Implement set difference for Features

Taking the difference of two sets of Features allows to check if one
contains required bits that are unknown to the other.

Author: jkczyz

lightning/src/ln/features.rs

@@ -435,6 +435,22 @@ impl<T: sealed::Context> core::ops::BitOr for Features<T> {
 	}
 }
 
+impl<T: sealed::Context> core::ops::Sub for Features<T> {
+	type Output = Self;
+
+	fn sub(mut self, o: Self) -> Self {
+		for (byte, o_byte) in self.flags.iter_mut().zip(o.flags.iter()) {
+			*byte &= !*o_byte;
+		}
+
+		let last_non_zero_byte = self.flags.iter().rposition(|&byte| byte != 0);
+		let size = if let Some(offset) = last_non_zero_byte { offset + 1 } else { 0 };
+		self.flags.resize(size, 0u8);
+
+		self
+	}
+}
+
 impl<T: sealed::Context> Clone for Features<T> {
 	fn clone(&self) -> Self {
 		Self {
@@ -928,6 +944,35 @@ mod tests {
 		assert!(features.supports_payment_secret());
 	}
 
+	#[test]
+	fn feature_difference() {
+		let mut features1 = InitFeatures::empty();
+		features1.set_data_loss_protect_optional();
+		features1.set_initial_routing_sync_required();
+		features1.set_gossip_queries_optional();
+		features1.set_variable_length_onion_optional();
+		features1.set_payment_secret_optional();
+		assert_eq!(features1.flags[0], 0b10000110);
+		assert_eq!(features1.flags[1], 0b10000010);
+
+		let mut features2 = InitFeatures::empty();
+		features2.set_data_loss_protect_optional();
+		features2.set_upfront_shutdown_script_optional();
+		features2.set_gossip_queries_optional();
+		features2.set_variable_length_onion_optional();
+		features2.set_static_remote_key_optional();
+		assert_eq!(features2.flags[0], 0b10100010);
+		assert_eq!(features2.flags[1], 0b00100010);
+
+		let features1_minus2 = features1.clone() - features2.clone();
+		assert_eq!(features1_minus2.flags[0], 0b00000100);
+		assert_eq!(features1_minus2.flags[1], 0b10000000);
+
+		let features2_minus1 = features2 - features1;
+		assert_eq!(features2_minus1.flags[0], 0b00100000);
+		assert_eq!(features2_minus1.flags[1], 0b00100000);
+	}
+
 	#[test]
 	fn encodes_features_without_length() {
 		let features = OfferFeatures::from_le_bytes(vec![1, 2, 3, 4, 5, 42, 100, 101]);