[rust-lightning] / lightning / src / ln / features.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.

//! Feature flag definitions for the Lightning protocol according to [BOLT #9].
//!
//! Lightning nodes advertise a supported set of operation through feature flags. Features are
//! applicable for a specific context as indicated in some [messages]. [`Features`] encapsulates
//! behavior for specifying and checking feature flags for a particular context. Each feature is
//! defined internally by a trait specifying the corresponding flags (i.e., even and odd bits).
//!
//! Whether a feature is considered "known" or "unknown" is relative to the implementation, whereas
//! the term "supports" is used in reference to a particular set of [`Features`]. That is, a node
//! supports a feature if it advertises the feature (as either required or optional) to its peers.
//! And the implementation can interpret a feature if the feature is known to it.
//!
//! The following features are currently required in the LDK:
//! - `VariableLengthOnion` - requires/supports variable-length routing onion payloads
//!     (see [BOLT-4](https://github.com/lightning/bolts/blob/master/04-onion-routing.md) for more information).
//! - `StaticRemoteKey` - requires/supports static key for remote output
//!     (see [BOLT-3](https://github.com/lightning/bolts/blob/master/03-transactions.md) for more information).
//!
//! The following features are currently supported in the LDK:
//! - `DataLossProtect` - requires/supports that a node which has somehow fallen behind, e.g., has been restored from an old backup,
//!     can detect that it has fallen behind
//!     (see [BOLT-2](https://github.com/lightning/bolts/blob/master/02-peer-protocol.md) for more information).
//! - `InitialRoutingSync` - requires/supports that the sending node needs a complete routing information dump
//!     (see [BOLT-7](https://github.com/lightning/bolts/blob/master/07-routing-gossip.md#initial-sync) for more information).
//! - `UpfrontShutdownScript` - commits to a shutdown scriptpubkey when opening a channel
//!     (see [BOLT-2](https://github.com/lightning/bolts/blob/master/02-peer-protocol.md#the-open_channel-message) for more information).
//! - `GossipQueries` - requires/supports more sophisticated gossip control
//!     (see [BOLT-7](https://github.com/lightning/bolts/blob/master/07-routing-gossip.md) for more information).
//! - `PaymentSecret` - requires/supports that a node supports payment_secret field
//!     (see [BOLT-4](https://github.com/lightning/bolts/blob/master/04-onion-routing.md) for more information).
//! - `BasicMPP` - requires/supports that a node can receive basic multi-part payments
//!     (see [BOLT-4](https://github.com/lightning/bolts/blob/master/04-onion-routing.md#basic-multi-part-payments) for more information).
//! - `Wumbo` - requires/supports that a node create large channels. Called `option_support_large_channel` in the spec.
//!     (see [BOLT-2](https://github.com/lightning/bolts/blob/master/02-peer-protocol.md#the-open_channel-message) for more information).
//! - `ShutdownAnySegwit` - requires/supports that future segwit versions are allowed in `shutdown`
//!     (see [BOLT-2](https://github.com/lightning/bolts/blob/master/02-peer-protocol.md) for more information).
//! - `OnionMessages` - requires/supports forwarding onion messages
//!     (see [BOLT-7](https://github.com/lightning/bolts/pull/759/files) for more information).
//!     TODO: update link
//! - `ChannelType` - node supports the channel_type field in open/accept
//!     (see [BOLT-2](https://github.com/lightning/bolts/blob/master/02-peer-protocol.md) for more information).
//! - `SCIDPrivacy` - supply channel aliases for routing
//!     (see [BOLT-2](https://github.com/lightning/bolts/blob/master/02-peer-protocol.md) for more information).
//! - `Keysend` - send funds to a node without an invoice
//!     (see the [`Keysend` feature assignment proposal](https://github.com/lightning/bolts/issues/605#issuecomment-606679798) for more information).
//!
//! [BOLT #9]: https://github.com/lightning/bolts/blob/master/09-features.md
//! [messages]: crate::ln::msgs

use {io, io_extras};
use prelude::*;
use core::{cmp, fmt};
use core::hash::{Hash, Hasher};
use core::marker::PhantomData;

use bitcoin::bech32;
use bitcoin::bech32::{Base32Len, FromBase32, ToBase32, u5, WriteBase32};
use ln::msgs::DecodeError;
use util::ser::{Readable, Writeable, Writer};

mod sealed {
        use prelude::*;
        use ln::features::Features;

        /// The context in which [`Features`] are applicable. Defines which features are required and
        /// which are optional for the context.
        pub trait Context {
                /// Features that are known to the implementation, where a required feature is indicated by
                /// its even bit and an optional feature is indicated by its odd bit.
                const KNOWN_FEATURE_FLAGS: &'static [u8];

                /// Bitmask for selecting features that are known to the implementation, regardless of
                /// whether each feature is required or optional.
                const KNOWN_FEATURE_MASK: &'static [u8];
        }

        /// Defines a [`Context`] by stating which features it requires and which are optional. Features
        /// are specified as a comma-separated list of bytes where each byte is a pipe-delimited list of
        /// feature identifiers.
        macro_rules! define_context {
                ($context: ident {
                        required_features: [$( $( $required_feature: ident )|*, )*],
                        optional_features: [$( $( $optional_feature: ident )|*, )*],
                }) => {
                        #[derive(Eq, PartialEq)]
                        pub struct $context {}

                        impl Context for $context {
                                const KNOWN_FEATURE_FLAGS: &'static [u8] = &[
                                        // For each byte, use bitwise-OR to compute the applicable flags for known
                                        // required features `r_i` and optional features `o_j` for all `i` and `j` such
                                        // that the following slice is formed:
                                        //
                                        // [
                                        //  `r_0` | `r_1` | ... | `o_0` | `o_1` | ...,
                                        //  ...,
                                        // ]
                                        $(
                                                0b00_00_00_00 $(|
                                                        <Self as $required_feature>::REQUIRED_MASK)*
                                                $(|
                                                        <Self as $optional_feature>::OPTIONAL_MASK)*,
                                        )*
                                ];

                                const KNOWN_FEATURE_MASK: &'static [u8] = &[
                                        // Similar as above, but set both flags for each feature regardless of whether
                                        // the feature is required or optional.
                                        $(
                                                0b00_00_00_00 $(|
                                                        <Self as $required_feature>::REQUIRED_MASK |
                                                        <Self as $required_feature>::OPTIONAL_MASK)*
                                                $(|
                                                        <Self as $optional_feature>::REQUIRED_MASK |
                                                        <Self as $optional_feature>::OPTIONAL_MASK)*,
                                        )*
                                ];
                        }

                        impl alloc::fmt::Display for Features<$context> {
                                fn fmt(&self, fmt: &mut alloc::fmt::Formatter) -> Result<(), alloc::fmt::Error> {
                                        $(
                                                $(
                                                        fmt.write_fmt(format_args!("{}: {}, ", stringify!($required_feature),
                                                                if <$context as $required_feature>::requires_feature(&self.flags) { "required" }
                                                                else if <$context as $required_feature>::supports_feature(&self.flags) { "supported" }
                                                                else { "not supported" }))?;
                                                )*
                                                $(
                                                        fmt.write_fmt(format_args!("{}: {}, ", stringify!($optional_feature),
                                                                if <$context as $optional_feature>::requires_feature(&self.flags) { "required" }
                                                                else if <$context as $optional_feature>::supports_feature(&self.flags) { "supported" }
                                                                else { "not supported" }))?;
                                                )*
                                        )*
                                        fmt.write_fmt(format_args!("unknown flags: {}",
                                                if self.requires_unknown_bits() { "required" }
                                                else if self.supports_unknown_bits() { "supported" } else { "none" }))
                                }
                        }
                };
        }

        define_context!(InitContext {
                required_features: [
                        // Byte 0
                        ,
                        // Byte 1
                        VariableLengthOnion | StaticRemoteKey | PaymentSecret,
                        // Byte 2
                        ,
                        // Byte 3
                        ,
                        // Byte 4
                        ,
                        // Byte 5
                        ,
                        // Byte 6
                        ,
                ],
                optional_features: [
                        // Byte 0
                        DataLossProtect | InitialRoutingSync | UpfrontShutdownScript | GossipQueries,
                        // Byte 1
                        ,
                        // Byte 2
                        BasicMPP | Wumbo,
                        // Byte 3
                        ShutdownAnySegwit,
                        // Byte 4
                        OnionMessages,
                        // Byte 5
                        ChannelType | SCIDPrivacy,
                        // Byte 6
                        ZeroConf,
                ],
        });
        define_context!(NodeContext {
                required_features: [
                        // Byte 0
                        ,
                        // Byte 1
                        VariableLengthOnion | StaticRemoteKey | PaymentSecret,
                        // Byte 2
                        ,
                        // Byte 3
                        ,
                        // Byte 4
                        ,
                        // Byte 5
                        ,
                        // Byte 6
                        ,
                ],
                optional_features: [
                        // Byte 0
                        DataLossProtect | UpfrontShutdownScript | GossipQueries,
                        // Byte 1
                        ,
                        // Byte 2
                        BasicMPP | Wumbo,
                        // Byte 3
                        ShutdownAnySegwit,
                        // Byte 4
                        OnionMessages,
                        // Byte 5
                        ChannelType | SCIDPrivacy,
                        // Byte 6
                        ZeroConf | Keysend,
                ],
        });
        define_context!(ChannelContext {
                required_features: [],
                optional_features: [],
        });
        define_context!(InvoiceContext {
                required_features: [
                        // Byte 0
                        ,
                        // Byte 1
                        VariableLengthOnion | PaymentSecret,
                        // Byte 2
                        ,
                ],
                optional_features: [
                        // Byte 0
                        ,
                        // Byte 1
                        ,
                        // Byte 2
                        BasicMPP,
                ],
        });
        // This isn't a "real" feature context, and is only used in the channel_type field in an
        // `OpenChannel` message.
        define_context!(ChannelTypeContext {
                required_features: [
                        // Byte 0
                        ,
                        // Byte 1
                        StaticRemoteKey,
                        // Byte 2
                        ,
                        // Byte 3
                        ,
                        // Byte 4
                        ,
                        // Byte 5
                        SCIDPrivacy,
                        // Byte 6
                        ZeroConf,
                ],
                optional_features: [
                        // Byte 0
                        ,
                        // Byte 1
                        ,
                        // Byte 2
                        ,
                        // Byte 3
                        ,
                        // Byte 4
                        ,
                        // Byte 5
                        ,
                        // Byte 6
                        ,
                ],
        });

        /// Defines a feature with the given bits for the specified [`Context`]s. The generated trait is
        /// useful for manipulating feature flags.
        macro_rules! define_feature {
                ($odd_bit: expr, $feature: ident, [$($context: ty),+], $doc: expr, $optional_setter: ident,
                 $required_setter: ident, $supported_getter: ident) => {
                        #[doc = $doc]
                        ///
                        /// See [BOLT #9] for details.
                        ///
                        /// [BOLT #9]: https://github.com/lightning/bolts/blob/master/09-features.md
                        pub trait $feature: Context {
                                /// The bit used to signify that the feature is required.
                                const EVEN_BIT: usize = $odd_bit - 1;

                                /// The bit used to signify that the feature is optional.
                                const ODD_BIT: usize = $odd_bit;

                                /// Assertion that [`EVEN_BIT`] is actually even.
                                ///
                                /// [`EVEN_BIT`]: #associatedconstant.EVEN_BIT
                                const ASSERT_EVEN_BIT_PARITY: usize;

                                /// Assertion that [`ODD_BIT`] is actually odd.
                                ///
                                /// [`ODD_BIT`]: #associatedconstant.ODD_BIT
                                const ASSERT_ODD_BIT_PARITY: usize;

                                /// The byte where the feature is set.
                                const BYTE_OFFSET: usize = Self::EVEN_BIT / 8;

                                /// The bitmask for the feature's required flag relative to the [`BYTE_OFFSET`].
                                ///
                                /// [`BYTE_OFFSET`]: #associatedconstant.BYTE_OFFSET
                                const REQUIRED_MASK: u8 = 1 << (Self::EVEN_BIT - 8 * Self::BYTE_OFFSET);

                                /// The bitmask for the feature's optional flag relative to the [`BYTE_OFFSET`].
                                ///
                                /// [`BYTE_OFFSET`]: #associatedconstant.BYTE_OFFSET
                                const OPTIONAL_MASK: u8 = 1 << (Self::ODD_BIT - 8 * Self::BYTE_OFFSET);

                                /// Returns whether the feature is required by the given flags.
                                #[inline]
                                fn requires_feature(flags: &Vec<u8>) -> bool {
                                        flags.len() > Self::BYTE_OFFSET &&
                                                (flags[Self::BYTE_OFFSET] & Self::REQUIRED_MASK) != 0
                                }

                                /// Returns whether the feature is supported by the given flags.
                                #[inline]
                                fn supports_feature(flags: &Vec<u8>) -> bool {
                                        flags.len() > Self::BYTE_OFFSET &&
                                                (flags[Self::BYTE_OFFSET] & (Self::REQUIRED_MASK | Self::OPTIONAL_MASK)) != 0
                                }

                                /// Sets the feature's required (even) bit in the given flags.
                                #[inline]
                                fn set_required_bit(flags: &mut Vec<u8>) {
                                        if flags.len() <= Self::BYTE_OFFSET {
                                                flags.resize(Self::BYTE_OFFSET + 1, 0u8);
                                        }

                                        flags[Self::BYTE_OFFSET] |= Self::REQUIRED_MASK;
                                }

                                /// Sets the feature's optional (odd) bit in the given flags.
                                #[inline]
                                fn set_optional_bit(flags: &mut Vec<u8>) {
                                        if flags.len() <= Self::BYTE_OFFSET {
                                                flags.resize(Self::BYTE_OFFSET + 1, 0u8);
                                        }

                                        flags[Self::BYTE_OFFSET] |= Self::OPTIONAL_MASK;
                                }

                                /// Clears the feature's required (even) and optional (odd) bits from the given
                                /// flags.
                                #[inline]
                                fn clear_bits(flags: &mut Vec<u8>) {
                                        if flags.len() > Self::BYTE_OFFSET {
                                                flags[Self::BYTE_OFFSET] &= !Self::REQUIRED_MASK;
                                                flags[Self::BYTE_OFFSET] &= !Self::OPTIONAL_MASK;
                                        }

                                        let last_non_zero_byte = flags.iter().rposition(|&byte| byte != 0);
                                        let size = if let Some(offset) = last_non_zero_byte { offset + 1 } else { 0 };
                                        flags.resize(size, 0u8);
                                }
                        }

                        impl <T: $feature> Features<T> {
                                /// Set this feature as optional.
                                pub fn $optional_setter(&mut self) {
                                        <T as $feature>::set_optional_bit(&mut self.flags);
                                }

                                /// Set this feature as required.
                                pub fn $required_setter(&mut self) {
                                        <T as $feature>::set_required_bit(&mut self.flags);
                                }

                                /// Checks if this feature is supported.
                                pub fn $supported_getter(&self) -> bool {
                                        <T as $feature>::supports_feature(&self.flags)
                                }
                        }

                        $(
                                impl $feature for $context {
                                        // EVEN_BIT % 2 == 0
                                        const ASSERT_EVEN_BIT_PARITY: usize = 0 - (<Self as $feature>::EVEN_BIT % 2);

                                        // ODD_BIT % 2 == 1
                                        const ASSERT_ODD_BIT_PARITY: usize = (<Self as $feature>::ODD_BIT % 2) - 1;
                                }
                        )*
                };
                ($odd_bit: expr, $feature: ident, [$($context: ty),+], $doc: expr, $optional_setter: ident,
                 $required_setter: ident, $supported_getter: ident, $required_getter: ident) => {
                        define_feature!($odd_bit, $feature, [$($context),+], $doc, $optional_setter, $required_setter, $supported_getter);
                        impl <T: $feature> Features<T> {
                                /// Checks if this feature is required.
                                pub fn $required_getter(&self) -> bool {
                                        <T as $feature>::requires_feature(&self.flags)
                                }
                        }
                }
        }

        define_feature!(1, DataLossProtect, [InitContext, NodeContext],
                "Feature flags for `option_data_loss_protect`.", set_data_loss_protect_optional,
                set_data_loss_protect_required, supports_data_loss_protect, requires_data_loss_protect);
        // NOTE: Per Bolt #9, initial_routing_sync has no even bit.
        define_feature!(3, InitialRoutingSync, [InitContext], "Feature flags for `initial_routing_sync`.",
                set_initial_routing_sync_optional, set_initial_routing_sync_required,
                initial_routing_sync);
        define_feature!(5, UpfrontShutdownScript, [InitContext, NodeContext],
                "Feature flags for `option_upfront_shutdown_script`.", set_upfront_shutdown_script_optional,
                set_upfront_shutdown_script_required, supports_upfront_shutdown_script,
                requires_upfront_shutdown_script);
        define_feature!(7, GossipQueries, [InitContext, NodeContext],
                "Feature flags for `gossip_queries`.", set_gossip_queries_optional, set_gossip_queries_required,
                supports_gossip_queries, requires_gossip_queries);
        define_feature!(9, VariableLengthOnion, [InitContext, NodeContext, InvoiceContext],
                "Feature flags for `var_onion_optin`.", set_variable_length_onion_optional,
                set_variable_length_onion_required, supports_variable_length_onion,
                requires_variable_length_onion);
        define_feature!(13, StaticRemoteKey, [InitContext, NodeContext, ChannelTypeContext],
                "Feature flags for `option_static_remotekey`.", set_static_remote_key_optional,
                set_static_remote_key_required, supports_static_remote_key, requires_static_remote_key);
        define_feature!(15, PaymentSecret, [InitContext, NodeContext, InvoiceContext],
                "Feature flags for `payment_secret`.", set_payment_secret_optional, set_payment_secret_required,
                supports_payment_secret, requires_payment_secret);
        define_feature!(17, BasicMPP, [InitContext, NodeContext, InvoiceContext],
                "Feature flags for `basic_mpp`.", set_basic_mpp_optional, set_basic_mpp_required,
                supports_basic_mpp, requires_basic_mpp);
        define_feature!(19, Wumbo, [InitContext, NodeContext],
                "Feature flags for `option_support_large_channel` (aka wumbo channels).", set_wumbo_optional, set_wumbo_required,
                supports_wumbo, requires_wumbo);
        define_feature!(27, ShutdownAnySegwit, [InitContext, NodeContext],
                "Feature flags for `opt_shutdown_anysegwit`.", set_shutdown_any_segwit_optional,
                set_shutdown_any_segwit_required, supports_shutdown_anysegwit, requires_shutdown_anysegwit);
        define_feature!(39, OnionMessages, [InitContext, NodeContext],
                "Feature flags for `option_onion_messages`.", set_onion_messages_optional,
                set_onion_messages_required, supports_onion_messages, requires_onion_messages);
        define_feature!(45, ChannelType, [InitContext, NodeContext],
                "Feature flags for `option_channel_type`.", set_channel_type_optional,
                set_channel_type_required, supports_channel_type, requires_channel_type);
        define_feature!(47, SCIDPrivacy, [InitContext, NodeContext, ChannelTypeContext],
                "Feature flags for only forwarding with SCID aliasing. Called `option_scid_alias` in the BOLTs",
                set_scid_privacy_optional, set_scid_privacy_required, supports_scid_privacy, requires_scid_privacy);
        define_feature!(51, ZeroConf, [InitContext, NodeContext, ChannelTypeContext],
                "Feature flags for accepting channels with zero confirmations. Called `option_zeroconf` in the BOLTs",
                set_zero_conf_optional, set_zero_conf_required, supports_zero_conf, requires_zero_conf);
        define_feature!(55, Keysend, [NodeContext],
                "Feature flags for keysend payments.", set_keysend_optional, set_keysend_required,
                supports_keysend, requires_keysend);

        #[cfg(test)]
        define_feature!(123456789, UnknownFeature, [NodeContext, ChannelContext, InvoiceContext],
                "Feature flags for an unknown feature used in testing.", set_unknown_feature_optional,
                set_unknown_feature_required, supports_unknown_test_feature, requires_unknown_test_feature);
}

/// Tracks the set of features which a node implements, templated by the context in which it
/// appears.
///
/// (C-not exported) as we map the concrete feature types below directly instead
#[derive(Eq)]
pub struct Features<T: sealed::Context> {
        /// Note that, for convenience, flags is LITTLE endian (despite being big-endian on the wire)
        flags: Vec<u8>,
        mark: PhantomData<T>,
}

impl <T: sealed::Context> Features<T> {
        pub(crate) fn or(mut self, o: Self) -> Self {
                let total_feature_len = cmp::max(self.flags.len(), o.flags.len());
                self.flags.resize(total_feature_len, 0u8);
                for (byte, o_byte) in self.flags.iter_mut().zip(o.flags.iter()) {
                        *byte |= *o_byte;
                }
                self
        }
}

impl<T: sealed::Context> Clone for Features<T> {
        fn clone(&self) -> Self {
                Self {
                        flags: self.flags.clone(),
                        mark: PhantomData,
                }
        }
}
impl<T: sealed::Context> Hash for Features<T> {
        fn hash<H: Hasher>(&self, hasher: &mut H) {
                self.flags.hash(hasher);
        }
}
impl<T: sealed::Context> PartialEq for Features<T> {
        fn eq(&self, o: &Self) -> bool {
                self.flags.eq(&o.flags)
        }
}
impl<T: sealed::Context> fmt::Debug for Features<T> {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> Result<(), fmt::Error> {
                self.flags.fmt(fmt)
        }
}

/// Features used within an `init` message.
pub type InitFeatures = Features<sealed::InitContext>;
/// Features used within a `node_announcement` message.
pub type NodeFeatures = Features<sealed::NodeContext>;
/// Features used within a `channel_announcement` message.
pub type ChannelFeatures = Features<sealed::ChannelContext>;
/// Features used within an invoice.
pub type InvoiceFeatures = Features<sealed::InvoiceContext>;

/// Features used within the channel_type field in an OpenChannel message.
///
/// A channel is always of some known "type", describing the transaction formats used and the exact
/// semantics of our interaction with our peer.
///
/// Note that because a channel is a specific type which is proposed by the opener and accepted by
/// the counterparty, only required features are allowed here.
///
/// This is serialized differently from other feature types - it is not prefixed by a length, and
/// thus must only appear inside a TLV where its length is known in advance.
pub type ChannelTypeFeatures = Features<sealed::ChannelTypeContext>;

impl InitFeatures {
        /// Writes all features present up to, and including, 13.
        pub(crate) fn write_up_to_13<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
                let len = cmp::min(2, self.flags.len());
                (len as u16).write(w)?;
                for i in (0..len).rev() {
                        if i == 0 {
                                self.flags[i].write(w)?;
                        } else {
                                // On byte 1, we want up-to-and-including-bit-13, 0-indexed, which is
                                // up-to-and-including-bit-5, 0-indexed, on this byte:
                                (self.flags[i] & 0b00_11_11_11).write(w)?;
                        }
                }
                Ok(())
        }

        /// Converts `InitFeatures` to `Features<C>`. Only known `InitFeatures` relevant to context `C`
        /// are included in the result.
        pub(crate) fn to_context<C: sealed::Context>(&self) -> Features<C> {
                self.to_context_internal()
        }
}

impl InvoiceFeatures {
        /// Converts `InvoiceFeatures` to `Features<C>`. Only known `InvoiceFeatures` relevant to
        /// context `C` are included in the result.
        pub(crate) fn to_context<C: sealed::Context>(&self) -> Features<C> {
                self.to_context_internal()
        }

        /// Getting a route for a keysend payment to a private node requires providing the payee's
        /// features (since they were not announced in a node announcement). However, keysend payments
        /// don't have an invoice to pull the payee's features from, so this method is provided for use in
        /// [`PaymentParameters::for_keysend`], thus omitting the need for payers to manually construct an
        /// `InvoiceFeatures` for [`find_route`].
        ///
        /// [`PaymentParameters::for_keysend`]: crate::routing::router::PaymentParameters::for_keysend
        /// [`find_route`]: crate::routing::router::find_route
        pub(crate) fn for_keysend() -> InvoiceFeatures {
                let mut res = InvoiceFeatures::empty();
                res.set_variable_length_onion_optional();
                res
        }
}

impl ChannelTypeFeatures {
        /// Constructs the implicit channel type based on the common supported types between us and our
        /// counterparty
        pub(crate) fn from_counterparty_init(counterparty_init: &InitFeatures) -> Self {
                let mut ret = counterparty_init.to_context_internal();
                // ChannelTypeFeatures must only contain required bits, so we OR the required forms of all
                // optional bits and then AND out the optional ones.
                for byte in ret.flags.iter_mut() {
                        *byte |= (*byte & 0b10_10_10_10) >> 1;
                        *byte &= 0b01_01_01_01;
                }
                ret
        }

        /// Constructs a ChannelTypeFeatures with only static_remotekey set
        pub(crate) fn only_static_remote_key() -> Self {
                let mut ret = Self::empty();
                <sealed::ChannelTypeContext as sealed::StaticRemoteKey>::set_required_bit(&mut ret.flags);
                ret
        }
}

impl ToBase32 for InvoiceFeatures {
        fn write_base32<W: WriteBase32>(&self, writer: &mut W) -> Result<(), <W as WriteBase32>::Err> {
                // Explanation for the "4": the normal way to round up when dividing is to add the divisor
                // minus one before dividing
                let length_u5s = (self.flags.len() * 8 + 4) / 5 as usize;
                let mut res_u5s: Vec<u5> = vec![u5::try_from_u8(0).unwrap(); length_u5s];
                for (byte_idx, byte) in self.flags.iter().enumerate() {
                        let bit_pos_from_left_0_indexed = byte_idx * 8;
                        let new_u5_idx = length_u5s - (bit_pos_from_left_0_indexed / 5) as usize - 1;
                        let new_bit_pos = bit_pos_from_left_0_indexed % 5;
                        let shifted_chunk_u16 = (*byte as u16) << new_bit_pos;
                        let curr_u5_as_u8 = res_u5s[new_u5_idx].to_u8();
                        res_u5s[new_u5_idx] = u5::try_from_u8(curr_u5_as_u8 | ((shifted_chunk_u16 & 0x001f) as u8)).unwrap();
                        if new_u5_idx > 0 {
                                let curr_u5_as_u8 = res_u5s[new_u5_idx - 1].to_u8();
                                res_u5s[new_u5_idx - 1] = u5::try_from_u8(curr_u5_as_u8 | (((shifted_chunk_u16 >> 5) & 0x001f) as u8)).unwrap();
                        }
                        if new_u5_idx > 1 {
                                let curr_u5_as_u8 = res_u5s[new_u5_idx - 2].to_u8();
                                res_u5s[new_u5_idx - 2] = u5::try_from_u8(curr_u5_as_u8 | (((shifted_chunk_u16 >> 10) & 0x001f) as u8)).unwrap();
                        }
                }
                // Trim the highest feature bits.
                while !res_u5s.is_empty() && res_u5s[0] == u5::try_from_u8(0).unwrap() {
                        res_u5s.remove(0);
                }
                writer.write(&res_u5s)
        }
}

impl Base32Len for InvoiceFeatures {
        fn base32_len(&self) -> usize {
                self.to_base32().len()
        }
}

impl FromBase32 for InvoiceFeatures {
        type Err = bech32::Error;

        fn from_base32(field_data: &[u5]) -> Result<InvoiceFeatures, bech32::Error> {
                // Explanation for the "7": the normal way to round up when dividing is to add the divisor
                // minus one before dividing
                let length_bytes = (field_data.len() * 5 + 7) / 8 as usize;
                let mut res_bytes: Vec<u8> = vec![0; length_bytes];
                for (u5_idx, chunk) in field_data.iter().enumerate() {
                        let bit_pos_from_right_0_indexed = (field_data.len() - u5_idx - 1) * 5;
                        let new_byte_idx = (bit_pos_from_right_0_indexed / 8) as usize;
                        let new_bit_pos = bit_pos_from_right_0_indexed % 8;
                        let chunk_u16 = chunk.to_u8() as u16;
                        res_bytes[new_byte_idx] |= ((chunk_u16 << new_bit_pos) & 0xff) as u8;
                        if new_byte_idx != length_bytes - 1 {
                                res_bytes[new_byte_idx + 1] |= ((chunk_u16 >> (8-new_bit_pos)) & 0xff) as u8;
                        }
                }
                // Trim the highest feature bits.
                while !res_bytes.is_empty() && res_bytes[res_bytes.len() - 1] == 0 {
                        res_bytes.pop();
                }
                Ok(InvoiceFeatures::from_le_bytes(res_bytes))
        }
}

impl<T: sealed::Context> Features<T> {
        /// Create a blank Features with no features set
        pub fn empty() -> Self {
                Features {
                        flags: Vec::new(),
                        mark: PhantomData,
                }
        }

        /// Converts `Features<T>` to `Features<C>`. Only known `T` features relevant to context `C` are
        /// included in the result.
        fn to_context_internal<C: sealed::Context>(&self) -> Features<C> {
                let from_byte_count = T::KNOWN_FEATURE_MASK.len();
                let to_byte_count = C::KNOWN_FEATURE_MASK.len();
                let mut flags = Vec::new();
                for (i, byte) in self.flags.iter().enumerate() {
                        if i < from_byte_count && i < to_byte_count {
                                let from_known_features = T::KNOWN_FEATURE_MASK[i];
                                let to_known_features = C::KNOWN_FEATURE_MASK[i];
                                flags.push(byte & from_known_features & to_known_features);
                        }
                }
                Features::<C> { flags, mark: PhantomData, }
        }

        /// Create a Features given a set of flags, in little-endian. This is in reverse byte order from
        /// most on-the-wire encodings.
        /// (C-not exported) as we don't support export across multiple T
        pub fn from_le_bytes(flags: Vec<u8>) -> Features<T> {
                Features {
                        flags,
                        mark: PhantomData,
                }
        }

        #[cfg(test)]
        /// Gets the underlying flags set, in LE.
        pub fn le_flags(&self) -> &Vec<u8> {
                &self.flags
        }

        fn write_be<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
                for f in self.flags.iter().rev() { // Swap back to big-endian
                        f.write(w)?;
                }
                Ok(())
        }

        fn from_be_bytes(mut flags: Vec<u8>) -> Features<T> {
                flags.reverse(); // Swap to little-endian
                Self {
                        flags,
                        mark: PhantomData,
                }
        }

        pub(crate) fn supports_any_optional_bits(&self) -> bool {
                self.flags.iter().any(|&byte| (byte & 0b10_10_10_10) != 0)
        }

        /// Returns true if this `Features` object contains unknown feature flags which are set as
        /// "required".
        pub fn requires_unknown_bits(&self) -> bool {
                // Bitwise AND-ing with all even bits set except for known features will select required
                // unknown features.
                let byte_count = T::KNOWN_FEATURE_MASK.len();
                self.flags.iter().enumerate().any(|(i, &byte)| {
                        let required_features = 0b01_01_01_01;
                        let unknown_features = if i < byte_count {
                                !T::KNOWN_FEATURE_MASK[i]
                        } else {
                                0b11_11_11_11
                        };
                        (byte & (required_features & unknown_features)) != 0
                })
        }

        pub(crate) fn supports_unknown_bits(&self) -> bool {
                // Bitwise AND-ing with all even and odd bits set except for known features will select
                // both required and optional unknown features.
                let byte_count = T::KNOWN_FEATURE_MASK.len();
                self.flags.iter().enumerate().any(|(i, &byte)| {
                        let unknown_features = if i < byte_count {
                                !T::KNOWN_FEATURE_MASK[i]
                        } else {
                                0b11_11_11_11
                        };
                        (byte & unknown_features) != 0
                })
        }
}

impl<T: sealed::UpfrontShutdownScript> Features<T> {
        #[cfg(test)]
        pub(crate) fn clear_upfront_shutdown_script(mut self) -> Self {
                <T as sealed::UpfrontShutdownScript>::clear_bits(&mut self.flags);
                self
        }
}

impl<T: sealed::ShutdownAnySegwit> Features<T> {
        #[cfg(test)]
        pub(crate) fn clear_shutdown_anysegwit(mut self) -> Self {
                <T as sealed::ShutdownAnySegwit>::clear_bits(&mut self.flags);
                self
        }
}

impl<T: sealed::Wumbo> Features<T> {
        #[cfg(test)]
        pub(crate) fn clear_wumbo(mut self) -> Self {
                <T as sealed::Wumbo>::clear_bits(&mut self.flags);
                self
        }
}

macro_rules! impl_feature_len_prefixed_write {
        ($features: ident) => {
                impl Writeable for $features {
                        fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
                                (self.flags.len() as u16).write(w)?;
                                self.write_be(w)
                        }
                }
                impl Readable for $features {
                        fn read<R: io::Read>(r: &mut R) -> Result<Self, DecodeError> {
                                Ok(Self::from_be_bytes(Vec::<u8>::read(r)?))
                        }
                }
        }
}
impl_feature_len_prefixed_write!(InitFeatures);
impl_feature_len_prefixed_write!(ChannelFeatures);
impl_feature_len_prefixed_write!(NodeFeatures);
impl_feature_len_prefixed_write!(InvoiceFeatures);

// Because ChannelTypeFeatures only appears inside of TLVs, it doesn't have a length prefix when
// serialized. Thus, we can't use `impl_feature_len_prefixed_write`, above, and have to write our
// own serialization.
impl Writeable for ChannelTypeFeatures {
        fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
                self.write_be(w)
        }
}
impl Readable for ChannelTypeFeatures {
        fn read<R: io::Read>(r: &mut R) -> Result<Self, DecodeError> {
                let v = io_extras::read_to_end(r)?;
                Ok(Self::from_be_bytes(v))
        }
}

#[cfg(test)]
mod tests {
        use super::{ChannelFeatures, ChannelTypeFeatures, InitFeatures, InvoiceFeatures, NodeFeatures, sealed};
        use bitcoin::bech32::{Base32Len, FromBase32, ToBase32, u5};

        #[test]
        fn sanity_test_unknown_bits() {
                let features = ChannelFeatures::empty();
                assert!(!features.requires_unknown_bits());
                assert!(!features.supports_unknown_bits());

                let mut features = ChannelFeatures::empty();
                features.set_unknown_feature_required();
                assert!(features.requires_unknown_bits());
                assert!(features.supports_unknown_bits());

                let mut features = ChannelFeatures::empty();
                features.set_unknown_feature_optional();
                assert!(!features.requires_unknown_bits());
                assert!(features.supports_unknown_bits());
        }

        #[test]
        fn convert_to_context_with_relevant_flags() {
                let mut init_features = InitFeatures::empty();
                // Set a bunch of features we use, plus initial_routing_sync_required (which shouldn't get
                // converted as it's only relevant in an init context).
                init_features.set_initial_routing_sync_required();
                init_features.set_data_loss_protect_optional();
                init_features.set_variable_length_onion_required();
                init_features.set_static_remote_key_required();
                init_features.set_payment_secret_required();
                init_features.set_basic_mpp_optional();
                init_features.set_wumbo_optional();
                init_features.set_shutdown_any_segwit_optional();
                init_features.set_onion_messages_optional();
                init_features.set_channel_type_optional();
                init_features.set_scid_privacy_optional();
                init_features.set_zero_conf_optional();

                assert!(init_features.initial_routing_sync());
                assert!(!init_features.supports_upfront_shutdown_script());
                assert!(!init_features.supports_gossip_queries());

                let node_features: NodeFeatures = init_features.to_context();
                {
                        // Check that the flags are as expected:
                        // - option_data_loss_protect
                        // - var_onion_optin (req) | static_remote_key (req) | payment_secret(req)
                        // - basic_mpp | wumbo
                        // - opt_shutdown_anysegwit
                        // - onion_messages
                        // - option_channel_type | option_scid_alias
                        // - option_zeroconf
                        assert_eq!(node_features.flags.len(), 7);
                        assert_eq!(node_features.flags[0], 0b00000010);
                        assert_eq!(node_features.flags[1], 0b01010001);
                        assert_eq!(node_features.flags[2], 0b00001010);
                        assert_eq!(node_features.flags[3], 0b00001000);
                        assert_eq!(node_features.flags[4], 0b10000000);
                        assert_eq!(node_features.flags[5], 0b10100000);
                        assert_eq!(node_features.flags[6], 0b00001000);
                }

                // Check that cleared flags are kept blank when converting back:
                // - initial_routing_sync was not applicable to NodeContext
                // - upfront_shutdown_script was cleared before converting
                // - gossip_queries was cleared before converting
                let features: InitFeatures = node_features.to_context_internal();
                assert!(!features.initial_routing_sync());
                assert!(!features.supports_upfront_shutdown_script());
                assert!(!init_features.supports_gossip_queries());
        }

        #[test]
        fn convert_to_context_with_unknown_flags() {
                // Ensure the `from` context has fewer known feature bytes than the `to` context.
                assert!(<sealed::InvoiceContext as sealed::Context>::KNOWN_FEATURE_MASK.len() <
                        <sealed::NodeContext as sealed::Context>::KNOWN_FEATURE_MASK.len());
                let mut invoice_features = InvoiceFeatures::empty();
                invoice_features.set_unknown_feature_optional();
                assert!(invoice_features.supports_unknown_bits());
                let node_features: NodeFeatures = invoice_features.to_context();
                assert!(!node_features.supports_unknown_bits());
        }

        #[test]
        fn set_feature_bits() {
                let mut features = InvoiceFeatures::empty();
                features.set_basic_mpp_optional();
                features.set_payment_secret_required();
                assert!(features.supports_basic_mpp());
                assert!(!features.requires_basic_mpp());
                assert!(features.requires_payment_secret());
                assert!(features.supports_payment_secret());
        }

        #[test]
        fn invoice_features_encoding() {
                let features_as_u5s = vec![
                        u5::try_from_u8(6).unwrap(),
                        u5::try_from_u8(10).unwrap(),
                        u5::try_from_u8(25).unwrap(),
                        u5::try_from_u8(1).unwrap(),
                        u5::try_from_u8(10).unwrap(),
                        u5::try_from_u8(0).unwrap(),
                        u5::try_from_u8(20).unwrap(),
                        u5::try_from_u8(2).unwrap(),
                        u5::try_from_u8(0).unwrap(),
                        u5::try_from_u8(6).unwrap(),
                        u5::try_from_u8(0).unwrap(),
                        u5::try_from_u8(16).unwrap(),
                        u5::try_from_u8(1).unwrap(),
                ];
                let features = InvoiceFeatures::from_le_bytes(vec![1, 2, 3, 4, 5, 42, 100, 101]);

                // Test length calculation.
                assert_eq!(features.base32_len(), 13);

                // Test serialization.
                let features_serialized = features.to_base32();
                assert_eq!(features_as_u5s, features_serialized);

                // Test deserialization.
                let features_deserialized = InvoiceFeatures::from_base32(&features_as_u5s).unwrap();
                assert_eq!(features, features_deserialized);
        }

        #[test]
        fn test_channel_type_mapping() {
                // If we map an InvoiceFeatures with StaticRemoteKey optional, it should map into a
                // required-StaticRemoteKey ChannelTypeFeatures.
                let mut init_features = InitFeatures::empty();
                init_features.set_static_remote_key_optional();
                let converted_features = ChannelTypeFeatures::from_counterparty_init(&init_features);
                assert_eq!(converted_features, ChannelTypeFeatures::only_static_remote_key());
                assert!(!converted_features.supports_any_optional_bits());
                assert!(converted_features.requires_static_remote_key());
        }
}