Add Features::requires_unknown_bits_from

When checking features, rather than checking against which features LDK
knows about, it is more useful to check against a peer's features. Add
Features::requires_unknown_bits_from such that the given features are
used instead.

diff --git a/lightning/src/ln/features.rs b/lightning/src/ln/features.rs
index f0e2f112af33cb4c363aaf83f84388d23baa07e6..4b49a56eacf73d5f4fe39c033a2206affbd889db 100644 (file)
--- a/lightning/src/ln/features.rs
+++ b/lightning/src/ln/features.rs
@@ -679,6 +679,25 @@ impl<T: sealed::Context> Features<T> {
                self.flags.iter().any(|&byte| (byte & 0b10_10_10_10) != 0)
        }
 
+       /// Returns true if this `Features` object contains required features unknown by `other`.
+       pub fn requires_unknown_bits_from(&self, other: &Features<T>) -> bool {
+               // Bitwise AND-ing with all even bits set except for known features will select required
+               // unknown features.
+               self.flags.iter().enumerate().any(|(i, &byte)| {
+                       const REQUIRED_FEATURES: u8 = 0b01_01_01_01;
+                       const OPTIONAL_FEATURES: u8 = 0b10_10_10_10;
+                       let unknown_features = if i < other.flags.len() {
+                               // Form a mask similar to !T::KNOWN_FEATURE_MASK only for `other`
+                               !(other.flags[i]
+                                       | ((other.flags[i] >> 1) & REQUIRED_FEATURES)
+                                       | ((other.flags[i] << 1) & OPTIONAL_FEATURES))
+                       } else {
+                               0b11_11_11_11
+                       };
+                       (byte & (REQUIRED_FEATURES & unknown_features)) != 0
+               })
+       }
+
        /// Returns true if this `Features` object contains unknown feature flags which are set as
        /// "required".
        pub fn requires_unknown_bits(&self) -> bool {
@@ -852,6 +871,43 @@ mod tests {
                assert!(features.supports_unknown_bits());
        }
 
+       #[test]
+       fn requires_unknown_bits_from() {
+               let mut features1 = InitFeatures::empty();
+               let mut features2 = InitFeatures::empty();
+               assert!(!features1.requires_unknown_bits_from(&features2));
+               assert!(!features2.requires_unknown_bits_from(&features1));
+
+               features1.set_data_loss_protect_required();
+               assert!(features1.requires_unknown_bits_from(&features2));
+               assert!(!features2.requires_unknown_bits_from(&features1));
+
+               features2.set_data_loss_protect_optional();
+               assert!(!features1.requires_unknown_bits_from(&features2));
+               assert!(!features2.requires_unknown_bits_from(&features1));
+
+               features2.set_gossip_queries_required();
+               assert!(!features1.requires_unknown_bits_from(&features2));
+               assert!(features2.requires_unknown_bits_from(&features1));
+
+               features1.set_gossip_queries_optional();
+               assert!(!features1.requires_unknown_bits_from(&features2));
+               assert!(!features2.requires_unknown_bits_from(&features1));
+
+               features1.set_variable_length_onion_required();
+               assert!(features1.requires_unknown_bits_from(&features2));
+               assert!(!features2.requires_unknown_bits_from(&features1));
+
+               features2.set_variable_length_onion_optional();
+               assert!(!features1.requires_unknown_bits_from(&features2));
+               assert!(!features2.requires_unknown_bits_from(&features1));
+
+               features1.set_basic_mpp_required();
+               features2.set_wumbo_required();
+               assert!(features1.requires_unknown_bits_from(&features2));
+               assert!(features2.requires_unknown_bits_from(&features1));
+       }
+
        #[test]
        fn convert_to_context_with_relevant_flags() {
                let mut init_features = InitFeatures::empty();