Add TlvStream::intersection

When we drop non-matching custom TLVs on multi-part receives, since we
can assume the TLV streams are sorted, we can implement faster algorithm
to get the intersection of the two streams.

Author: alecchendev

lightning/src/offers/merkle.rs

@@ -185,6 +185,23 @@ impl<'a> TlvStream<'a> {
 	fn skip_signatures(self) -> core::iter::Filter<TlvStream<'a>, fn(&TlvRecord) -> bool> {
 		self.filter(|record| !SIGNATURE_TYPES.contains(&record.r#type))
 	}
+
+	pub(crate) fn intersection(self, rhs: Self) -> Vec<u8> {
+		let mut rhs_iter = rhs.into_iter();
+		let mut rhs_tlv = rhs_iter.next();
+		let mut intersection = Vec::new();
+		for tlv in self.into_iter() {
+			while let Some(ref rhs_tlv_inner) = rhs_tlv {
+				if rhs_tlv_inner == &tlv {
+					tlv.write(&mut intersection).expect("Should be able to write TLV");
+				} else if rhs_tlv_inner.r#type > tlv.r#type {
+					break;
+				}
+				rhs_tlv = rhs_iter.next();
+			}
+		}
+		intersection
+	}
 }
 
 /// A slice into a [`TlvStream`] for a record.