}

#[cfg(test)]
mod tests {
        use crate::routing::gossip::{NetworkGraph, P2PGossipSync, NodeId, EffectiveCapacity};
        use crate::routing::router::{get_route, build_route_from_hops_internal, add_random_cltv_offset, default_node_features,
                PaymentParameters, Route, RouteHint, RouteHintHop, RouteHop, RoutingFees,
                DEFAULT_MAX_TOTAL_CLTV_EXPIRY_DELTA, MAX_PATH_LENGTH_ESTIMATE};
        use crate::routing::scoring::{ChannelUsage, Score, ProbabilisticScorer, ProbabilisticScoringParameters};
        use crate::routing::test_utils::{add_channel, add_or_update_node, build_graph, build_line_graph, id_to_feature_flags, get_nodes, update_channel};
        use crate::chain::transaction::OutPoint;
        use crate::chain::keysinterface::{EntropySource, KeysInterface};
        use crate::ln::features::{ChannelFeatures, InitFeatures, NodeFeatures};
        use crate::ln::msgs::{ErrorAction, LightningError, UnsignedChannelUpdate, MAX_VALUE_MSAT};
        use crate::ln::channelmanager;
        use crate::util::test_utils as ln_test_utils;
        use crate::util::chacha20::ChaCha20;
        #[cfg(c_bindings)]
        use crate::util::ser::{Writeable, Writer};

        use bitcoin::hashes::Hash;
        use bitcoin::network::constants::Network;
        use bitcoin::blockdata::constants::genesis_block;
        use bitcoin::blockdata::script::Builder;
        use bitcoin::blockdata::opcodes;
        use bitcoin::blockdata::transaction::TxOut;

        use hex;

        use bitcoin::secp256k1::{PublicKey,SecretKey};
        use bitcoin::secp256k1::Secp256k1;

        use crate::prelude::*;
        use crate::sync::Arc;

        use core::convert::TryInto;

        fn get_channel_details(short_channel_id: Option<u64>, node_id: PublicKey,
                        features: InitFeatures, outbound_capacity_msat: u64) -> channelmanager::ChannelDetails {
                channelmanager::ChannelDetails {
                        channel_id: [0; 32],
                        counterparty: channelmanager::ChannelCounterparty {
                                features,
                                node_id,
                                unspendable_punishment_reserve: 0,
                                forwarding_info: None,
                                outbound_htlc_minimum_msat: None,
                                outbound_htlc_maximum_msat: None,
                        },
                        funding_txo: Some(OutPoint { txid: bitcoin::Txid::from_slice(&[0; 32]).unwrap(), index: 0 }),
                        channel_type: None,
                        short_channel_id,
                        outbound_scid_alias: None,
                        inbound_scid_alias: None,
                        channel_value_satoshis: 0,
                        user_channel_id: 0,
                        balance_msat: 0,
                        outbound_capacity_msat,
                        next_outbound_htlc_limit_msat: outbound_capacity_msat,
                        inbound_capacity_msat: 42,
                        unspendable_punishment_reserve: None,
                        confirmations_required: None,
                        confirmations: None,
                        force_close_spend_delay: None,
                        is_outbound: true, is_channel_ready: true,
                        is_usable: true, is_public: true,
                        inbound_htlc_minimum_msat: None,
                        inbound_htlc_maximum_msat: None,
                        config: None,
                }
        }

        #[test]
        fn simple_route_test() {
                let (secp_ctx, network_graph, _, _, logger) = build_graph();
                let (_, our_id, _, nodes) = get_nodes(&secp_ctx);
                let payment_params = PaymentParameters::from_node_id(nodes[2]);
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();

                // Simple route to 2 via 1

                if let Err(LightningError{err, action: ErrorAction::IgnoreError}) = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 0, 42, Arc::clone(&logger), &scorer, &random_seed_bytes) {
                        assert_eq!(err, "Cannot send a payment of 0 msat");
                } else { panic!(); }

                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 100, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                assert_eq!(route.paths[0].len(), 2);

                assert_eq!(route.paths[0][0].pubkey, nodes[1]);
                assert_eq!(route.paths[0][0].short_channel_id, 2);
                assert_eq!(route.paths[0][0].fee_msat, 100);
                assert_eq!(route.paths[0][0].cltv_expiry_delta, (4 << 4) | 1);
                assert_eq!(route.paths[0][0].node_features.le_flags(), &id_to_feature_flags(2));
                assert_eq!(route.paths[0][0].channel_features.le_flags(), &id_to_feature_flags(2));

                assert_eq!(route.paths[0][1].pubkey, nodes[2]);
                assert_eq!(route.paths[0][1].short_channel_id, 4);
                assert_eq!(route.paths[0][1].fee_msat, 100);
                assert_eq!(route.paths[0][1].cltv_expiry_delta, 42);
                assert_eq!(route.paths[0][1].node_features.le_flags(), &id_to_feature_flags(3));
                assert_eq!(route.paths[0][1].channel_features.le_flags(), &id_to_feature_flags(4));
        }

        #[test]
        fn invalid_first_hop_test() {
                let (secp_ctx, network_graph, _, _, logger) = build_graph();
                let (_, our_id, _, nodes) = get_nodes(&secp_ctx);
                let payment_params = PaymentParameters::from_node_id(nodes[2]);
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();

                // Simple route to 2 via 1

                let our_chans = vec![get_channel_details(Some(2), our_id, InitFeatures::from_le_bytes(vec![0b11]), 100000)];

                if let Err(LightningError{err, action: ErrorAction::IgnoreError}) =
                        get_route(&our_id, &payment_params, &network_graph.read_only(), Some(&our_chans.iter().collect::<Vec<_>>()), 100, 42, Arc::clone(&logger), &scorer, &random_seed_bytes) {
                        assert_eq!(err, "First hop cannot have our_node_pubkey as a destination.");
                } else { panic!(); }

                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 100, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                assert_eq!(route.paths[0].len(), 2);
        }

        #[test]
        fn htlc_minimum_test() {
                let (secp_ctx, network_graph, gossip_sync, _, logger) = build_graph();
                let (our_privkey, our_id, privkeys, nodes) = get_nodes(&secp_ctx);
                let payment_params = PaymentParameters::from_node_id(nodes[2]);
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();

                // Simple route to 2 via 1

                // Disable other paths
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 12,
                        timestamp: 2,
                        flags: 2, // to disable
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[0], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 3,
                        timestamp: 2,
                        flags: 2, // to disable
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[7], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 13,
                        timestamp: 2,
                        flags: 2, // to disable
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[2], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 6,
                        timestamp: 2,
                        flags: 2, // to disable
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[2], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 7,
                        timestamp: 2,
                        flags: 2, // to disable
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Check against amount_to_transfer_over_msat.
                // Set minimal HTLC of 200_000_000 msat.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 2,
                        timestamp: 3,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 200_000_000,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Second hop only allows to forward 199_999_999 at most, thus not allowing the first hop to
                // be used.
                update_channel(&gossip_sync, &secp_ctx, &privkeys[1], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 4,
                        timestamp: 3,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 199_999_999,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Not possible to send 199_999_999, because the minimum on channel=2 is 200_000_000.
                if let Err(LightningError{err, action: ErrorAction::IgnoreError}) = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 199_999_999, 42, Arc::clone(&logger), &scorer, &random_seed_bytes) {
                        assert_eq!(err, "Failed to find a path to the given destination");
                } else { panic!(); }

                // Lift the restriction on the first hop.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 2,
                        timestamp: 4,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // A payment above the minimum should pass
                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 199_999_999, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                assert_eq!(route.paths[0].len(), 2);
        }

        #[test]
        fn htlc_minimum_overpay_test() {
                let (secp_ctx, network_graph, gossip_sync, _, logger) = build_graph();
                let (our_privkey, our_id, privkeys, nodes) = get_nodes(&secp_ctx);
                let payment_params = PaymentParameters::from_node_id(nodes[2]).with_features(channelmanager::provided_invoice_features());
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();

                // A route to node#2 via two paths.
                // One path allows transferring 35-40 sats, another one also allows 35-40 sats.
                // Thus, they can't send 60 without overpaying.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 2,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 35_000,
                        htlc_maximum_msat: 40_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 12,
                        timestamp: 3,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 35_000,
                        htlc_maximum_msat: 40_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Make 0 fee.
                update_channel(&gossip_sync, &secp_ctx, &privkeys[7], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 13,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[1], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 4,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Disable other paths
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 1,
                        timestamp: 3,
                        flags: 2, // to disable
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 60_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                // Overpay fees to hit htlc_minimum_msat.
                let overpaid_fees = route.paths[0][0].fee_msat + route.paths[1][0].fee_msat;
                // TODO: this could be better balanced to overpay 10k and not 15k.
                assert_eq!(overpaid_fees, 15_000);

                // Now, test that if there are 2 paths, a "cheaper" by fee path wouldn't be prioritized
                // while taking even more fee to match htlc_minimum_msat.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 12,
                        timestamp: 4,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 65_000,
                        htlc_maximum_msat: 80_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 2,
                        timestamp: 3,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[1], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 4,
                        timestamp: 4,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 100_000,
                        excess_data: Vec::new()
                });

                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 60_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                // Fine to overpay for htlc_minimum_msat if it allows us to save fee.
                assert_eq!(route.paths.len(), 1);
                assert_eq!(route.paths[0][0].short_channel_id, 12);
                let fees = route.paths[0][0].fee_msat;
                assert_eq!(fees, 5_000);

                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 50_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                // Not fine to overpay for htlc_minimum_msat if it requires paying more than fee on
                // the other channel.
                assert_eq!(route.paths.len(), 1);
                assert_eq!(route.paths[0][0].short_channel_id, 2);
                let fees = route.paths[0][0].fee_msat;
                assert_eq!(fees, 5_000);
        }

        #[test]
        fn disable_channels_test() {
                let (secp_ctx, network_graph, gossip_sync, _, logger) = build_graph();
                let (our_privkey, our_id, privkeys, nodes) = get_nodes(&secp_ctx);
                let payment_params = PaymentParameters::from_node_id(nodes[2]);
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();

                // // Disable channels 4 and 12 by flags=2
                update_channel(&gossip_sync, &secp_ctx, &privkeys[1], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 4,
                        timestamp: 2,
                        flags: 2, // to disable
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 12,
                        timestamp: 2,
                        flags: 2, // to disable
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // If all the channels require some features we don't understand, route should fail
                if let Err(LightningError{err, action: ErrorAction::IgnoreError}) = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 100, 42, Arc::clone(&logger), &scorer, &random_seed_bytes) {
                        assert_eq!(err, "Failed to find a path to the given destination");
                } else { panic!(); }

                // If we specify a channel to node7, that overrides our local channel view and that gets used
                let our_chans = vec![get_channel_details(Some(42), nodes[7].clone(), InitFeatures::from_le_bytes(vec![0b11]), 250_000_000)];
                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), Some(&our_chans.iter().collect::<Vec<_>>()), 100, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                assert_eq!(route.paths[0].len(), 2);

                assert_eq!(route.paths[0][0].pubkey, nodes[7]);
                assert_eq!(route.paths[0][0].short_channel_id, 42);
                assert_eq!(route.paths[0][0].fee_msat, 200);
                assert_eq!(route.paths[0][0].cltv_expiry_delta, (13 << 4) | 1);
                assert_eq!(route.paths[0][0].node_features.le_flags(), &vec![0b11]); // it should also override our view of their features
                assert_eq!(route.paths[0][0].channel_features.le_flags(), &Vec::<u8>::new()); // No feature flags will meet the relevant-to-channel conversion

                assert_eq!(route.paths[0][1].pubkey, nodes[2]);
                assert_eq!(route.paths[0][1].short_channel_id, 13);
                assert_eq!(route.paths[0][1].fee_msat, 100);
                assert_eq!(route.paths[0][1].cltv_expiry_delta, 42);
                assert_eq!(route.paths[0][1].node_features.le_flags(), &id_to_feature_flags(3));
                assert_eq!(route.paths[0][1].channel_features.le_flags(), &id_to_feature_flags(13));
        }

        #[test]
        fn disable_node_test() {
                let (secp_ctx, network_graph, gossip_sync, _, logger) = build_graph();
                let (_, our_id, privkeys, nodes) = get_nodes(&secp_ctx);
                let payment_params = PaymentParameters::from_node_id(nodes[2]);
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();

                // Disable nodes 1, 2, and 8 by requiring unknown feature bits
                let mut unknown_features = NodeFeatures::empty();
                unknown_features.set_unknown_feature_required();
                add_or_update_node(&gossip_sync, &secp_ctx, &privkeys[0], unknown_features.clone(), 1);
                add_or_update_node(&gossip_sync, &secp_ctx, &privkeys[1], unknown_features.clone(), 1);
                add_or_update_node(&gossip_sync, &secp_ctx, &privkeys[7], unknown_features.clone(), 1);

                // If all nodes require some features we don't understand, route should fail
                if let Err(LightningError{err, action: ErrorAction::IgnoreError}) = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 100, 42, Arc::clone(&logger), &scorer, &random_seed_bytes) {
                        assert_eq!(err, "Failed to find a path to the given destination");
                } else { panic!(); }

                // If we specify a channel to node7, that overrides our local channel view and that gets used
                let our_chans = vec![get_channel_details(Some(42), nodes[7].clone(), InitFeatures::from_le_bytes(vec![0b11]), 250_000_000)];
                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), Some(&our_chans.iter().collect::<Vec<_>>()), 100, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                assert_eq!(route.paths[0].len(), 2);

                assert_eq!(route.paths[0][0].pubkey, nodes[7]);
                assert_eq!(route.paths[0][0].short_channel_id, 42);
                assert_eq!(route.paths[0][0].fee_msat, 200);
                assert_eq!(route.paths[0][0].cltv_expiry_delta, (13 << 4) | 1);
                assert_eq!(route.paths[0][0].node_features.le_flags(), &vec![0b11]); // it should also override our view of their features
                assert_eq!(route.paths[0][0].channel_features.le_flags(), &Vec::<u8>::new()); // No feature flags will meet the relevant-to-channel conversion

                assert_eq!(route.paths[0][1].pubkey, nodes[2]);
                assert_eq!(route.paths[0][1].short_channel_id, 13);
                assert_eq!(route.paths[0][1].fee_msat, 100);
                assert_eq!(route.paths[0][1].cltv_expiry_delta, 42);
                assert_eq!(route.paths[0][1].node_features.le_flags(), &id_to_feature_flags(3));
                assert_eq!(route.paths[0][1].channel_features.le_flags(), &id_to_feature_flags(13));

                // Note that we don't test disabling node 3 and failing to route to it, as we (somewhat
                // naively) assume that the user checked the feature bits on the invoice, which override
                // the node_announcement.
        }

        #[test]
        fn our_chans_test() {
                let (secp_ctx, network_graph, _, _, logger) = build_graph();
                let (_, our_id, _, nodes) = get_nodes(&secp_ctx);
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();

                // Route to 1 via 2 and 3 because our channel to 1 is disabled
                let payment_params = PaymentParameters::from_node_id(nodes[0]);
                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 100, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                assert_eq!(route.paths[0].len(), 3);

                assert_eq!(route.paths[0][0].pubkey, nodes[1]);
                assert_eq!(route.paths[0][0].short_channel_id, 2);
                assert_eq!(route.paths[0][0].fee_msat, 200);
                assert_eq!(route.paths[0][0].cltv_expiry_delta, (4 << 4) | 1);
                assert_eq!(route.paths[0][0].node_features.le_flags(), &id_to_feature_flags(2));
                assert_eq!(route.paths[0][0].channel_features.le_flags(), &id_to_feature_flags(2));

                assert_eq!(route.paths[0][1].pubkey, nodes[2]);
                assert_eq!(route.paths[0][1].short_channel_id, 4);
                assert_eq!(route.paths[0][1].fee_msat, 100);
                assert_eq!(route.paths[0][1].cltv_expiry_delta, (3 << 4) | 2);
                assert_eq!(route.paths[0][1].node_features.le_flags(), &id_to_feature_flags(3));
                assert_eq!(route.paths[0][1].channel_features.le_flags(), &id_to_feature_flags(4));

                assert_eq!(route.paths[0][2].pubkey, nodes[0]);
                assert_eq!(route.paths[0][2].short_channel_id, 3);
                assert_eq!(route.paths[0][2].fee_msat, 100);
                assert_eq!(route.paths[0][2].cltv_expiry_delta, 42);
                assert_eq!(route.paths[0][2].node_features.le_flags(), &id_to_feature_flags(1));
                assert_eq!(route.paths[0][2].channel_features.le_flags(), &id_to_feature_flags(3));

                // If we specify a channel to node7, that overrides our local channel view and that gets used
                let payment_params = PaymentParameters::from_node_id(nodes[2]);
                let our_chans = vec![get_channel_details(Some(42), nodes[7].clone(), InitFeatures::from_le_bytes(vec![0b11]), 250_000_000)];
                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), Some(&our_chans.iter().collect::<Vec<_>>()), 100, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                assert_eq!(route.paths[0].len(), 2);

                assert_eq!(route.paths[0][0].pubkey, nodes[7]);
                assert_eq!(route.paths[0][0].short_channel_id, 42);
                assert_eq!(route.paths[0][0].fee_msat, 200);
                assert_eq!(route.paths[0][0].cltv_expiry_delta, (13 << 4) | 1);
                assert_eq!(route.paths[0][0].node_features.le_flags(), &vec![0b11]);
                assert_eq!(route.paths[0][0].channel_features.le_flags(), &Vec::<u8>::new()); // No feature flags will meet the relevant-to-channel conversion

                assert_eq!(route.paths[0][1].pubkey, nodes[2]);
                assert_eq!(route.paths[0][1].short_channel_id, 13);
                assert_eq!(route.paths[0][1].fee_msat, 100);
                assert_eq!(route.paths[0][1].cltv_expiry_delta, 42);
                assert_eq!(route.paths[0][1].node_features.le_flags(), &id_to_feature_flags(3));
                assert_eq!(route.paths[0][1].channel_features.le_flags(), &id_to_feature_flags(13));
        }

        fn last_hops(nodes: &Vec<PublicKey>) -> Vec<RouteHint> {
                let zero_fees = RoutingFees {
                        base_msat: 0,
                        proportional_millionths: 0,
                };
                vec![RouteHint(vec![RouteHintHop {
                        src_node_id: nodes[3],
                        short_channel_id: 8,
                        fees: zero_fees,
                        cltv_expiry_delta: (8 << 4) | 1,
                        htlc_minimum_msat: None,
                        htlc_maximum_msat: None,
                }
                ]), RouteHint(vec![RouteHintHop {
                        src_node_id: nodes[4],
                        short_channel_id: 9,
                        fees: RoutingFees {
                                base_msat: 1001,
                                proportional_millionths: 0,
                        },
                        cltv_expiry_delta: (9 << 4) | 1,
                        htlc_minimum_msat: None,
                        htlc_maximum_msat: None,
                }]), RouteHint(vec![RouteHintHop {
                        src_node_id: nodes[5],
                        short_channel_id: 10,
                        fees: zero_fees,
                        cltv_expiry_delta: (10 << 4) | 1,
                        htlc_minimum_msat: None,
                        htlc_maximum_msat: None,
                }])]
        }

        fn last_hops_multi_private_channels(nodes: &Vec<PublicKey>) -> Vec<RouteHint> {
                let zero_fees = RoutingFees {
                        base_msat: 0,
                        proportional_millionths: 0,
                };
                vec![RouteHint(vec![RouteHintHop {
                        src_node_id: nodes[2],
                        short_channel_id: 5,
                        fees: RoutingFees {
                                base_msat: 100,
                                proportional_millionths: 0,
                        },
                        cltv_expiry_delta: (5 << 4) | 1,
                        htlc_minimum_msat: None,
                        htlc_maximum_msat: None,
                }, RouteHintHop {
                        src_node_id: nodes[3],
                        short_channel_id: 8,
                        fees: zero_fees,
                        cltv_expiry_delta: (8 << 4) | 1,
                        htlc_minimum_msat: None,
                        htlc_maximum_msat: None,
                }
                ]), RouteHint(vec![RouteHintHop {
                        src_node_id: nodes[4],
                        short_channel_id: 9,
                        fees: RoutingFees {
                                base_msat: 1001,
                                proportional_millionths: 0,
                        },
                        cltv_expiry_delta: (9 << 4) | 1,
                        htlc_minimum_msat: None,
                        htlc_maximum_msat: None,
                }]), RouteHint(vec![RouteHintHop {
                        src_node_id: nodes[5],
                        short_channel_id: 10,
                        fees: zero_fees,
                        cltv_expiry_delta: (10 << 4) | 1,
                        htlc_minimum_msat: None,
                        htlc_maximum_msat: None,
                }])]
        }

        #[test]
        fn partial_route_hint_test() {
                let (secp_ctx, network_graph, _, _, logger) = build_graph();
                let (_, our_id, _, nodes) = get_nodes(&secp_ctx);
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();

                // Simple test across 2, 3, 5, and 4 via a last_hop channel
                // Tests the behaviour when the RouteHint contains a suboptimal hop.
                // RouteHint may be partially used by the algo to build the best path.

                // First check that last hop can't have its source as the payee.
                let invalid_last_hop = RouteHint(vec![RouteHintHop {
                        src_node_id: nodes[6],
                        short_channel_id: 8,
                        fees: RoutingFees {
                                base_msat: 1000,
                                proportional_millionths: 0,
                        },
                        cltv_expiry_delta: (8 << 4) | 1,
                        htlc_minimum_msat: None,
                        htlc_maximum_msat: None,
                }]);

                let mut invalid_last_hops = last_hops_multi_private_channels(&nodes);
                invalid_last_hops.push(invalid_last_hop);
                {
                        let payment_params = PaymentParameters::from_node_id(nodes[6]).with_route_hints(invalid_last_hops);
                        if let Err(LightningError{err, action: ErrorAction::IgnoreError}) = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 100, 42, Arc::clone(&logger), &scorer, &random_seed_bytes) {
                                assert_eq!(err, "Route hint cannot have the payee as the source.");
                        } else { panic!(); }
                }

                let payment_params = PaymentParameters::from_node_id(nodes[6]).with_route_hints(last_hops_multi_private_channels(&nodes));
                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 100, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                assert_eq!(route.paths[0].len(), 5);

                assert_eq!(route.paths[0][0].pubkey, nodes[1]);
                assert_eq!(route.paths[0][0].short_channel_id, 2);
                assert_eq!(route.paths[0][0].fee_msat, 100);
                assert_eq!(route.paths[0][0].cltv_expiry_delta, (4 << 4) | 1);
                assert_eq!(route.paths[0][0].node_features.le_flags(), &id_to_feature_flags(2));
                assert_eq!(route.paths[0][0].channel_features.le_flags(), &id_to_feature_flags(2));

                assert_eq!(route.paths[0][1].pubkey, nodes[2]);
                assert_eq!(route.paths[0][1].short_channel_id, 4);
                assert_eq!(route.paths[0][1].fee_msat, 0);
                assert_eq!(route.paths[0][1].cltv_expiry_delta, (6 << 4) | 1);
                assert_eq!(route.paths[0][1].node_features.le_flags(), &id_to_feature_flags(3));
                assert_eq!(route.paths[0][1].channel_features.le_flags(), &id_to_feature_flags(4));

                assert_eq!(route.paths[0][2].pubkey, nodes[4]);
                assert_eq!(route.paths[0][2].short_channel_id, 6);
                assert_eq!(route.paths[0][2].fee_msat, 0);
                assert_eq!(route.paths[0][2].cltv_expiry_delta, (11 << 4) | 1);
                assert_eq!(route.paths[0][2].node_features.le_flags(), &id_to_feature_flags(5));
                assert_eq!(route.paths[0][2].channel_features.le_flags(), &id_to_feature_flags(6));

                assert_eq!(route.paths[0][3].pubkey, nodes[3]);
                assert_eq!(route.paths[0][3].short_channel_id, 11);
                assert_eq!(route.paths[0][3].fee_msat, 0);
                assert_eq!(route.paths[0][3].cltv_expiry_delta, (8 << 4) | 1);
                // If we have a peer in the node map, we'll use their features here since we don't have
                // a way of figuring out their features from the invoice:
                assert_eq!(route.paths[0][3].node_features.le_flags(), &id_to_feature_flags(4));
                assert_eq!(route.paths[0][3].channel_features.le_flags(), &id_to_feature_flags(11));

                assert_eq!(route.paths[0][4].pubkey, nodes[6]);
                assert_eq!(route.paths[0][4].short_channel_id, 8);
                assert_eq!(route.paths[0][4].fee_msat, 100);
                assert_eq!(route.paths[0][4].cltv_expiry_delta, 42);
                assert_eq!(route.paths[0][4].node_features.le_flags(), default_node_features().le_flags()); // We dont pass flags in from invoices yet
                assert_eq!(route.paths[0][4].channel_features.le_flags(), &Vec::<u8>::new()); // We can't learn any flags from invoices, sadly
        }

        fn empty_last_hop(nodes: &Vec<PublicKey>) -> Vec<RouteHint> {
                let zero_fees = RoutingFees {
                        base_msat: 0,
                        proportional_millionths: 0,
                };
                vec![RouteHint(vec![RouteHintHop {
                        src_node_id: nodes[3],
                        short_channel_id: 8,
                        fees: zero_fees,
                        cltv_expiry_delta: (8 << 4) | 1,
                        htlc_minimum_msat: None,
                        htlc_maximum_msat: None,
                }]), RouteHint(vec![

                ]), RouteHint(vec![RouteHintHop {
                        src_node_id: nodes[5],
                        short_channel_id: 10,
                        fees: zero_fees,
                        cltv_expiry_delta: (10 << 4) | 1,
                        htlc_minimum_msat: None,
                        htlc_maximum_msat: None,
                }])]
        }

        #[test]
        fn ignores_empty_last_hops_test() {
                let (secp_ctx, network_graph, _, _, logger) = build_graph();
                let (_, our_id, _, nodes) = get_nodes(&secp_ctx);
                let payment_params = PaymentParameters::from_node_id(nodes[6]).with_route_hints(empty_last_hop(&nodes));
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();

                // Test handling of an empty RouteHint passed in Invoice.

                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 100, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                assert_eq!(route.paths[0].len(), 5);

                assert_eq!(route.paths[0][0].pubkey, nodes[1]);
                assert_eq!(route.paths[0][0].short_channel_id, 2);
                assert_eq!(route.paths[0][0].fee_msat, 100);
                assert_eq!(route.paths[0][0].cltv_expiry_delta, (4 << 4) | 1);
                assert_eq!(route.paths[0][0].node_features.le_flags(), &id_to_feature_flags(2));
                assert_eq!(route.paths[0][0].channel_features.le_flags(), &id_to_feature_flags(2));

                assert_eq!(route.paths[0][1].pubkey, nodes[2]);
                assert_eq!(route.paths[0][1].short_channel_id, 4);
                assert_eq!(route.paths[0][1].fee_msat, 0);
                assert_eq!(route.paths[0][1].cltv_expiry_delta, (6 << 4) | 1);
                assert_eq!(route.paths[0][1].node_features.le_flags(), &id_to_feature_flags(3));
                assert_eq!(route.paths[0][1].channel_features.le_flags(), &id_to_feature_flags(4));

                assert_eq!(route.paths[0][2].pubkey, nodes[4]);
                assert_eq!(route.paths[0][2].short_channel_id, 6);
                assert_eq!(route.paths[0][2].fee_msat, 0);
                assert_eq!(route.paths[0][2].cltv_expiry_delta, (11 << 4) | 1);
                assert_eq!(route.paths[0][2].node_features.le_flags(), &id_to_feature_flags(5));
                assert_eq!(route.paths[0][2].channel_features.le_flags(), &id_to_feature_flags(6));

                assert_eq!(route.paths[0][3].pubkey, nodes[3]);
                assert_eq!(route.paths[0][3].short_channel_id, 11);
                assert_eq!(route.paths[0][3].fee_msat, 0);
                assert_eq!(route.paths[0][3].cltv_expiry_delta, (8 << 4) | 1);
                // If we have a peer in the node map, we'll use their features here since we don't have
                // a way of figuring out their features from the invoice:
                assert_eq!(route.paths[0][3].node_features.le_flags(), &id_to_feature_flags(4));
                assert_eq!(route.paths[0][3].channel_features.le_flags(), &id_to_feature_flags(11));

                assert_eq!(route.paths[0][4].pubkey, nodes[6]);
                assert_eq!(route.paths[0][4].short_channel_id, 8);
                assert_eq!(route.paths[0][4].fee_msat, 100);
                assert_eq!(route.paths[0][4].cltv_expiry_delta, 42);
                assert_eq!(route.paths[0][4].node_features.le_flags(), default_node_features().le_flags()); // We dont pass flags in from invoices yet
                assert_eq!(route.paths[0][4].channel_features.le_flags(), &Vec::<u8>::new()); // We can't learn any flags from invoices, sadly
        }

        /// Builds a trivial last-hop hint that passes through the two nodes given, with channel 0xff00
        /// and 0xff01.
        fn multi_hop_last_hops_hint(hint_hops: [PublicKey; 2]) -> Vec<RouteHint> {
                let zero_fees = RoutingFees {
                        base_msat: 0,
                        proportional_millionths: 0,
                };
                vec![RouteHint(vec![RouteHintHop {
                        src_node_id: hint_hops[0],
                        short_channel_id: 0xff00,
                        fees: RoutingFees {
                                base_msat: 100,
                                proportional_millionths: 0,
                        },
                        cltv_expiry_delta: (5 << 4) | 1,
                        htlc_minimum_msat: None,
                        htlc_maximum_msat: None,
                }, RouteHintHop {
                        src_node_id: hint_hops[1],
                        short_channel_id: 0xff01,
                        fees: zero_fees,
                        cltv_expiry_delta: (8 << 4) | 1,
                        htlc_minimum_msat: None,
                        htlc_maximum_msat: None,
                }])]
        }

        #[test]
        fn multi_hint_last_hops_test() {
                let (secp_ctx, network_graph, gossip_sync, _, logger) = build_graph();
                let (_, our_id, privkeys, nodes) = get_nodes(&secp_ctx);
                let last_hops = multi_hop_last_hops_hint([nodes[2], nodes[3]]);
                let payment_params = PaymentParameters::from_node_id(nodes[6]).with_route_hints(last_hops.clone());
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();
                // Test through channels 2, 3, 0xff00, 0xff01.
                // Test shows that multiple hop hints are considered.

                // Disabling channels 6 & 7 by flags=2
                update_channel(&gossip_sync, &secp_ctx, &privkeys[2], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 6,
                        timestamp: 2,
                        flags: 2, // to disable
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[2], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 7,
                        timestamp: 2,
                        flags: 2, // to disable
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 100, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                assert_eq!(route.paths[0].len(), 4);

                assert_eq!(route.paths[0][0].pubkey, nodes[1]);
                assert_eq!(route.paths[0][0].short_channel_id, 2);
                assert_eq!(route.paths[0][0].fee_msat, 200);
                assert_eq!(route.paths[0][0].cltv_expiry_delta, 65);
                assert_eq!(route.paths[0][0].node_features.le_flags(), &id_to_feature_flags(2));
                assert_eq!(route.paths[0][0].channel_features.le_flags(), &id_to_feature_flags(2));

                assert_eq!(route.paths[0][1].pubkey, nodes[2]);
                assert_eq!(route.paths[0][1].short_channel_id, 4);
                assert_eq!(route.paths[0][1].fee_msat, 100);
                assert_eq!(route.paths[0][1].cltv_expiry_delta, 81);
                assert_eq!(route.paths[0][1].node_features.le_flags(), &id_to_feature_flags(3));
                assert_eq!(route.paths[0][1].channel_features.le_flags(), &id_to_feature_flags(4));

                assert_eq!(route.paths[0][2].pubkey, nodes[3]);
                assert_eq!(route.paths[0][2].short_channel_id, last_hops[0].0[0].short_channel_id);
                assert_eq!(route.paths[0][2].fee_msat, 0);
                assert_eq!(route.paths[0][2].cltv_expiry_delta, 129);
                assert_eq!(route.paths[0][2].node_features.le_flags(), &id_to_feature_flags(4));
                assert_eq!(route.paths[0][2].channel_features.le_flags(), &Vec::<u8>::new()); // We can't learn any flags from invoices, sadly

                assert_eq!(route.paths[0][3].pubkey, nodes[6]);
                assert_eq!(route.paths[0][3].short_channel_id, last_hops[0].0[1].short_channel_id);
                assert_eq!(route.paths[0][3].fee_msat, 100);
                assert_eq!(route.paths[0][3].cltv_expiry_delta, 42);
                assert_eq!(route.paths[0][3].node_features.le_flags(), default_node_features().le_flags()); // We dont pass flags in from invoices yet
                assert_eq!(route.paths[0][3].channel_features.le_flags(), &Vec::<u8>::new()); // We can't learn any flags from invoices, sadly
        }

        #[test]
        fn private_multi_hint_last_hops_test() {
                let (secp_ctx, network_graph, gossip_sync, _, logger) = build_graph();
                let (_, our_id, privkeys, nodes) = get_nodes(&secp_ctx);

                let non_announced_privkey = SecretKey::from_slice(&hex::decode(format!("{:02x}", 0xf0).repeat(32)).unwrap()[..]).unwrap();
                let non_announced_pubkey = PublicKey::from_secret_key(&secp_ctx, &non_announced_privkey);

                let last_hops = multi_hop_last_hops_hint([nodes[2], non_announced_pubkey]);
                let payment_params = PaymentParameters::from_node_id(nodes[6]).with_route_hints(last_hops.clone());
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                // Test through channels 2, 3, 0xff00, 0xff01.
                // Test shows that multiple hop hints are considered.

                // Disabling channels 6 & 7 by flags=2
                update_channel(&gossip_sync, &secp_ctx, &privkeys[2], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 6,
                        timestamp: 2,
                        flags: 2, // to disable
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[2], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 7,
                        timestamp: 2,
                        flags: 2, // to disable
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 100, 42, Arc::clone(&logger), &scorer, &[42u8; 32]).unwrap();
                assert_eq!(route.paths[0].len(), 4);

                assert_eq!(route.paths[0][0].pubkey, nodes[1]);
                assert_eq!(route.paths[0][0].short_channel_id, 2);
                assert_eq!(route.paths[0][0].fee_msat, 200);
                assert_eq!(route.paths[0][0].cltv_expiry_delta, 65);
                assert_eq!(route.paths[0][0].node_features.le_flags(), &id_to_feature_flags(2));
                assert_eq!(route.paths[0][0].channel_features.le_flags(), &id_to_feature_flags(2));

                assert_eq!(route.paths[0][1].pubkey, nodes[2]);
                assert_eq!(route.paths[0][1].short_channel_id, 4);
                assert_eq!(route.paths[0][1].fee_msat, 100);
                assert_eq!(route.paths[0][1].cltv_expiry_delta, 81);
                assert_eq!(route.paths[0][1].node_features.le_flags(), &id_to_feature_flags(3));
                assert_eq!(route.paths[0][1].channel_features.le_flags(), &id_to_feature_flags(4));

                assert_eq!(route.paths[0][2].pubkey, non_announced_pubkey);
                assert_eq!(route.paths[0][2].short_channel_id, last_hops[0].0[0].short_channel_id);
                assert_eq!(route.paths[0][2].fee_msat, 0);
                assert_eq!(route.paths[0][2].cltv_expiry_delta, 129);
                assert_eq!(route.paths[0][2].node_features.le_flags(), default_node_features().le_flags()); // We dont pass flags in from invoices yet
                assert_eq!(route.paths[0][2].channel_features.le_flags(), &Vec::<u8>::new()); // We can't learn any flags from invoices, sadly

                assert_eq!(route.paths[0][3].pubkey, nodes[6]);
                assert_eq!(route.paths[0][3].short_channel_id, last_hops[0].0[1].short_channel_id);
                assert_eq!(route.paths[0][3].fee_msat, 100);
                assert_eq!(route.paths[0][3].cltv_expiry_delta, 42);
                assert_eq!(route.paths[0][3].node_features.le_flags(), default_node_features().le_flags()); // We dont pass flags in from invoices yet
                assert_eq!(route.paths[0][3].channel_features.le_flags(), &Vec::<u8>::new()); // We can't learn any flags from invoices, sadly
        }

        fn last_hops_with_public_channel(nodes: &Vec<PublicKey>) -> Vec<RouteHint> {
                let zero_fees = RoutingFees {
                        base_msat: 0,
                        proportional_millionths: 0,
                };
                vec![RouteHint(vec![RouteHintHop {
                        src_node_id: nodes[4],
                        short_channel_id: 11,
                        fees: zero_fees,
                        cltv_expiry_delta: (11 << 4) | 1,
                        htlc_minimum_msat: None,
                        htlc_maximum_msat: None,
                }, RouteHintHop {
                        src_node_id: nodes[3],
                        short_channel_id: 8,
                        fees: zero_fees,
                        cltv_expiry_delta: (8 << 4) | 1,
                        htlc_minimum_msat: None,
                        htlc_maximum_msat: None,
                }]), RouteHint(vec![RouteHintHop {
                        src_node_id: nodes[4],
                        short_channel_id: 9,
                        fees: RoutingFees {
                                base_msat: 1001,
                                proportional_millionths: 0,
                        },
                        cltv_expiry_delta: (9 << 4) | 1,
                        htlc_minimum_msat: None,
                        htlc_maximum_msat: None,
                }]), RouteHint(vec![RouteHintHop {
                        src_node_id: nodes[5],
                        short_channel_id: 10,
                        fees: zero_fees,
                        cltv_expiry_delta: (10 << 4) | 1,
                        htlc_minimum_msat: None,
                        htlc_maximum_msat: None,
                }])]
        }

        #[test]
        fn last_hops_with_public_channel_test() {
                let (secp_ctx, network_graph, _, _, logger) = build_graph();
                let (_, our_id, _, nodes) = get_nodes(&secp_ctx);
                let payment_params = PaymentParameters::from_node_id(nodes[6]).with_route_hints(last_hops_with_public_channel(&nodes));
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();
                // This test shows that public routes can be present in the invoice
                // which would be handled in the same manner.

                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 100, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                assert_eq!(route.paths[0].len(), 5);

                assert_eq!(route.paths[0][0].pubkey, nodes[1]);
                assert_eq!(route.paths[0][0].short_channel_id, 2);
                assert_eq!(route.paths[0][0].fee_msat, 100);
                assert_eq!(route.paths[0][0].cltv_expiry_delta, (4 << 4) | 1);
                assert_eq!(route.paths[0][0].node_features.le_flags(), &id_to_feature_flags(2));
                assert_eq!(route.paths[0][0].channel_features.le_flags(), &id_to_feature_flags(2));

                assert_eq!(route.paths[0][1].pubkey, nodes[2]);
                assert_eq!(route.paths[0][1].short_channel_id, 4);
                assert_eq!(route.paths[0][1].fee_msat, 0);
                assert_eq!(route.paths[0][1].cltv_expiry_delta, (6 << 4) | 1);
                assert_eq!(route.paths[0][1].node_features.le_flags(), &id_to_feature_flags(3));
                assert_eq!(route.paths[0][1].channel_features.le_flags(), &id_to_feature_flags(4));

                assert_eq!(route.paths[0][2].pubkey, nodes[4]);
                assert_eq!(route.paths[0][2].short_channel_id, 6);
                assert_eq!(route.paths[0][2].fee_msat, 0);
                assert_eq!(route.paths[0][2].cltv_expiry_delta, (11 << 4) | 1);
                assert_eq!(route.paths[0][2].node_features.le_flags(), &id_to_feature_flags(5));
                assert_eq!(route.paths[0][2].channel_features.le_flags(), &id_to_feature_flags(6));

                assert_eq!(route.paths[0][3].pubkey, nodes[3]);
                assert_eq!(route.paths[0][3].short_channel_id, 11);
                assert_eq!(route.paths[0][3].fee_msat, 0);
                assert_eq!(route.paths[0][3].cltv_expiry_delta, (8 << 4) | 1);
                // If we have a peer in the node map, we'll use their features here since we don't have
                // a way of figuring out their features from the invoice:
                assert_eq!(route.paths[0][3].node_features.le_flags(), &id_to_feature_flags(4));
                assert_eq!(route.paths[0][3].channel_features.le_flags(), &id_to_feature_flags(11));

                assert_eq!(route.paths[0][4].pubkey, nodes[6]);
                assert_eq!(route.paths[0][4].short_channel_id, 8);
                assert_eq!(route.paths[0][4].fee_msat, 100);
                assert_eq!(route.paths[0][4].cltv_expiry_delta, 42);
                assert_eq!(route.paths[0][4].node_features.le_flags(), default_node_features().le_flags()); // We dont pass flags in from invoices yet
                assert_eq!(route.paths[0][4].channel_features.le_flags(), &Vec::<u8>::new()); // We can't learn any flags from invoices, sadly
        }

        #[test]
        fn our_chans_last_hop_connect_test() {
                let (secp_ctx, network_graph, _, _, logger) = build_graph();
                let (_, our_id, _, nodes) = get_nodes(&secp_ctx);
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();

                // Simple test with outbound channel to 4 to test that last_hops and first_hops connect
                let our_chans = vec![get_channel_details(Some(42), nodes[3].clone(), InitFeatures::from_le_bytes(vec![0b11]), 250_000_000)];
                let mut last_hops = last_hops(&nodes);
                let payment_params = PaymentParameters::from_node_id(nodes[6]).with_route_hints(last_hops.clone());
                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), Some(&our_chans.iter().collect::<Vec<_>>()), 100, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                assert_eq!(route.paths[0].len(), 2);

                assert_eq!(route.paths[0][0].pubkey, nodes[3]);
                assert_eq!(route.paths[0][0].short_channel_id, 42);
                assert_eq!(route.paths[0][0].fee_msat, 0);
                assert_eq!(route.paths[0][0].cltv_expiry_delta, (8 << 4) | 1);
                assert_eq!(route.paths[0][0].node_features.le_flags(), &vec![0b11]);
                assert_eq!(route.paths[0][0].channel_features.le_flags(), &Vec::<u8>::new()); // No feature flags will meet the relevant-to-channel conversion

                assert_eq!(route.paths[0][1].pubkey, nodes[6]);
                assert_eq!(route.paths[0][1].short_channel_id, 8);
                assert_eq!(route.paths[0][1].fee_msat, 100);
                assert_eq!(route.paths[0][1].cltv_expiry_delta, 42);
                assert_eq!(route.paths[0][1].node_features.le_flags(), default_node_features().le_flags()); // We dont pass flags in from invoices yet
                assert_eq!(route.paths[0][1].channel_features.le_flags(), &Vec::<u8>::new()); // We can't learn any flags from invoices, sadly

                last_hops[0].0[0].fees.base_msat = 1000;

                // Revert to via 6 as the fee on 8 goes up
                let payment_params = PaymentParameters::from_node_id(nodes[6]).with_route_hints(last_hops);
                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 100, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                assert_eq!(route.paths[0].len(), 4);

                assert_eq!(route.paths[0][0].pubkey, nodes[1]);
                assert_eq!(route.paths[0][0].short_channel_id, 2);
                assert_eq!(route.paths[0][0].fee_msat, 200); // fee increased as its % of value transferred across node
                assert_eq!(route.paths[0][0].cltv_expiry_delta, (4 << 4) | 1);
                assert_eq!(route.paths[0][0].node_features.le_flags(), &id_to_feature_flags(2));
                assert_eq!(route.paths[0][0].channel_features.le_flags(), &id_to_feature_flags(2));

                assert_eq!(route.paths[0][1].pubkey, nodes[2]);
                assert_eq!(route.paths[0][1].short_channel_id, 4);
                assert_eq!(route.paths[0][1].fee_msat, 100);
                assert_eq!(route.paths[0][1].cltv_expiry_delta, (7 << 4) | 1);
                assert_eq!(route.paths[0][1].node_features.le_flags(), &id_to_feature_flags(3));
                assert_eq!(route.paths[0][1].channel_features.le_flags(), &id_to_feature_flags(4));

                assert_eq!(route.paths[0][2].pubkey, nodes[5]);
                assert_eq!(route.paths[0][2].short_channel_id, 7);
                assert_eq!(route.paths[0][2].fee_msat, 0);
                assert_eq!(route.paths[0][2].cltv_expiry_delta, (10 << 4) | 1);
                // If we have a peer in the node map, we'll use their features here since we don't have
                // a way of figuring out their features from the invoice:
                assert_eq!(route.paths[0][2].node_features.le_flags(), &id_to_feature_flags(6));
                assert_eq!(route.paths[0][2].channel_features.le_flags(), &id_to_feature_flags(7));

                assert_eq!(route.paths[0][3].pubkey, nodes[6]);
                assert_eq!(route.paths[0][3].short_channel_id, 10);
                assert_eq!(route.paths[0][3].fee_msat, 100);
                assert_eq!(route.paths[0][3].cltv_expiry_delta, 42);
                assert_eq!(route.paths[0][3].node_features.le_flags(), default_node_features().le_flags()); // We dont pass flags in from invoices yet
                assert_eq!(route.paths[0][3].channel_features.le_flags(), &Vec::<u8>::new()); // We can't learn any flags from invoices, sadly

                // ...but still use 8 for larger payments as 6 has a variable feerate
                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 2000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                assert_eq!(route.paths[0].len(), 5);

                assert_eq!(route.paths[0][0].pubkey, nodes[1]);
                assert_eq!(route.paths[0][0].short_channel_id, 2);
                assert_eq!(route.paths[0][0].fee_msat, 3000);
                assert_eq!(route.paths[0][0].cltv_expiry_delta, (4 << 4) | 1);
                assert_eq!(route.paths[0][0].node_features.le_flags(), &id_to_feature_flags(2));
                assert_eq!(route.paths[0][0].channel_features.le_flags(), &id_to_feature_flags(2));

                assert_eq!(route.paths[0][1].pubkey, nodes[2]);
                assert_eq!(route.paths[0][1].short_channel_id, 4);
                assert_eq!(route.paths[0][1].fee_msat, 0);
                assert_eq!(route.paths[0][1].cltv_expiry_delta, (6 << 4) | 1);
                assert_eq!(route.paths[0][1].node_features.le_flags(), &id_to_feature_flags(3));
                assert_eq!(route.paths[0][1].channel_features.le_flags(), &id_to_feature_flags(4));

                assert_eq!(route.paths[0][2].pubkey, nodes[4]);
                assert_eq!(route.paths[0][2].short_channel_id, 6);
                assert_eq!(route.paths[0][2].fee_msat, 0);
                assert_eq!(route.paths[0][2].cltv_expiry_delta, (11 << 4) | 1);
                assert_eq!(route.paths[0][2].node_features.le_flags(), &id_to_feature_flags(5));
                assert_eq!(route.paths[0][2].channel_features.le_flags(), &id_to_feature_flags(6));

                assert_eq!(route.paths[0][3].pubkey, nodes[3]);
                assert_eq!(route.paths[0][3].short_channel_id, 11);
                assert_eq!(route.paths[0][3].fee_msat, 1000);
                assert_eq!(route.paths[0][3].cltv_expiry_delta, (8 << 4) | 1);
                // If we have a peer in the node map, we'll use their features here since we don't have
                // a way of figuring out their features from the invoice:
                assert_eq!(route.paths[0][3].node_features.le_flags(), &id_to_feature_flags(4));
                assert_eq!(route.paths[0][3].channel_features.le_flags(), &id_to_feature_flags(11));

                assert_eq!(route.paths[0][4].pubkey, nodes[6]);
                assert_eq!(route.paths[0][4].short_channel_id, 8);
                assert_eq!(route.paths[0][4].fee_msat, 2000);
                assert_eq!(route.paths[0][4].cltv_expiry_delta, 42);
                assert_eq!(route.paths[0][4].node_features.le_flags(), default_node_features().le_flags()); // We dont pass flags in from invoices yet
                assert_eq!(route.paths[0][4].channel_features.le_flags(), &Vec::<u8>::new()); // We can't learn any flags from invoices, sadly
        }

        fn do_unannounced_path_test(last_hop_htlc_max: Option<u64>, last_hop_fee_prop: u32, outbound_capacity_msat: u64, route_val: u64) -> Result<Route, LightningError> {
                let source_node_id = PublicKey::from_secret_key(&Secp256k1::new(), &SecretKey::from_slice(&hex::decode(format!("{:02}", 41).repeat(32)).unwrap()[..]).unwrap());
                let middle_node_id = PublicKey::from_secret_key(&Secp256k1::new(), &SecretKey::from_slice(&hex::decode(format!("{:02}", 42).repeat(32)).unwrap()[..]).unwrap());
                let target_node_id = PublicKey::from_secret_key(&Secp256k1::new(), &SecretKey::from_slice(&hex::decode(format!("{:02}", 43).repeat(32)).unwrap()[..]).unwrap());

                // If we specify a channel to a middle hop, that overrides our local channel view and that gets used
                let last_hops = RouteHint(vec![RouteHintHop {
                        src_node_id: middle_node_id,
                        short_channel_id: 8,
                        fees: RoutingFees {
                                base_msat: 1000,
                                proportional_millionths: last_hop_fee_prop,
                        },
                        cltv_expiry_delta: (8 << 4) | 1,
                        htlc_minimum_msat: None,
                        htlc_maximum_msat: last_hop_htlc_max,
                }]);
                let payment_params = PaymentParameters::from_node_id(target_node_id).with_route_hints(vec![last_hops]);
                let our_chans = vec![get_channel_details(Some(42), middle_node_id, InitFeatures::from_le_bytes(vec![0b11]), outbound_capacity_msat)];
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();
                let genesis_hash = genesis_block(Network::Testnet).header.block_hash();
                let logger = ln_test_utils::TestLogger::new();
                let network_graph = NetworkGraph::new(genesis_hash, &logger);
                let route = get_route(&source_node_id, &payment_params, &network_graph.read_only(),
                                Some(&our_chans.iter().collect::<Vec<_>>()), route_val, 42, &logger, &scorer, &random_seed_bytes);
                route
        }

        #[test]
        fn unannounced_path_test() {
                // We should be able to send a payment to a destination without any help of a routing graph
                // if we have a channel with a common counterparty that appears in the first and last hop
                // hints.
                let route = do_unannounced_path_test(None, 1, 2000000, 1000000).unwrap();

                let middle_node_id = PublicKey::from_secret_key(&Secp256k1::new(), &SecretKey::from_slice(&hex::decode(format!("{:02}", 42).repeat(32)).unwrap()[..]).unwrap());
                let target_node_id = PublicKey::from_secret_key(&Secp256k1::new(), &SecretKey::from_slice(&hex::decode(format!("{:02}", 43).repeat(32)).unwrap()[..]).unwrap());
                assert_eq!(route.paths[0].len(), 2);

                assert_eq!(route.paths[0][0].pubkey, middle_node_id);
                assert_eq!(route.paths[0][0].short_channel_id, 42);
                assert_eq!(route.paths[0][0].fee_msat, 1001);
                assert_eq!(route.paths[0][0].cltv_expiry_delta, (8 << 4) | 1);
                assert_eq!(route.paths[0][0].node_features.le_flags(), &[0b11]);
                assert_eq!(route.paths[0][0].channel_features.le_flags(), &[0; 0]); // We can't learn any flags from invoices, sadly

                assert_eq!(route.paths[0][1].pubkey, target_node_id);
                assert_eq!(route.paths[0][1].short_channel_id, 8);
                assert_eq!(route.paths[0][1].fee_msat, 1000000);
                assert_eq!(route.paths[0][1].cltv_expiry_delta, 42);
                assert_eq!(route.paths[0][1].node_features.le_flags(), default_node_features().le_flags()); // We dont pass flags in from invoices yet
                assert_eq!(route.paths[0][1].channel_features.le_flags(), &[0; 0]); // We can't learn any flags from invoices, sadly
        }

        #[test]
        fn overflow_unannounced_path_test_liquidity_underflow() {
                // Previously, when we had a last-hop hint connected directly to a first-hop channel, where
                // the last-hop had a fee which overflowed a u64, we'd panic.
                // This was due to us adding the first-hop from us unconditionally, causing us to think
                // we'd built a path (as our node is in the "best candidate" set), when we had not.
                // In this test, we previously hit a subtraction underflow due to having less available
                // liquidity at the last hop than 0.
                assert!(do_unannounced_path_test(Some(21_000_000_0000_0000_000), 0, 21_000_000_0000_0000_000, 21_000_000_0000_0000_000).is_err());
        }

        #[test]
        fn overflow_unannounced_path_test_feerate_overflow() {
                // This tests for the same case as above, except instead of hitting a subtraction
                // underflow, we hit a case where the fee charged at a hop overflowed.
                assert!(do_unannounced_path_test(Some(21_000_000_0000_0000_000), 50000, 21_000_000_0000_0000_000, 21_000_000_0000_0000_000).is_err());
        }

        #[test]
        fn available_amount_while_routing_test() {
                // Tests whether we choose the correct available channel amount while routing.

                let (secp_ctx, network_graph, mut gossip_sync, chain_monitor, logger) = build_graph();
                let (our_privkey, our_id, privkeys, nodes) = get_nodes(&secp_ctx);
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();
                let payment_params = PaymentParameters::from_node_id(nodes[2]).with_features(channelmanager::provided_invoice_features());

                // We will use a simple single-path route from
                // our node to node2 via node0: channels {1, 3}.

                // First disable all other paths.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 2,
                        timestamp: 2,
                        flags: 2,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 12,
                        timestamp: 2,
                        flags: 2,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Make the first channel (#1) very permissive,
                // and we will be testing all limits on the second channel.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 1,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 1_000_000_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // First, let's see if routing works if we have absolutely no idea about the available amount.
                // In this case, it should be set to 250_000 sats.
                update_channel(&gossip_sync, &secp_ctx, &privkeys[0], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 3,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 250_000_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                {
                        // Attempt to route more than available results in a failure.
                        if let Err(LightningError{err, action: ErrorAction::IgnoreError}) = get_route(
                                        &our_id, &payment_params, &network_graph.read_only(), None, 250_000_001, 42, Arc::clone(&logger), &scorer, &random_seed_bytes) {
                                assert_eq!(err, "Failed to find a sufficient route to the given destination");
                        } else { panic!(); }
                }

                {
                        // Now, attempt to route an exact amount we have should be fine.
                        let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 250_000_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 1);
                        let path = route.paths.last().unwrap();
                        assert_eq!(path.len(), 2);
                        assert_eq!(path.last().unwrap().pubkey, nodes[2]);
                        assert_eq!(path.last().unwrap().fee_msat, 250_000_000);
                }

                // Check that setting next_outbound_htlc_limit_msat in first_hops limits the channels.
                // Disable channel #1 and use another first hop.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 1,
                        timestamp: 3,
                        flags: 2,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 1_000_000_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Now, limit the first_hop by the next_outbound_htlc_limit_msat of 200_000 sats.
                let our_chans = vec![get_channel_details(Some(42), nodes[0].clone(), InitFeatures::from_le_bytes(vec![0b11]), 200_000_000)];

                {
                        // Attempt to route more than available results in a failure.
                        if let Err(LightningError{err, action: ErrorAction::IgnoreError}) = get_route(
                                        &our_id, &payment_params, &network_graph.read_only(), Some(&our_chans.iter().collect::<Vec<_>>()), 200_000_001, 42, Arc::clone(&logger), &scorer, &random_seed_bytes) {
                                assert_eq!(err, "Failed to find a sufficient route to the given destination");
                        } else { panic!(); }
                }

                {
                        // Now, attempt to route an exact amount we have should be fine.
                        let route = get_route(&our_id, &payment_params, &network_graph.read_only(), Some(&our_chans.iter().collect::<Vec<_>>()), 200_000_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 1);
                        let path = route.paths.last().unwrap();
                        assert_eq!(path.len(), 2);
                        assert_eq!(path.last().unwrap().pubkey, nodes[2]);
                        assert_eq!(path.last().unwrap().fee_msat, 200_000_000);
                }

                // Enable channel #1 back.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 1,
                        timestamp: 4,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 1_000_000_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });


                // Now let's see if routing works if we know only htlc_maximum_msat.
                update_channel(&gossip_sync, &secp_ctx, &privkeys[0], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 3,
                        timestamp: 3,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 15_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                {
                        // Attempt to route more than available results in a failure.
                        if let Err(LightningError{err, action: ErrorAction::IgnoreError}) = get_route(
                                        &our_id, &payment_params, &network_graph.read_only(), None, 15_001, 42, Arc::clone(&logger), &scorer, &random_seed_bytes) {
                                assert_eq!(err, "Failed to find a sufficient route to the given destination");
                        } else { panic!(); }
                }

                {
                        // Now, attempt to route an exact amount we have should be fine.
                        let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 15_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 1);
                        let path = route.paths.last().unwrap();
                        assert_eq!(path.len(), 2);
                        assert_eq!(path.last().unwrap().pubkey, nodes[2]);
                        assert_eq!(path.last().unwrap().fee_msat, 15_000);
                }

                // Now let's see if routing works if we know only capacity from the UTXO.

                // We can't change UTXO capacity on the fly, so we'll disable
                // the existing channel and add another one with the capacity we need.
                update_channel(&gossip_sync, &secp_ctx, &privkeys[0], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 3,
                        timestamp: 4,
                        flags: 2,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                let good_script = Builder::new().push_opcode(opcodes::all::OP_PUSHNUM_2)
                .push_slice(&PublicKey::from_secret_key(&secp_ctx, &privkeys[0]).serialize())
                .push_slice(&PublicKey::from_secret_key(&secp_ctx, &privkeys[2]).serialize())
                .push_opcode(opcodes::all::OP_PUSHNUM_2)
                .push_opcode(opcodes::all::OP_CHECKMULTISIG).into_script().to_v0_p2wsh();

                *chain_monitor.utxo_ret.lock().unwrap() = Ok(TxOut { value: 15, script_pubkey: good_script.clone() });
                gossip_sync.add_chain_access(Some(chain_monitor));

                add_channel(&gossip_sync, &secp_ctx, &privkeys[0], &privkeys[2], ChannelFeatures::from_le_bytes(id_to_feature_flags(3)), 333);
                update_channel(&gossip_sync, &secp_ctx, &privkeys[0], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 333,
                        timestamp: 1,
                        flags: 0,
                        cltv_expiry_delta: (3 << 4) | 1,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 15_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[2], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 333,
                        timestamp: 1,
                        flags: 1,
                        cltv_expiry_delta: (3 << 4) | 2,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 15_000,
                        fee_base_msat: 100,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                {
                        // Attempt to route more than available results in a failure.
                        if let Err(LightningError{err, action: ErrorAction::IgnoreError}) = get_route(
                                        &our_id, &payment_params, &network_graph.read_only(), None, 15_001, 42, Arc::clone(&logger), &scorer, &random_seed_bytes) {
                                assert_eq!(err, "Failed to find a sufficient route to the given destination");
                        } else { panic!(); }
                }

                {
                        // Now, attempt to route an exact amount we have should be fine.
                        let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 15_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 1);
                        let path = route.paths.last().unwrap();
                        assert_eq!(path.len(), 2);
                        assert_eq!(path.last().unwrap().pubkey, nodes[2]);
                        assert_eq!(path.last().unwrap().fee_msat, 15_000);
                }

                // Now let's see if routing chooses htlc_maximum_msat over UTXO capacity.
                update_channel(&gossip_sync, &secp_ctx, &privkeys[0], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 333,
                        timestamp: 6,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 10_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                {
                        // Attempt to route more than available results in a failure.
                        if let Err(LightningError{err, action: ErrorAction::IgnoreError}) = get_route(
                                        &our_id, &payment_params, &network_graph.read_only(), None, 10_001, 42, Arc::clone(&logger), &scorer, &random_seed_bytes) {
                                assert_eq!(err, "Failed to find a sufficient route to the given destination");
                        } else { panic!(); }
                }

                {
                        // Now, attempt to route an exact amount we have should be fine.
                        let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 10_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 1);
                        let path = route.paths.last().unwrap();
                        assert_eq!(path.len(), 2);
                        assert_eq!(path.last().unwrap().pubkey, nodes[2]);
                        assert_eq!(path.last().unwrap().fee_msat, 10_000);
                }
        }

        #[test]
        fn available_liquidity_last_hop_test() {
                // Check that available liquidity properly limits the path even when only
                // one of the latter hops is limited.
                let (secp_ctx, network_graph, gossip_sync, _, logger) = build_graph();
                let (our_privkey, our_id, privkeys, nodes) = get_nodes(&secp_ctx);
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();
                let payment_params = PaymentParameters::from_node_id(nodes[3]).with_features(channelmanager::provided_invoice_features());

                // Path via {node7, node2, node4} is channels {12, 13, 6, 11}.
                // {12, 13, 11} have the capacities of 100, {6} has a capacity of 50.
                // Total capacity: 50 sats.

                // Disable other potential paths.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 2,
                        timestamp: 2,
                        flags: 2,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[2], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 7,
                        timestamp: 2,
                        flags: 2,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Limit capacities

                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 12,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[7], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 13,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                update_channel(&gossip_sync, &secp_ctx, &privkeys[2], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 6,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 50_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[4], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 11,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                {
                        // Attempt to route more than available results in a failure.
                        if let Err(LightningError{err, action: ErrorAction::IgnoreError}) = get_route(
                                        &our_id, &payment_params, &network_graph.read_only(), None, 60_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes) {
                                assert_eq!(err, "Failed to find a sufficient route to the given destination");
                        } else { panic!(); }
                }

                {
                        // Now, attempt to route 49 sats (just a bit below the capacity).
                        let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 49_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 1);
                        let mut total_amount_paid_msat = 0;
                        for path in &route.paths {
                                assert_eq!(path.len(), 4);
                                assert_eq!(path.last().unwrap().pubkey, nodes[3]);
                                total_amount_paid_msat += path.last().unwrap().fee_msat;
                        }
                        assert_eq!(total_amount_paid_msat, 49_000);
                }

                {
                        // Attempt to route an exact amount is also fine
                        let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 50_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 1);
                        let mut total_amount_paid_msat = 0;
                        for path in &route.paths {
                                assert_eq!(path.len(), 4);
                                assert_eq!(path.last().unwrap().pubkey, nodes[3]);
                                total_amount_paid_msat += path.last().unwrap().fee_msat;
                        }
                        assert_eq!(total_amount_paid_msat, 50_000);
                }
        }

        #[test]
        fn ignore_fee_first_hop_test() {
                let (secp_ctx, network_graph, gossip_sync, _, logger) = build_graph();
                let (our_privkey, our_id, privkeys, nodes) = get_nodes(&secp_ctx);
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();
                let payment_params = PaymentParameters::from_node_id(nodes[2]);

                // Path via node0 is channels {1, 3}. Limit them to 100 and 50 sats (total limit 50).
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 1,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 1_000_000,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[0], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 3,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 50_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                {
                        let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 50_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 1);
                        let mut total_amount_paid_msat = 0;
                        for path in &route.paths {
                                assert_eq!(path.len(), 2);
                                assert_eq!(path.last().unwrap().pubkey, nodes[2]);
                                total_amount_paid_msat += path.last().unwrap().fee_msat;
                        }
                        assert_eq!(total_amount_paid_msat, 50_000);
                }
        }

        #[test]
        fn simple_mpp_route_test() {
                let (secp_ctx, network_graph, gossip_sync, _, logger) = build_graph();
                let (our_privkey, our_id, privkeys, nodes) = get_nodes(&secp_ctx);
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();
                let payment_params = PaymentParameters::from_node_id(nodes[2])
                        .with_features(channelmanager::provided_invoice_features());

                // We need a route consisting of 3 paths:
                // From our node to node2 via node0, node7, node1 (three paths one hop each).
                // To achieve this, the amount being transferred should be around
                // the total capacity of these 3 paths.

                // First, we set limits on these (previously unlimited) channels.
                // Their aggregate capacity will be 50 + 60 + 180 = 290 sats.

                // Path via node0 is channels {1, 3}. Limit them to 100 and 50 sats (total limit 50).
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 1,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[0], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 3,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 50_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Path via node7 is channels {12, 13}. Limit them to 60 and 60 sats
                // (total limit 60).
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 12,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 60_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[7], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 13,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 60_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Path via node1 is channels {2, 4}. Limit them to 200 and 180 sats
                // (total capacity 180 sats).
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 2,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 200_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[1], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 4,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 180_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                {
                        // Attempt to route more than available results in a failure.
                        if let Err(LightningError{err, action: ErrorAction::IgnoreError}) = get_route(
                                &our_id, &payment_params, &network_graph.read_only(), None, 300_000, 42,
                                Arc::clone(&logger), &scorer, &random_seed_bytes) {
                                        assert_eq!(err, "Failed to find a sufficient route to the given destination");
                        } else { panic!(); }
                }

                {
                        // Attempt to route while setting max_path_count to 0 results in a failure.
                        let zero_payment_params = payment_params.clone().with_max_path_count(0);
                        if let Err(LightningError{err, action: ErrorAction::IgnoreError}) = get_route(
                                &our_id, &zero_payment_params, &network_graph.read_only(), None, 100, 42,
                                Arc::clone(&logger), &scorer, &random_seed_bytes) {
                                        assert_eq!(err, "Can't find a route with no paths allowed.");
                        } else { panic!(); }
                }

                {
                        // Attempt to route while setting max_path_count to 3 results in a failure.
                        // This is the case because the minimal_value_contribution_msat would require each path
                        // to account for 1/3 of the total value, which is violated by 2 out of 3 paths.
                        let fail_payment_params = payment_params.clone().with_max_path_count(3);
                        if let Err(LightningError{err, action: ErrorAction::IgnoreError}) = get_route(
                                &our_id, &fail_payment_params, &network_graph.read_only(), None, 250_000, 42,
                                Arc::clone(&logger), &scorer, &random_seed_bytes) {
                                        assert_eq!(err, "Failed to find a sufficient route to the given destination");
                        } else { panic!(); }
                }

                {
                        // Now, attempt to route 250 sats (just a bit below the capacity).
                        // Our algorithm should provide us with these 3 paths.
                        let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None,
                                250_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 3);
                        let mut total_amount_paid_msat = 0;
                        for path in &route.paths {
                                assert_eq!(path.len(), 2);
                                assert_eq!(path.last().unwrap().pubkey, nodes[2]);
                                total_amount_paid_msat += path.last().unwrap().fee_msat;
                        }
                        assert_eq!(total_amount_paid_msat, 250_000);
                }

                {
                        // Attempt to route an exact amount is also fine
                        let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None,
                                290_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 3);
                        let mut total_amount_paid_msat = 0;
                        for path in &route.paths {
                                assert_eq!(path.len(), 2);
                                assert_eq!(path.last().unwrap().pubkey, nodes[2]);
                                total_amount_paid_msat += path.last().unwrap().fee_msat;
                        }
                        assert_eq!(total_amount_paid_msat, 290_000);
                }
        }

        #[test]
        fn long_mpp_route_test() {
                let (secp_ctx, network_graph, gossip_sync, _, logger) = build_graph();
                let (our_privkey, our_id, privkeys, nodes) = get_nodes(&secp_ctx);
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();
                let payment_params = PaymentParameters::from_node_id(nodes[3]).with_features(channelmanager::provided_invoice_features());

                // We need a route consisting of 3 paths:
                // From our node to node3 via {node0, node2}, {node7, node2, node4} and {node7, node2}.
                // Note that these paths overlap (channels 5, 12, 13).
                // We will route 300 sats.
                // Each path will have 100 sats capacity, those channels which
                // are used twice will have 200 sats capacity.

                // Disable other potential paths.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 2,
                        timestamp: 2,
                        flags: 2,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[2], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 7,
                        timestamp: 2,
                        flags: 2,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Path via {node0, node2} is channels {1, 3, 5}.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 1,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[0], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 3,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Capacity of 200 sats because this channel will be used by 3rd path as well.
                add_channel(&gossip_sync, &secp_ctx, &privkeys[2], &privkeys[3], ChannelFeatures::from_le_bytes(id_to_feature_flags(5)), 5);
                update_channel(&gossip_sync, &secp_ctx, &privkeys[2], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 5,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 200_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Path via {node7, node2, node4} is channels {12, 13, 6, 11}.
                // Add 100 sats to the capacities of {12, 13}, because these channels
                // are also used for 3rd path. 100 sats for the rest. Total capacity: 100 sats.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 12,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 200_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[7], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 13,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 200_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                update_channel(&gossip_sync, &secp_ctx, &privkeys[2], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 6,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[4], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 11,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Path via {node7, node2} is channels {12, 13, 5}.
                // We already limited them to 200 sats (they are used twice for 100 sats).
                // Nothing to do here.

                {
                        // Attempt to route more than available results in a failure.
                        if let Err(LightningError{err, action: ErrorAction::IgnoreError}) = get_route(
                                        &our_id, &payment_params, &network_graph.read_only(), None, 350_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes) {
                                assert_eq!(err, "Failed to find a sufficient route to the given destination");
                        } else { panic!(); }
                }

                {
                        // Now, attempt to route 300 sats (exact amount we can route).
                        // Our algorithm should provide us with these 3 paths, 100 sats each.
                        let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 300_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 3);

                        let mut total_amount_paid_msat = 0;
                        for path in &route.paths {
                                assert_eq!(path.last().unwrap().pubkey, nodes[3]);
                                total_amount_paid_msat += path.last().unwrap().fee_msat;
                        }
                        assert_eq!(total_amount_paid_msat, 300_000);
                }

        }

        #[test]
        fn mpp_cheaper_route_test() {
                let (secp_ctx, network_graph, gossip_sync, _, logger) = build_graph();
                let (our_privkey, our_id, privkeys, nodes) = get_nodes(&secp_ctx);
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();
                let payment_params = PaymentParameters::from_node_id(nodes[3]).with_features(channelmanager::provided_invoice_features());

                // This test checks that if we have two cheaper paths and one more expensive path,
                // so that liquidity-wise any 2 of 3 combination is sufficient,
                // two cheaper paths will be taken.
                // These paths have equal available liquidity.

                // We need a combination of 3 paths:
                // From our node to node3 via {node0, node2}, {node7, node2, node4} and {node7, node2}.
                // Note that these paths overlap (channels 5, 12, 13).
                // Each path will have 100 sats capacity, those channels which
                // are used twice will have 200 sats capacity.

                // Disable other potential paths.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 2,
                        timestamp: 2,
                        flags: 2,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[2], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 7,
                        timestamp: 2,
                        flags: 2,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Path via {node0, node2} is channels {1, 3, 5}.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 1,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[0], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 3,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Capacity of 200 sats because this channel will be used by 3rd path as well.
                add_channel(&gossip_sync, &secp_ctx, &privkeys[2], &privkeys[3], ChannelFeatures::from_le_bytes(id_to_feature_flags(5)), 5);
                update_channel(&gossip_sync, &secp_ctx, &privkeys[2], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 5,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 200_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Path via {node7, node2, node4} is channels {12, 13, 6, 11}.
                // Add 100 sats to the capacities of {12, 13}, because these channels
                // are also used for 3rd path. 100 sats for the rest. Total capacity: 100 sats.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 12,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 200_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[7], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 13,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 200_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                update_channel(&gossip_sync, &secp_ctx, &privkeys[2], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 6,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 1_000,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[4], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 11,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Path via {node7, node2} is channels {12, 13, 5}.
                // We already limited them to 200 sats (they are used twice for 100 sats).
                // Nothing to do here.

                {
                        // Now, attempt to route 180 sats.
                        // Our algorithm should provide us with these 2 paths.
                        let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 180_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 2);

                        let mut total_value_transferred_msat = 0;
                        let mut total_paid_msat = 0;
                        for path in &route.paths {
                                assert_eq!(path.last().unwrap().pubkey, nodes[3]);
                                total_value_transferred_msat += path.last().unwrap().fee_msat;
                                for hop in path {
                                        total_paid_msat += hop.fee_msat;
                                }
                        }
                        // If we paid fee, this would be higher.
                        assert_eq!(total_value_transferred_msat, 180_000);
                        let total_fees_paid = total_paid_msat - total_value_transferred_msat;
                        assert_eq!(total_fees_paid, 0);
                }
        }

        #[test]
        fn fees_on_mpp_route_test() {
                // This test makes sure that MPP algorithm properly takes into account
                // fees charged on the channels, by making the fees impactful:
                // if the fee is not properly accounted for, the behavior is different.
                let (secp_ctx, network_graph, gossip_sync, _, logger) = build_graph();
                let (our_privkey, our_id, privkeys, nodes) = get_nodes(&secp_ctx);
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();
                let payment_params = PaymentParameters::from_node_id(nodes[3]).with_features(channelmanager::provided_invoice_features());

                // We need a route consisting of 2 paths:
                // From our node to node3 via {node0, node2} and {node7, node2, node4}.
                // We will route 200 sats, Each path will have 100 sats capacity.

                // This test is not particularly stable: e.g.,
                // there's a way to route via {node0, node2, node4}.
                // It works while pathfinding is deterministic, but can be broken otherwise.
                // It's fine to ignore this concern for now.

                // Disable other potential paths.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 2,
                        timestamp: 2,
                        flags: 2,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                update_channel(&gossip_sync, &secp_ctx, &privkeys[2], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 7,
                        timestamp: 2,
                        flags: 2,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Path via {node0, node2} is channels {1, 3, 5}.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 1,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[0], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 3,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                add_channel(&gossip_sync, &secp_ctx, &privkeys[2], &privkeys[3], ChannelFeatures::from_le_bytes(id_to_feature_flags(5)), 5);
                update_channel(&gossip_sync, &secp_ctx, &privkeys[2], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 5,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Path via {node7, node2, node4} is channels {12, 13, 6, 11}.
                // All channels should be 100 sats capacity. But for the fee experiment,
                // we'll add absolute fee of 150 sats paid for the use channel 6 (paid to node2 on channel 13).
                // Since channel 12 allows to deliver only 250 sats to channel 13, channel 13 can transfer only
                // 100 sats (and pay 150 sats in fees for the use of channel 6),
                // so no matter how large are other channels,
                // the whole path will be limited by 100 sats with just these 2 conditions:
                // - channel 12 capacity is 250 sats
                // - fee for channel 6 is 150 sats
                // Let's test this by enforcing these 2 conditions and removing other limits.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 12,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 250_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[7], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 13,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                update_channel(&gossip_sync, &secp_ctx, &privkeys[2], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 6,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 150_000,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[4], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 11,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                {
                        // Attempt to route more than available results in a failure.
                        if let Err(LightningError{err, action: ErrorAction::IgnoreError}) = get_route(
                                        &our_id, &payment_params, &network_graph.read_only(), None, 210_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes) {
                                assert_eq!(err, "Failed to find a sufficient route to the given destination");
                        } else { panic!(); }
                }

                {
                        // Now, attempt to route 200 sats (exact amount we can route).
                        let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 200_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 2);

                        let mut total_amount_paid_msat = 0;
                        for path in &route.paths {
                                assert_eq!(path.last().unwrap().pubkey, nodes[3]);
                                total_amount_paid_msat += path.last().unwrap().fee_msat;
                        }
                        assert_eq!(total_amount_paid_msat, 200_000);
                        assert_eq!(route.get_total_fees(), 150_000);
                }
        }

        #[test]
        fn mpp_with_last_hops() {
                // Previously, if we tried to send an MPP payment to a destination which was only reachable
                // via a single last-hop route hint, we'd fail to route if we first collected routes
                // totaling close but not quite enough to fund the full payment.
                //
                // This was because we considered last-hop hints to have exactly the sought payment amount
                // instead of the amount we were trying to collect, needlessly limiting our path searching
                // at the very first hop.
                //
                // Specifically, this interacted with our "all paths must fund at least 5% of total target"
                // criterion to cause us to refuse all routes at the last hop hint which would be considered
                // to only have the remaining to-collect amount in available liquidity.
                //
                // This bug appeared in production in some specific channel configurations.
                let (secp_ctx, network_graph, gossip_sync, _, logger) = build_graph();
                let (our_privkey, our_id, privkeys, nodes) = get_nodes(&secp_ctx);
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();
                let payment_params = PaymentParameters::from_node_id(PublicKey::from_slice(&[02; 33]).unwrap()).with_features(channelmanager::provided_invoice_features())
                        .with_route_hints(vec![RouteHint(vec![RouteHintHop {
                                src_node_id: nodes[2],
                                short_channel_id: 42,
                                fees: RoutingFees { base_msat: 0, proportional_millionths: 0 },
                                cltv_expiry_delta: 42,
                                htlc_minimum_msat: None,
                                htlc_maximum_msat: None,
                        }])]).with_max_channel_saturation_power_of_half(0);

                // Keep only two paths from us to nodes[2], both with a 99sat HTLC maximum, with one with
                // no fee and one with a 1msat fee. Previously, trying to route 100 sats to nodes[2] here
                // would first use the no-fee route and then fail to find a path along the second route as
                // we think we can only send up to 1 additional sat over the last-hop but refuse to as its
                // under 5% of our payment amount.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 1,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: (5 << 4) | 5,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 99_000,
                        fee_base_msat: u32::max_value(),
                        fee_proportional_millionths: u32::max_value(),
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 2,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: (5 << 4) | 3,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 99_000,
                        fee_base_msat: u32::max_value(),
                        fee_proportional_millionths: u32::max_value(),
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[1], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 4,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: (4 << 4) | 1,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 1,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[7], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 13,
                        timestamp: 2,
                        flags: 0|2, // Channel disabled
                        cltv_expiry_delta: (13 << 4) | 1,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 2000000,
                        excess_data: Vec::new()
                });

                // Get a route for 100 sats and check that we found the MPP route no problem and didn't
                // overpay at all.
                let mut route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 100_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                assert_eq!(route.paths.len(), 2);
                route.paths.sort_by_key(|path| path[0].short_channel_id);
                // Paths are manually ordered ordered by SCID, so:
                // * the first is channel 1 (0 fee, but 99 sat maximum) -> channel 3 -> channel 42
                // * the second is channel 2 (1 msat fee) -> channel 4 -> channel 42
                assert_eq!(route.paths[0][0].short_channel_id, 1);
                assert_eq!(route.paths[0][0].fee_msat, 0);
                assert_eq!(route.paths[0][2].fee_msat, 99_000);
                assert_eq!(route.paths[1][0].short_channel_id, 2);
                assert_eq!(route.paths[1][0].fee_msat, 1);
                assert_eq!(route.paths[1][2].fee_msat, 1_000);
                assert_eq!(route.get_total_fees(), 1);
                assert_eq!(route.get_total_amount(), 100_000);
        }

        #[test]
        fn drop_lowest_channel_mpp_route_test() {
                // This test checks that low-capacity channel is dropped when after
                // path finding we realize that we found more capacity than we need.
                let (secp_ctx, network_graph, gossip_sync, _, logger) = build_graph();
                let (our_privkey, our_id, privkeys, nodes) = get_nodes(&secp_ctx);
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();
                let payment_params = PaymentParameters::from_node_id(nodes[2]).with_features(channelmanager::provided_invoice_features())
                        .with_max_channel_saturation_power_of_half(0);

                // We need a route consisting of 3 paths:
                // From our node to node2 via node0, node7, node1 (three paths one hop each).

                // The first and the second paths should be sufficient, but the third should be
                // cheaper, so that we select it but drop later.

                // First, we set limits on these (previously unlimited) channels.
                // Their aggregate capacity will be 50 + 60 + 20 = 130 sats.

                // Path via node0 is channels {1, 3}. Limit them to 100 and 50 sats (total limit 50);
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 1,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 100_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[0], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 3,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 50_000,
                        fee_base_msat: 100,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Path via node7 is channels {12, 13}. Limit them to 60 and 60 sats (total limit 60);
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 12,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 60_000,
                        fee_base_msat: 100,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[7], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 13,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 60_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                // Path via node1 is channels {2, 4}. Limit them to 20 and 20 sats (total capacity 20 sats).
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 2,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 20_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[1], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 4,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 20_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                {
                        // Attempt to route more than available results in a failure.
                        if let Err(LightningError{err, action: ErrorAction::IgnoreError}) = get_route(
                                        &our_id, &payment_params, &network_graph.read_only(), None, 150_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes) {
                                assert_eq!(err, "Failed to find a sufficient route to the given destination");
                        } else { panic!(); }
                }

                {
                        // Now, attempt to route 125 sats (just a bit below the capacity of 3 channels).
                        // Our algorithm should provide us with these 3 paths.
                        let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 125_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 3);
                        let mut total_amount_paid_msat = 0;
                        for path in &route.paths {
                                assert_eq!(path.len(), 2);
                                assert_eq!(path.last().unwrap().pubkey, nodes[2]);
                                total_amount_paid_msat += path.last().unwrap().fee_msat;
                        }
                        assert_eq!(total_amount_paid_msat, 125_000);
                }

                {
                        // Attempt to route without the last small cheap channel
                        let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 90_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 2);
                        let mut total_amount_paid_msat = 0;
                        for path in &route.paths {
                                assert_eq!(path.len(), 2);
                                assert_eq!(path.last().unwrap().pubkey, nodes[2]);
                                total_amount_paid_msat += path.last().unwrap().fee_msat;
                        }
                        assert_eq!(total_amount_paid_msat, 90_000);
                }
        }

        #[test]
        fn min_criteria_consistency() {
                // Test that we don't use an inconsistent metric between updating and walking nodes during
                // our Dijkstra's pass. In the initial version of MPP, the "best source" for a given node
                // was updated with a different criterion from the heap sorting, resulting in loops in
                // calculated paths. We test for that specific case here.

                // We construct a network that looks like this:
                //
                //            node2 -1(3)2- node3
                //              2          2
                //               (2)     (4)
                //                  1   1
                //    node1 -1(5)2- node4 -1(1)2- node6
                //    2
                //   (6)
                //        1
                // our_node
                //
                // We create a loop on the side of our real path - our destination is node 6, with a
                // previous hop of node 4. From 4, the cheapest previous path is channel 2 from node 2,
                // followed by node 3 over channel 3. Thereafter, the cheapest next-hop is back to node 4
                // (this time over channel 4). Channel 4 has 0 htlc_minimum_msat whereas channel 1 (the
                // other channel with a previous-hop of node 4) has a high (but irrelevant to the overall
                // payment) htlc_minimum_msat. In the original algorithm, this resulted in node4's
                // "previous hop" being set to node 3, creating a loop in the path.
                let secp_ctx = Secp256k1::new();
                let genesis_hash = genesis_block(Network::Testnet).header.block_hash();
                let logger = Arc::new(ln_test_utils::TestLogger::new());
                let network = Arc::new(NetworkGraph::new(genesis_hash, Arc::clone(&logger)));
                let gossip_sync = P2PGossipSync::new(Arc::clone(&network), None, Arc::clone(&logger));
                let (our_privkey, our_id, privkeys, nodes) = get_nodes(&secp_ctx);
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();
                let payment_params = PaymentParameters::from_node_id(nodes[6]);

                add_channel(&gossip_sync, &secp_ctx, &our_privkey, &privkeys[1], ChannelFeatures::from_le_bytes(id_to_feature_flags(6)), 6);
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 6,
                        timestamp: 1,
                        flags: 0,
                        cltv_expiry_delta: (6 << 4) | 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                add_or_update_node(&gossip_sync, &secp_ctx, &privkeys[1], NodeFeatures::from_le_bytes(id_to_feature_flags(1)), 0);

                add_channel(&gossip_sync, &secp_ctx, &privkeys[1], &privkeys[4], ChannelFeatures::from_le_bytes(id_to_feature_flags(5)), 5);
                update_channel(&gossip_sync, &secp_ctx, &privkeys[1], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 5,
                        timestamp: 1,
                        flags: 0,
                        cltv_expiry_delta: (5 << 4) | 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 100,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                add_or_update_node(&gossip_sync, &secp_ctx, &privkeys[4], NodeFeatures::from_le_bytes(id_to_feature_flags(4)), 0);

                add_channel(&gossip_sync, &secp_ctx, &privkeys[4], &privkeys[3], ChannelFeatures::from_le_bytes(id_to_feature_flags(4)), 4);
                update_channel(&gossip_sync, &secp_ctx, &privkeys[4], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 4,
                        timestamp: 1,
                        flags: 0,
                        cltv_expiry_delta: (4 << 4) | 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                add_or_update_node(&gossip_sync, &secp_ctx, &privkeys[3], NodeFeatures::from_le_bytes(id_to_feature_flags(3)), 0);

                add_channel(&gossip_sync, &secp_ctx, &privkeys[3], &privkeys[2], ChannelFeatures::from_le_bytes(id_to_feature_flags(3)), 3);
                update_channel(&gossip_sync, &secp_ctx, &privkeys[3], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 3,
                        timestamp: 1,
                        flags: 0,
                        cltv_expiry_delta: (3 << 4) | 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                add_or_update_node(&gossip_sync, &secp_ctx, &privkeys[2], NodeFeatures::from_le_bytes(id_to_feature_flags(2)), 0);

                add_channel(&gossip_sync, &secp_ctx, &privkeys[2], &privkeys[4], ChannelFeatures::from_le_bytes(id_to_feature_flags(2)), 2);
                update_channel(&gossip_sync, &secp_ctx, &privkeys[2], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 2,
                        timestamp: 1,
                        flags: 0,
                        cltv_expiry_delta: (2 << 4) | 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                add_channel(&gossip_sync, &secp_ctx, &privkeys[4], &privkeys[6], ChannelFeatures::from_le_bytes(id_to_feature_flags(1)), 1);
                update_channel(&gossip_sync, &secp_ctx, &privkeys[4], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 1,
                        timestamp: 1,
                        flags: 0,
                        cltv_expiry_delta: (1 << 4) | 0,
                        htlc_minimum_msat: 100,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                add_or_update_node(&gossip_sync, &secp_ctx, &privkeys[6], NodeFeatures::from_le_bytes(id_to_feature_flags(6)), 0);

                {
                        // Now ensure the route flows simply over nodes 1 and 4 to 6.
                        let route = get_route(&our_id, &payment_params, &network.read_only(), None, 10_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 1);
                        assert_eq!(route.paths[0].len(), 3);

                        assert_eq!(route.paths[0][0].pubkey, nodes[1]);
                        assert_eq!(route.paths[0][0].short_channel_id, 6);
                        assert_eq!(route.paths[0][0].fee_msat, 100);
                        assert_eq!(route.paths[0][0].cltv_expiry_delta, (5 << 4) | 0);
                        assert_eq!(route.paths[0][0].node_features.le_flags(), &id_to_feature_flags(1));
                        assert_eq!(route.paths[0][0].channel_features.le_flags(), &id_to_feature_flags(6));

                        assert_eq!(route.paths[0][1].pubkey, nodes[4]);
                        assert_eq!(route.paths[0][1].short_channel_id, 5);
                        assert_eq!(route.paths[0][1].fee_msat, 0);
                        assert_eq!(route.paths[0][1].cltv_expiry_delta, (1 << 4) | 0);
                        assert_eq!(route.paths[0][1].node_features.le_flags(), &id_to_feature_flags(4));
                        assert_eq!(route.paths[0][1].channel_features.le_flags(), &id_to_feature_flags(5));

                        assert_eq!(route.paths[0][2].pubkey, nodes[6]);
                        assert_eq!(route.paths[0][2].short_channel_id, 1);
                        assert_eq!(route.paths[0][2].fee_msat, 10_000);
                        assert_eq!(route.paths[0][2].cltv_expiry_delta, 42);
                        assert_eq!(route.paths[0][2].node_features.le_flags(), &id_to_feature_flags(6));
                        assert_eq!(route.paths[0][2].channel_features.le_flags(), &id_to_feature_flags(1));
                }
        }


        #[test]
        fn exact_fee_liquidity_limit() {
                // Test that if, while walking the graph, we find a hop that has exactly enough liquidity
                // for us, including later hop fees, we take it. In the first version of our MPP algorithm
                // we calculated fees on a higher value, resulting in us ignoring such paths.
                let (secp_ctx, network_graph, gossip_sync, _, logger) = build_graph();
                let (our_privkey, our_id, _, nodes) = get_nodes(&secp_ctx);
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();
                let payment_params = PaymentParameters::from_node_id(nodes[2]);

                // We modify the graph to set the htlc_maximum of channel 2 to below the value we wish to
                // send.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 2,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 85_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 12,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: (4 << 4) | 1,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 270_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 1000000,
                        excess_data: Vec::new()
                });

                {
                        // Now, attempt to route 90 sats, which is exactly 90 sats at the last hop, plus the
                        // 200% fee charged channel 13 in the 1-to-2 direction.
                        let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 90_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 1);
                        assert_eq!(route.paths[0].len(), 2);

                        assert_eq!(route.paths[0][0].pubkey, nodes[7]);
                        assert_eq!(route.paths[0][0].short_channel_id, 12);
                        assert_eq!(route.paths[0][0].fee_msat, 90_000*2);
                        assert_eq!(route.paths[0][0].cltv_expiry_delta, (13 << 4) | 1);
                        assert_eq!(route.paths[0][0].node_features.le_flags(), &id_to_feature_flags(8));
                        assert_eq!(route.paths[0][0].channel_features.le_flags(), &id_to_feature_flags(12));

                        assert_eq!(route.paths[0][1].pubkey, nodes[2]);
                        assert_eq!(route.paths[0][1].short_channel_id, 13);
                        assert_eq!(route.paths[0][1].fee_msat, 90_000);
                        assert_eq!(route.paths[0][1].cltv_expiry_delta, 42);
                        assert_eq!(route.paths[0][1].node_features.le_flags(), &id_to_feature_flags(3));
                        assert_eq!(route.paths[0][1].channel_features.le_flags(), &id_to_feature_flags(13));
                }
        }

        #[test]
        fn htlc_max_reduction_below_min() {
                // Test that if, while walking the graph, we reduce the value being sent to meet an
                // htlc_maximum_msat, we don't end up undershooting a later htlc_minimum_msat. In the
                // initial version of MPP we'd accept such routes but reject them while recalculating fees,
                // resulting in us thinking there is no possible path, even if other paths exist.
                let (secp_ctx, network_graph, gossip_sync, _, logger) = build_graph();
                let (our_privkey, our_id, privkeys, nodes) = get_nodes(&secp_ctx);
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();
                let payment_params = PaymentParameters::from_node_id(nodes[2]).with_features(channelmanager::provided_invoice_features());

                // We modify the graph to set the htlc_minimum of channel 2 and 4 as needed - channel 2
                // gets an htlc_maximum_msat of 80_000 and channel 4 an htlc_minimum_msat of 90_000. We
                // then try to send 90_000.
                update_channel(&gossip_sync, &secp_ctx, &our_privkey, UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 2,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: 0,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 80_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[1], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 4,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: (4 << 4) | 1,
                        htlc_minimum_msat: 90_000,
                        htlc_maximum_msat: MAX_VALUE_MSAT,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                {
                        // Now, attempt to route 90 sats, hitting the htlc_minimum on channel 4, but
                        // overshooting the htlc_maximum on channel 2. Thus, we should pick the (absurdly
                        // expensive) channels 12-13 path.
                        let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 90_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 1);
                        assert_eq!(route.paths[0].len(), 2);

                        assert_eq!(route.paths[0][0].pubkey, nodes[7]);
                        assert_eq!(route.paths[0][0].short_channel_id, 12);
                        assert_eq!(route.paths[0][0].fee_msat, 90_000*2);
                        assert_eq!(route.paths[0][0].cltv_expiry_delta, (13 << 4) | 1);
                        assert_eq!(route.paths[0][0].node_features.le_flags(), &id_to_feature_flags(8));
                        assert_eq!(route.paths[0][0].channel_features.le_flags(), &id_to_feature_flags(12));

                        assert_eq!(route.paths[0][1].pubkey, nodes[2]);
                        assert_eq!(route.paths[0][1].short_channel_id, 13);
                        assert_eq!(route.paths[0][1].fee_msat, 90_000);
                        assert_eq!(route.paths[0][1].cltv_expiry_delta, 42);
                        assert_eq!(route.paths[0][1].node_features.le_flags(), channelmanager::provided_invoice_features().le_flags());
                        assert_eq!(route.paths[0][1].channel_features.le_flags(), &id_to_feature_flags(13));
                }
        }

        #[test]
        fn multiple_direct_first_hops() {
                // Previously we'd only ever considered one first hop path per counterparty.
                // However, as we don't restrict users to one channel per peer, we really need to support
                // looking at all first hop paths.
                // Here we test that we do not ignore all-but-the-last first hop paths per counterparty (as
                // we used to do by overwriting the `first_hop_targets` hashmap entry) and that we can MPP
                // route over multiple channels with the same first hop.
                let secp_ctx = Secp256k1::new();
                let (_, our_id, _, nodes) = get_nodes(&secp_ctx);
                let genesis_hash = genesis_block(Network::Testnet).header.block_hash();
                let logger = Arc::new(ln_test_utils::TestLogger::new());
                let network_graph = NetworkGraph::new(genesis_hash, Arc::clone(&logger));
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let payment_params = PaymentParameters::from_node_id(nodes[0]).with_features(channelmanager::provided_invoice_features());
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();

                {
                        let route = get_route(&our_id, &payment_params, &network_graph.read_only(), Some(&[
                                &get_channel_details(Some(3), nodes[0], channelmanager::provided_init_features(), 200_000),
                                &get_channel_details(Some(2), nodes[0], channelmanager::provided_init_features(), 10_000),
                        ]), 100_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 1);
                        assert_eq!(route.paths[0].len(), 1);

                        assert_eq!(route.paths[0][0].pubkey, nodes[0]);
                        assert_eq!(route.paths[0][0].short_channel_id, 3);
                        assert_eq!(route.paths[0][0].fee_msat, 100_000);
                }
                {
                        let route = get_route(&our_id, &payment_params, &network_graph.read_only(), Some(&[
                                &get_channel_details(Some(3), nodes[0], channelmanager::provided_init_features(), 50_000),
                                &get_channel_details(Some(2), nodes[0], channelmanager::provided_init_features(), 50_000),
                        ]), 100_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 2);
                        assert_eq!(route.paths[0].len(), 1);
                        assert_eq!(route.paths[1].len(), 1);

                        assert!((route.paths[0][0].short_channel_id == 3 && route.paths[1][0].short_channel_id == 2) ||
                                (route.paths[0][0].short_channel_id == 2 && route.paths[1][0].short_channel_id == 3));

                        assert_eq!(route.paths[0][0].pubkey, nodes[0]);
                        assert_eq!(route.paths[0][0].fee_msat, 50_000);

                        assert_eq!(route.paths[1][0].pubkey, nodes[0]);
                        assert_eq!(route.paths[1][0].fee_msat, 50_000);
                }

                {
                        // If we have a bunch of outbound channels to the same node, where most are not
                        // sufficient to pay the full payment, but one is, we should default to just using the
                        // one single channel that has sufficient balance, avoiding MPP.
                        //
                        // If we have several options above the 3xpayment value threshold, we should pick the
                        // smallest of them, avoiding further fragmenting our available outbound balance to
                        // this node.
                        let route = get_route(&our_id, &payment_params, &network_graph.read_only(), Some(&[
                                &get_channel_details(Some(2), nodes[0], channelmanager::provided_init_features(), 50_000),
                                &get_channel_details(Some(3), nodes[0], channelmanager::provided_init_features(), 50_000),
                                &get_channel_details(Some(5), nodes[0], channelmanager::provided_init_features(), 50_000),
                                &get_channel_details(Some(6), nodes[0], channelmanager::provided_init_features(), 300_000),
                                &get_channel_details(Some(7), nodes[0], channelmanager::provided_init_features(), 50_000),
                                &get_channel_details(Some(8), nodes[0], channelmanager::provided_init_features(), 50_000),
                                &get_channel_details(Some(9), nodes[0], channelmanager::provided_init_features(), 50_000),
                                &get_channel_details(Some(4), nodes[0], channelmanager::provided_init_features(), 1_000_000),
                        ]), 100_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                        assert_eq!(route.paths.len(), 1);
                        assert_eq!(route.paths[0].len(), 1);

                        assert_eq!(route.paths[0][0].pubkey, nodes[0]);
                        assert_eq!(route.paths[0][0].short_channel_id, 6);
                        assert_eq!(route.paths[0][0].fee_msat, 100_000);
                }
        }

        #[test]
        fn prefers_shorter_route_with_higher_fees() {
                let (secp_ctx, network_graph, _, _, logger) = build_graph();
                let (_, our_id, _, nodes) = get_nodes(&secp_ctx);
                let payment_params = PaymentParameters::from_node_id(nodes[6]).with_route_hints(last_hops(&nodes));

                // Without penalizing each hop 100 msats, a longer path with lower fees is chosen.
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();
                let route = get_route(
                        &our_id, &payment_params, &network_graph.read_only(), None, 100, 42,
                        Arc::clone(&logger), &scorer, &random_seed_bytes
                ).unwrap();
                let path = route.paths[0].iter().map(|hop| hop.short_channel_id).collect::<Vec<_>>();

                assert_eq!(route.get_total_fees(), 100);
                assert_eq!(route.get_total_amount(), 100);
                assert_eq!(path, vec![2, 4, 6, 11, 8]);

                // Applying a 100 msat penalty to each hop results in taking channels 7 and 10 to nodes[6]
                // from nodes[2] rather than channel 6, 11, and 8, even though the longer path is cheaper.
                let scorer = ln_test_utils::TestScorer::with_penalty(100);
                let route = get_route(
                        &our_id, &payment_params, &network_graph.read_only(), None, 100, 42,
                        Arc::clone(&logger), &scorer, &random_seed_bytes
                ).unwrap();
                let path = route.paths[0].iter().map(|hop| hop.short_channel_id).collect::<Vec<_>>();

                assert_eq!(route.get_total_fees(), 300);
                assert_eq!(route.get_total_amount(), 100);
                assert_eq!(path, vec![2, 4, 7, 10]);
        }

        struct BadChannelScorer {
                short_channel_id: u64,
        }

        #[cfg(c_bindings)]
        impl Writeable for BadChannelScorer {
                fn write<W: Writer>(&self, _w: &mut W) -> Result<(), crate::io::Error> { unimplemented!() }
        }
        impl Score for BadChannelScorer {
                fn channel_penalty_msat(&self, short_channel_id: u64, _: &NodeId, _: &NodeId, _: ChannelUsage) -> u64 {
                        if short_channel_id == self.short_channel_id { u64::max_value() } else { 0 }
                }

                fn payment_path_failed(&mut self, _path: &[&RouteHop], _short_channel_id: u64) {}
                fn payment_path_successful(&mut self, _path: &[&RouteHop]) {}
                fn probe_failed(&mut self, _path: &[&RouteHop], _short_channel_id: u64) {}
                fn probe_successful(&mut self, _path: &[&RouteHop]) {}
        }

        struct BadNodeScorer {
                node_id: NodeId,
        }

        #[cfg(c_bindings)]
        impl Writeable for BadNodeScorer {
                fn write<W: Writer>(&self, _w: &mut W) -> Result<(), crate::io::Error> { unimplemented!() }
        }

        impl Score for BadNodeScorer {
                fn channel_penalty_msat(&self, _: u64, _: &NodeId, target: &NodeId, _: ChannelUsage) -> u64 {
                        if *target == self.node_id { u64::max_value() } else { 0 }
                }

                fn payment_path_failed(&mut self, _path: &[&RouteHop], _short_channel_id: u64) {}
                fn payment_path_successful(&mut self, _path: &[&RouteHop]) {}
                fn probe_failed(&mut self, _path: &[&RouteHop], _short_channel_id: u64) {}
                fn probe_successful(&mut self, _path: &[&RouteHop]) {}
        }

        #[test]
        fn avoids_routing_through_bad_channels_and_nodes() {
                let (secp_ctx, network, _, _, logger) = build_graph();
                let (_, our_id, _, nodes) = get_nodes(&secp_ctx);
                let payment_params = PaymentParameters::from_node_id(nodes[6]).with_route_hints(last_hops(&nodes));
                let network_graph = network.read_only();

                // A path to nodes[6] exists when no penalties are applied to any channel.
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();
                let route = get_route(
                        &our_id, &payment_params, &network_graph, None, 100, 42,
                        Arc::clone(&logger), &scorer, &random_seed_bytes
                ).unwrap();
                let path = route.paths[0].iter().map(|hop| hop.short_channel_id).collect::<Vec<_>>();

                assert_eq!(route.get_total_fees(), 100);
                assert_eq!(route.get_total_amount(), 100);
                assert_eq!(path, vec![2, 4, 6, 11, 8]);

                // A different path to nodes[6] exists if channel 6 cannot be routed over.
                let scorer = BadChannelScorer { short_channel_id: 6 };
                let route = get_route(
                        &our_id, &payment_params, &network_graph, None, 100, 42,
                        Arc::clone(&logger), &scorer, &random_seed_bytes
                ).unwrap();
                let path = route.paths[0].iter().map(|hop| hop.short_channel_id).collect::<Vec<_>>();

                assert_eq!(route.get_total_fees(), 300);
                assert_eq!(route.get_total_amount(), 100);
                assert_eq!(path, vec![2, 4, 7, 10]);

                // A path to nodes[6] does not exist if nodes[2] cannot be routed through.
                let scorer = BadNodeScorer { node_id: NodeId::from_pubkey(&nodes[2]) };
                match get_route(
                        &our_id, &payment_params, &network_graph, None, 100, 42,
                        Arc::clone(&logger), &scorer, &random_seed_bytes
                ) {
                        Err(LightningError { err, .. } ) => {
                                assert_eq!(err, "Failed to find a path to the given destination");
                        },
                        Ok(_) => panic!("Expected error"),
                }
        }

        #[test]
        fn total_fees_single_path() {
                let route = Route {
                        paths: vec![vec![
                                RouteHop {
                                        pubkey: PublicKey::from_slice(&hex::decode("02eec7245d6b7d2ccb30380bfbe2a3648cd7a942653f5aa340edcea1f283686619").unwrap()[..]).unwrap(),
                                        channel_features: ChannelFeatures::empty(), node_features: NodeFeatures::empty(),
                                        short_channel_id: 0, fee_msat: 100, cltv_expiry_delta: 0
                                },
                                RouteHop {
                                        pubkey: PublicKey::from_slice(&hex::decode("0324653eac434488002cc06bbfb7f10fe18991e35f9fe4302dbea6d2353dc0ab1c").unwrap()[..]).unwrap(),
                                        channel_features: ChannelFeatures::empty(), node_features: NodeFeatures::empty(),
                                        short_channel_id: 0, fee_msat: 150, cltv_expiry_delta: 0
                                },
                                RouteHop {
                                        pubkey: PublicKey::from_slice(&hex::decode("027f31ebc5462c1fdce1b737ecff52d37d75dea43ce11c74d25aa297165faa2007").unwrap()[..]).unwrap(),
                                        channel_features: ChannelFeatures::empty(), node_features: NodeFeatures::empty(),
                                        short_channel_id: 0, fee_msat: 225, cltv_expiry_delta: 0
                                },
                        ]],
                        payment_params: None,
                };

                assert_eq!(route.get_total_fees(), 250);
                assert_eq!(route.get_total_amount(), 225);
        }

        #[test]
        fn total_fees_multi_path() {
                let route = Route {
                        paths: vec![vec![
                                RouteHop {
                                        pubkey: PublicKey::from_slice(&hex::decode("02eec7245d6b7d2ccb30380bfbe2a3648cd7a942653f5aa340edcea1f283686619").unwrap()[..]).unwrap(),
                                        channel_features: ChannelFeatures::empty(), node_features: NodeFeatures::empty(),
                                        short_channel_id: 0, fee_msat: 100, cltv_expiry_delta: 0
                                },
                                RouteHop {
                                        pubkey: PublicKey::from_slice(&hex::decode("0324653eac434488002cc06bbfb7f10fe18991e35f9fe4302dbea6d2353dc0ab1c").unwrap()[..]).unwrap(),
                                        channel_features: ChannelFeatures::empty(), node_features: NodeFeatures::empty(),
                                        short_channel_id: 0, fee_msat: 150, cltv_expiry_delta: 0
                                },
                        ],vec![
                                RouteHop {
                                        pubkey: PublicKey::from_slice(&hex::decode("02eec7245d6b7d2ccb30380bfbe2a3648cd7a942653f5aa340edcea1f283686619").unwrap()[..]).unwrap(),
                                        channel_features: ChannelFeatures::empty(), node_features: NodeFeatures::empty(),
                                        short_channel_id: 0, fee_msat: 100, cltv_expiry_delta: 0
                                },
                                RouteHop {
                                        pubkey: PublicKey::from_slice(&hex::decode("0324653eac434488002cc06bbfb7f10fe18991e35f9fe4302dbea6d2353dc0ab1c").unwrap()[..]).unwrap(),
                                        channel_features: ChannelFeatures::empty(), node_features: NodeFeatures::empty(),
                                        short_channel_id: 0, fee_msat: 150, cltv_expiry_delta: 0
                                },
                        ]],
                        payment_params: None,
                };

                assert_eq!(route.get_total_fees(), 200);
                assert_eq!(route.get_total_amount(), 300);
        }

        #[test]
        fn total_empty_route_no_panic() {
                // In an earlier version of `Route::get_total_fees` and `Route::get_total_amount`, they
                // would both panic if the route was completely empty. We test to ensure they return 0
                // here, even though its somewhat nonsensical as a route.
                let route = Route { paths: Vec::new(), payment_params: None };

                assert_eq!(route.get_total_fees(), 0);
                assert_eq!(route.get_total_amount(), 0);
        }

        #[test]
        fn limits_total_cltv_delta() {
                let (secp_ctx, network, _, _, logger) = build_graph();
                let (_, our_id, _, nodes) = get_nodes(&secp_ctx);
                let network_graph = network.read_only();

                let scorer = ln_test_utils::TestScorer::with_penalty(0);

                // Make sure that generally there is at least one route available
                let feasible_max_total_cltv_delta = 1008;
                let feasible_payment_params = PaymentParameters::from_node_id(nodes[6]).with_route_hints(last_hops(&nodes))
                        .with_max_total_cltv_expiry_delta(feasible_max_total_cltv_delta);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();
                let route = get_route(&our_id, &feasible_payment_params, &network_graph, None, 100, 0, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                let path = route.paths[0].iter().map(|hop| hop.short_channel_id).collect::<Vec<_>>();
                assert_ne!(path.len(), 0);

                // But not if we exclude all paths on the basis of their accumulated CLTV delta
                let fail_max_total_cltv_delta = 23;
                let fail_payment_params = PaymentParameters::from_node_id(nodes[6]).with_route_hints(last_hops(&nodes))
                        .with_max_total_cltv_expiry_delta(fail_max_total_cltv_delta);
                match get_route(&our_id, &fail_payment_params, &network_graph, None, 100, 0, Arc::clone(&logger), &scorer, &random_seed_bytes)
                {
                        Err(LightningError { err, .. } ) => {
                                assert_eq!(err, "Failed to find a path to the given destination");
                        },
                        Ok(_) => panic!("Expected error"),
                }
        }

        #[test]
        fn avoids_recently_failed_paths() {
                // Ensure that the router always avoids all of the `previously_failed_channels` channels by
                // randomly inserting channels into it until we can't find a route anymore.
                let (secp_ctx, network, _, _, logger) = build_graph();
                let (_, our_id, _, nodes) = get_nodes(&secp_ctx);
                let network_graph = network.read_only();

                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let mut payment_params = PaymentParameters::from_node_id(nodes[6]).with_route_hints(last_hops(&nodes))
                        .with_max_path_count(1);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();

                // We should be able to find a route initially, and then after we fail a few random
                // channels eventually we won't be able to any longer.
                assert!(get_route(&our_id, &payment_params, &network_graph, None, 100, 0, Arc::clone(&logger), &scorer, &random_seed_bytes).is_ok());
                loop {
                        if let Ok(route) = get_route(&our_id, &payment_params, &network_graph, None, 100, 0, Arc::clone(&logger), &scorer, &random_seed_bytes) {
                                for chan in route.paths[0].iter() {
                                        assert!(!payment_params.previously_failed_channels.contains(&chan.short_channel_id));
                                }
                                let victim = (u64::from_ne_bytes(random_seed_bytes[0..8].try_into().unwrap()) as usize)
                                        % route.paths[0].len();
                                payment_params.previously_failed_channels.push(route.paths[0][victim].short_channel_id);
                        } else { break; }
                }
        }

        #[test]
        fn limits_path_length() {
                let (secp_ctx, network, _, _, logger) = build_line_graph();
                let (_, our_id, _, nodes) = get_nodes(&secp_ctx);
                let network_graph = network.read_only();

                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();

                // First check we can actually create a long route on this graph.
                let feasible_payment_params = PaymentParameters::from_node_id(nodes[18]);
                let route = get_route(&our_id, &feasible_payment_params, &network_graph, None, 100, 0,
                        Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                let path = route.paths[0].iter().map(|hop| hop.short_channel_id).collect::<Vec<_>>();
                assert!(path.len() == MAX_PATH_LENGTH_ESTIMATE.into());

                // But we can't create a path surpassing the MAX_PATH_LENGTH_ESTIMATE limit.
                let fail_payment_params = PaymentParameters::from_node_id(nodes[19]);
                match get_route(&our_id, &fail_payment_params, &network_graph, None, 100, 0,
                        Arc::clone(&logger), &scorer, &random_seed_bytes)
                {
                        Err(LightningError { err, .. } ) => {
                                assert_eq!(err, "Failed to find a path to the given destination");
                        },
                        Ok(_) => panic!("Expected error"),
                }
        }

        #[test]
        fn adds_and_limits_cltv_offset() {
                let (secp_ctx, network_graph, _, _, logger) = build_graph();
                let (_, our_id, _, nodes) = get_nodes(&secp_ctx);

                let scorer = ln_test_utils::TestScorer::with_penalty(0);

                let payment_params = PaymentParameters::from_node_id(nodes[6]).with_route_hints(last_hops(&nodes));
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();
                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 100, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                assert_eq!(route.paths.len(), 1);

                let cltv_expiry_deltas_before = route.paths[0].iter().map(|h| h.cltv_expiry_delta).collect::<Vec<u32>>();

                // Check whether the offset added to the last hop by default is in [1 .. DEFAULT_MAX_TOTAL_CLTV_EXPIRY_DELTA]
                let mut route_default = route.clone();
                add_random_cltv_offset(&mut route_default, &payment_params, &network_graph.read_only(), &random_seed_bytes);
                let cltv_expiry_deltas_default = route_default.paths[0].iter().map(|h| h.cltv_expiry_delta).collect::<Vec<u32>>();
                assert_eq!(cltv_expiry_deltas_before.split_last().unwrap().1, cltv_expiry_deltas_default.split_last().unwrap().1);
                assert!(cltv_expiry_deltas_default.last() > cltv_expiry_deltas_before.last());
                assert!(cltv_expiry_deltas_default.last().unwrap() <= &DEFAULT_MAX_TOTAL_CLTV_EXPIRY_DELTA);

                // Check that no offset is added when we restrict the max_total_cltv_expiry_delta
                let mut route_limited = route.clone();
                let limited_max_total_cltv_expiry_delta = cltv_expiry_deltas_before.iter().sum();
                let limited_payment_params = payment_params.with_max_total_cltv_expiry_delta(limited_max_total_cltv_expiry_delta);
                add_random_cltv_offset(&mut route_limited, &limited_payment_params, &network_graph.read_only(), &random_seed_bytes);
                let cltv_expiry_deltas_limited = route_limited.paths[0].iter().map(|h| h.cltv_expiry_delta).collect::<Vec<u32>>();
                assert_eq!(cltv_expiry_deltas_before, cltv_expiry_deltas_limited);
        }

        #[test]
        fn adds_plausible_cltv_offset() {
                let (secp_ctx, network, _, _, logger) = build_graph();
                let (_, our_id, _, nodes) = get_nodes(&secp_ctx);
                let network_graph = network.read_only();
                let network_nodes = network_graph.nodes();
                let network_channels = network_graph.channels();
                let scorer = ln_test_utils::TestScorer::with_penalty(0);
                let payment_params = PaymentParameters::from_node_id(nodes[3]);
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[4u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();

                let mut route = get_route(&our_id, &payment_params, &network_graph, None, 100, 0,
                                                                  Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                add_random_cltv_offset(&mut route, &payment_params, &network_graph, &random_seed_bytes);

                let mut path_plausibility = vec![];

                for p in route.paths {
                        // 1. Select random observation point
                        let mut prng = ChaCha20::new(&random_seed_bytes, &[0u8; 12]);
                        let mut random_bytes = [0u8; ::core::mem::size_of::<usize>()];

                        prng.process_in_place(&mut random_bytes);
                        let random_path_index = usize::from_be_bytes(random_bytes).wrapping_rem(p.len());
                        let observation_point = NodeId::from_pubkey(&p.get(random_path_index).unwrap().pubkey);

                        // 2. Calculate what CLTV expiry delta we would observe there
                        let observed_cltv_expiry_delta: u32 = p[random_path_index..].iter().map(|h| h.cltv_expiry_delta).sum();

                        // 3. Starting from the observation point, find candidate paths
                        let mut candidates: VecDeque<(NodeId, Vec<u32>)> = VecDeque::new();
                        candidates.push_back((observation_point, vec![]));

                        let mut found_plausible_candidate = false;

                        'candidate_loop: while let Some((cur_node_id, cur_path_cltv_deltas)) = candidates.pop_front() {
                                if let Some(remaining) = observed_cltv_expiry_delta.checked_sub(cur_path_cltv_deltas.iter().sum::<u32>()) {
                                        if remaining == 0 || remaining.wrapping_rem(40) == 0 || remaining.wrapping_rem(144) == 0 {
                                                found_plausible_candidate = true;
                                                break 'candidate_loop;
                                        }
                                }

                                if let Some(cur_node) = network_nodes.get(&cur_node_id) {
                                        for channel_id in &cur_node.channels {
                                                if let Some(channel_info) = network_channels.get(&channel_id) {
                                                        if let Some((dir_info, next_id)) = channel_info.as_directed_from(&cur_node_id) {
                                                                let next_cltv_expiry_delta = dir_info.direction().cltv_expiry_delta as u32;
                                                                if cur_path_cltv_deltas.iter().sum::<u32>()
                                                                        .saturating_add(next_cltv_expiry_delta) <= observed_cltv_expiry_delta {
                                                                        let mut new_path_cltv_deltas = cur_path_cltv_deltas.clone();
                                                                        new_path_cltv_deltas.push(next_cltv_expiry_delta);
                                                                        candidates.push_back((*next_id, new_path_cltv_deltas));
                                                                }
                                                        }
                                                }
                                        }
                                }
                        }

                        path_plausibility.push(found_plausible_candidate);
                }
                assert!(path_plausibility.iter().all(|x| *x));
        }

        #[test]
        fn builds_correct_path_from_hops() {
                let (secp_ctx, network, _, _, logger) = build_graph();
                let (_, our_id, _, nodes) = get_nodes(&secp_ctx);
                let network_graph = network.read_only();

                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();

                let payment_params = PaymentParameters::from_node_id(nodes[3]);
                let hops = [nodes[1], nodes[2], nodes[4], nodes[3]];
                let route = build_route_from_hops_internal(&our_id, &hops, &payment_params,
                         &network_graph, 100, 0, Arc::clone(&logger), &random_seed_bytes).unwrap();
                let route_hop_pubkeys = route.paths[0].iter().map(|hop| hop.pubkey).collect::<Vec<_>>();
                assert_eq!(hops.len(), route.paths[0].len());
                for (idx, hop_pubkey) in hops.iter().enumerate() {
                        assert!(*hop_pubkey == route_hop_pubkeys[idx]);
                }
        }

        #[test]
        fn avoids_saturating_channels() {
                let (secp_ctx, network_graph, gossip_sync, _, logger) = build_graph();
                let (_, our_id, privkeys, nodes) = get_nodes(&secp_ctx);

                let scorer = ProbabilisticScorer::new(Default::default(), &*network_graph, Arc::clone(&logger));

                // Set the fee on channel 13 to 100% to match channel 4 giving us two equivalent paths (us
                // -> node 7 -> node2 and us -> node 1 -> node 2) which we should balance over.
                update_channel(&gossip_sync, &secp_ctx, &privkeys[1], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 4,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: (4 << 4) | 1,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 250_000_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });
                update_channel(&gossip_sync, &secp_ctx, &privkeys[7], UnsignedChannelUpdate {
                        chain_hash: genesis_block(Network::Testnet).header.block_hash(),
                        short_channel_id: 13,
                        timestamp: 2,
                        flags: 0,
                        cltv_expiry_delta: (13 << 4) | 1,
                        htlc_minimum_msat: 0,
                        htlc_maximum_msat: 250_000_000,
                        fee_base_msat: 0,
                        fee_proportional_millionths: 0,
                        excess_data: Vec::new()
                });

                let payment_params = PaymentParameters::from_node_id(nodes[2]).with_features(channelmanager::provided_invoice_features());
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();
                // 100,000 sats is less than the available liquidity on each channel, set above.
                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 100_000_000, 42, Arc::clone(&logger), &scorer, &random_seed_bytes).unwrap();
                assert_eq!(route.paths.len(), 2);
                assert!((route.paths[0][1].short_channel_id == 4 && route.paths[1][1].short_channel_id == 13) ||
                        (route.paths[1][1].short_channel_id == 4 && route.paths[0][1].short_channel_id == 13));
        }

        #[cfg(not(feature = "no-std"))]
        pub(super) fn random_init_seed() -> u64 {
                // Because the default HashMap in std pulls OS randomness, we can use it as a (bad) RNG.
                use core::hash::{BuildHasher, Hasher};
                let seed = std::collections::hash_map::RandomState::new().build_hasher().finish();
                println!("Using seed of {}", seed);
                seed
        }
        #[cfg(not(feature = "no-std"))]
        use crate::util::ser::ReadableArgs;

        #[test]
        #[cfg(not(feature = "no-std"))]
        fn generate_routes() {
                use crate::routing::scoring::{ProbabilisticScorer, ProbabilisticScoringParameters};

                let mut d = match super::bench_utils::get_route_file() {
                        Ok(f) => f,
                        Err(e) => {
                                eprintln!("{}", e);
                                return;
                        },
                };
                let logger = ln_test_utils::TestLogger::new();
                let graph = NetworkGraph::read(&mut d, &logger).unwrap();
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();

                // First, get 100 (source, destination) pairs for which route-getting actually succeeds...
                let mut seed = random_init_seed() as usize;
                let nodes = graph.read_only().nodes().clone();
                'load_endpoints: for _ in 0..10 {
                        loop {
                                seed = seed.overflowing_mul(0xdeadbeef).0;
                                let src = &PublicKey::from_slice(nodes.keys().skip(seed % nodes.len()).next().unwrap().as_slice()).unwrap();
                                seed = seed.overflowing_mul(0xdeadbeef).0;
                                let dst = PublicKey::from_slice(nodes.keys().skip(seed % nodes.len()).next().unwrap().as_slice()).unwrap();
                                let payment_params = PaymentParameters::from_node_id(dst);
                                let amt = seed as u64 % 200_000_000;
                                let params = ProbabilisticScoringParameters::default();
                                let scorer = ProbabilisticScorer::new(params, &graph, &logger);
                                if get_route(src, &payment_params, &graph.read_only(), None, amt, 42, &logger, &scorer, &random_seed_bytes).is_ok() {
                                        continue 'load_endpoints;
                                }
                        }
                }
        }

        #[test]
        #[cfg(not(feature = "no-std"))]
        fn generate_routes_mpp() {
                use crate::routing::scoring::{ProbabilisticScorer, ProbabilisticScoringParameters};

                let mut d = match super::bench_utils::get_route_file() {
                        Ok(f) => f,
                        Err(e) => {
                                eprintln!("{}", e);
                                return;
                        },
                };
                let logger = ln_test_utils::TestLogger::new();
                let graph = NetworkGraph::read(&mut d, &logger).unwrap();
                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();

                // First, get 100 (source, destination) pairs for which route-getting actually succeeds...
                let mut seed = random_init_seed() as usize;
                let nodes = graph.read_only().nodes().clone();
                'load_endpoints: for _ in 0..10 {
                        loop {
                                seed = seed.overflowing_mul(0xdeadbeef).0;
                                let src = &PublicKey::from_slice(nodes.keys().skip(seed % nodes.len()).next().unwrap().as_slice()).unwrap();
                                seed = seed.overflowing_mul(0xdeadbeef).0;
                                let dst = PublicKey::from_slice(nodes.keys().skip(seed % nodes.len()).next().unwrap().as_slice()).unwrap();
                                let payment_params = PaymentParameters::from_node_id(dst).with_features(channelmanager::provided_invoice_features());
                                let amt = seed as u64 % 200_000_000;
                                let params = ProbabilisticScoringParameters::default();
                                let scorer = ProbabilisticScorer::new(params, &graph, &logger);
                                if get_route(src, &payment_params, &graph.read_only(), None, amt, 42, &logger, &scorer, &random_seed_bytes).is_ok() {
                                        continue 'load_endpoints;
                                }
                        }
                }
        }

        #[test]
        fn honors_manual_penalties() {
                let (secp_ctx, network_graph, _, _, logger) = build_line_graph();
                let (_, our_id, _, nodes) = get_nodes(&secp_ctx);

                let keys_manager = ln_test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();

                let scorer_params = ProbabilisticScoringParameters::default();
                let mut scorer = ProbabilisticScorer::new(scorer_params, Arc::clone(&network_graph), Arc::clone(&logger));

                // First check set manual penalties are returned by the scorer.
                let usage = ChannelUsage {
                        amount_msat: 0,
                        inflight_htlc_msat: 0,
                        effective_capacity: EffectiveCapacity::Total { capacity_msat: 1_024_000, htlc_maximum_msat: 1_000 },
                };
                scorer.set_manual_penalty(&NodeId::from_pubkey(&nodes[3]), 123);
                scorer.set_manual_penalty(&NodeId::from_pubkey(&nodes[4]), 456);
                assert_eq!(scorer.channel_penalty_msat(42, &NodeId::from_pubkey(&nodes[3]), &NodeId::from_pubkey(&nodes[4]), usage), 456);

                // Then check we can get a normal route
                let payment_params = PaymentParameters::from_node_id(nodes[10]);
                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 100, 42, Arc::clone(&logger), &scorer, &random_seed_bytes);
                assert!(route.is_ok());

                // Then check that we can't get a route if we ban an intermediate node.
                scorer.add_banned(&NodeId::from_pubkey(&nodes[3]));
                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 100, 42, Arc::clone(&logger), &scorer, &random_seed_bytes);
                assert!(route.is_err());

                // Finally make sure we can route again, when we remove the ban.
                scorer.remove_banned(&NodeId::from_pubkey(&nodes[3]));
                let route = get_route(&our_id, &payment_params, &network_graph.read_only(), None, 100, 42, Arc::clone(&logger), &scorer, &random_seed_bytes);
                assert!(route.is_ok());
        }
}

#[cfg(all(test, not(feature = "no-std")))]
pub(crate) mod bench_utils {
        use std::fs::File;
        /// Tries to open a network graph file, or panics with a URL to fetch it.
        pub(crate) fn get_route_file() -> Result<std::fs::File, &'static str> {
                let res = File::open("net_graph-2021-05-31.bin") // By default we're run in RL/lightning
                        .or_else(|_| File::open("lightning/net_graph-2021-05-31.bin")) // We may be run manually in RL/
                        .or_else(|_| { // Fall back to guessing based on the binary location
                                // path is likely something like .../rust-lightning/target/debug/deps/lightning-...
                                let mut path = std::env::current_exe().unwrap();
                                path.pop(); // lightning-...
                                path.pop(); // deps
                                path.pop(); // debug
                                path.pop(); // target
                                path.push("lightning");
                                path.push("net_graph-2021-05-31.bin");
                                eprintln!("{}", path.to_str().unwrap());
                                File::open(path)
                        })
                .map_err(|_| "Please fetch https://bitcoin.ninja/ldk-net_graph-v0.0.15-2021-05-31.bin and place it at lightning/net_graph-2021-05-31.bin");
                #[cfg(require_route_graph_test)]
                return Ok(res.unwrap());
                #[cfg(not(require_route_graph_test))]
                return res;
        }
}

#[cfg(all(test, feature = "_bench_unstable", not(feature = "no-std")))]
mod benches {
        use super::*;
        use bitcoin::hashes::Hash;
        use bitcoin::secp256k1::{PublicKey, Secp256k1, SecretKey};
        use crate::chain::transaction::OutPoint;
        use crate::chain::keysinterface::{EntropySource, KeysManager,KeysInterface};
        use crate::ln::channelmanager::{self, ChannelCounterparty, ChannelDetails};
        use crate::ln::features::InvoiceFeatures;
        use crate::routing::gossip::NetworkGraph;
        use crate::routing::scoring::{FixedPenaltyScorer, ProbabilisticScorer, ProbabilisticScoringParameters};
        use crate::util::logger::{Logger, Record};
        use crate::util::ser::ReadableArgs;

        use test::Bencher;

        struct DummyLogger {}
        impl Logger for DummyLogger {
                fn log(&self, _record: &Record) {}
        }

        fn read_network_graph(logger: &DummyLogger) -> NetworkGraph<&DummyLogger> {
                let mut d = bench_utils::get_route_file().unwrap();
                NetworkGraph::read(&mut d, logger).unwrap()
        }

        fn payer_pubkey() -> PublicKey {
                let secp_ctx = Secp256k1::new();
                PublicKey::from_secret_key(&secp_ctx, &SecretKey::from_slice(&[42; 32]).unwrap())
        }

        #[inline]
        fn first_hop(node_id: PublicKey) -> ChannelDetails {
                ChannelDetails {
                        channel_id: [0; 32],
                        counterparty: ChannelCounterparty {
                                features: channelmanager::provided_init_features(),
                                node_id,
                                unspendable_punishment_reserve: 0,
                                forwarding_info: None,
                                outbound_htlc_minimum_msat: None,
                                outbound_htlc_maximum_msat: None,
                        },
                        funding_txo: Some(OutPoint {
                                txid: bitcoin::Txid::from_slice(&[0; 32]).unwrap(), index: 0
                        }),
                        channel_type: None,
                        short_channel_id: Some(1),
                        inbound_scid_alias: None,
                        outbound_scid_alias: None,
                        channel_value_satoshis: 10_000_000,
                        user_channel_id: 0,
                        balance_msat: 10_000_000,
                        outbound_capacity_msat: 10_000_000,
                        next_outbound_htlc_limit_msat: 10_000_000,
                        inbound_capacity_msat: 0,
                        unspendable_punishment_reserve: None,
                        confirmations_required: None,
                        confirmations: None,
                        force_close_spend_delay: None,
                        is_outbound: true,
                        is_channel_ready: true,
                        is_usable: true,
                        is_public: true,
                        inbound_htlc_minimum_msat: None,
                        inbound_htlc_maximum_msat: None,
                        config: None,
                }
        }

        #[bench]
        fn generate_routes_with_zero_penalty_scorer(bench: &mut Bencher) {
                let logger = DummyLogger {};
                let network_graph = read_network_graph(&logger);
                let scorer = FixedPenaltyScorer::with_penalty(0);
                generate_routes(bench, &network_graph, scorer, InvoiceFeatures::empty());
        }

        #[bench]
        fn generate_mpp_routes_with_zero_penalty_scorer(bench: &mut Bencher) {
                let logger = DummyLogger {};
                let network_graph = read_network_graph(&logger);
                let scorer = FixedPenaltyScorer::with_penalty(0);
                generate_routes(bench, &network_graph, scorer, channelmanager::provided_invoice_features());
        }

        #[bench]
        fn generate_routes_with_probabilistic_scorer(bench: &mut Bencher) {
                let logger = DummyLogger {};
                let network_graph = read_network_graph(&logger);
                let params = ProbabilisticScoringParameters::default();
                let scorer = ProbabilisticScorer::new(params, &network_graph, &logger);
                generate_routes(bench, &network_graph, scorer, InvoiceFeatures::empty());
        }

        #[bench]
        fn generate_mpp_routes_with_probabilistic_scorer(bench: &mut Bencher) {
                let logger = DummyLogger {};
                let network_graph = read_network_graph(&logger);
                let params = ProbabilisticScoringParameters::default();
                let scorer = ProbabilisticScorer::new(params, &network_graph, &logger);
                generate_routes(bench, &network_graph, scorer, channelmanager::provided_invoice_features());
        }

        fn generate_routes<S: Score>(
                bench: &mut Bencher, graph: &NetworkGraph<&DummyLogger>, mut scorer: S,
                features: InvoiceFeatures
        ) {
                let nodes = graph.read_only().nodes().clone();
                let payer = payer_pubkey();
                let keys_manager = KeysManager::new(&[0u8; 32], 42, 42);
                let random_seed_bytes = keys_manager.get_secure_random_bytes();

                // First, get 100 (source, destination) pairs for which route-getting actually succeeds...
                let mut routes = Vec::new();
                let mut route_endpoints = Vec::new();
                let mut seed: usize = 0xdeadbeef;
                'load_endpoints: for _ in 0..150 {
                        loop {
                                seed *= 0xdeadbeef;
                                let src = PublicKey::from_slice(nodes.keys().skip(seed % nodes.len()).next().unwrap().as_slice()).unwrap();
                                seed *= 0xdeadbeef;
                                let dst = PublicKey::from_slice(nodes.keys().skip(seed % nodes.len()).next().unwrap().as_slice()).unwrap();
                                let params = PaymentParameters::from_node_id(dst).with_features(features.clone());
                                let first_hop = first_hop(src);
                                let amt = seed as u64 % 1_000_000;
                                if let Ok(route) = get_route(&payer, &params, &graph.read_only(), Some(&[&first_hop]), amt, 42, &DummyLogger{}, &scorer, &random_seed_bytes) {
                                        routes.push(route);
                                        route_endpoints.push((first_hop, params, amt));
                                        continue 'load_endpoints;
                                }
                        }
                }

                // ...and seed the scorer with success and failure data...
                for route in routes {
                        let amount = route.get_total_amount();
                        if amount < 250_000 {
                                for path in route.paths {
                                        scorer.payment_path_successful(&path.iter().collect::<Vec<_>>());
                                }
                        } else if amount > 750_000 {
                                for path in route.paths {
                                        let short_channel_id = path[path.len() / 2].short_channel_id;
                                        scorer.payment_path_failed(&path.iter().collect::<Vec<_>>(), short_channel_id);
                                }
                        }
                }

                // Because we've changed channel scores, its possible we'll take different routes to the
                // selected destinations, possibly causing us to fail because, eg, the newly-selected path
                // requires a too-high CLTV delta.
                route_endpoints.retain(|(first_hop, params, amt)| {
                        get_route(&payer, params, &graph.read_only(), Some(&[first_hop]), *amt, 42, &DummyLogger{}, &scorer, &random_seed_bytes).is_ok()
                });
                route_endpoints.truncate(100);
                assert_eq!(route_endpoints.len(), 100);

                // ...then benchmark finding paths between the nodes we learned.
                let mut idx = 0;
                bench.iter(|| {
                        let (first_hop, params, amt) = &route_endpoints[idx % route_endpoints.len()];
                        assert!(get_route(&payer, params, &graph.read_only(), Some(&[first_hop]), *amt, 42, &DummyLogger{}, &scorer, &random_seed_bytes).is_ok());
                        idx += 1;
                });
        }
}