Add starts_with function to MultiLocation and Junctions (#4835)

* add matches_prefix function to MultiLocation and Junctions

* rename matches_prefix to starts_with

* remove unnecessary main in doc comment

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Make use of starts_with in match_and_split

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>
Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>

xcm/src/v0/multi_location.rs

@@ -356,17 +356,30 @@ impl MultiLocation {
 	/// # }
 	/// ```
 	pub fn match_and_split(&self, prefix: &MultiLocation) -> Option<&Junction> {
-		if prefix.len() + 1 != self.len() {
+		if prefix.len() + 1 != self.len() || !self.starts_with(prefix) {
 			return None
 		}
-		for i in 0..prefix.len() {
-			if prefix.at(i) != self.at(i) {
-				return None
-			}
-		}
 		return self.at(prefix.len())
 	}
 
+	/// Returns whether `self` begins with or is equal to `prefix`.
+	///
+	/// # Example
+	/// ```rust
+	/// # use xcm::v0::{Junction::*, MultiLocation::*};
+	/// let m = X4(Parent, PalletInstance(3), OnlyChild, OnlyChild);
+	/// assert!(m.starts_with(&X2(Parent, PalletInstance(3))));
+	/// assert!(m.starts_with(&m));
+	/// assert!(!m.starts_with(&X2(Parent, GeneralIndex(99))));
+	/// assert!(!m.starts_with(&X1(PalletInstance(3))));
+	/// ```
+	pub fn starts_with(&self, prefix: &MultiLocation) -> bool {
+		if self.len() < prefix.len() {
+			return false
+		}
+		prefix.iter().zip(self.iter()).all(|(l, r)| l == r)
+	}
+
 	/// Mutates `self`, suffixing it with `new`. Returns `Err` in case of overflow.
 	pub fn push(&mut self, new: Junction) -> result::Result<(), ()> {
 		let mut n = MultiLocation::Null;

xcm/src/v1/multilocation.rs

@@ -253,6 +253,24 @@ impl MultiLocation {
 		self.interior.match_and_split(&prefix.interior)
 	}
 
+	/// Returns whether `self` has the same number of parents as `prefix` and its junctions begins
+	/// with the junctions of `prefix`.
+	///
+	/// # Example
+	/// ```rust
+	/// # use xcm::v1::{Junctions::*, Junction::*, MultiLocation};
+	/// let m = MultiLocation::new(1, X3(PalletInstance(3), OnlyChild, OnlyChild));
+	/// assert!(m.starts_with(&MultiLocation::new(1, X1(PalletInstance(3)))));
+	/// assert!(!m.starts_with(&MultiLocation::new(1, X1(GeneralIndex(99)))));
+	/// assert!(!m.starts_with(&MultiLocation::new(0, X1(PalletInstance(3)))));
+	/// ```
+	pub fn starts_with(&self, prefix: &MultiLocation) -> bool {
+		if self.parents != prefix.parents {
+			return false
+		}
+		self.interior.starts_with(&prefix.interior)
+	}
+
 	/// Mutate `self` so that it is suffixed with `suffix`.
 	///
 	/// Does not modify `self` and returns `Err` with `suffix` in case of overflow.
@@ -816,15 +834,29 @@ impl Junctions {
 	/// # }
 	/// ```
 	pub fn match_and_split(&self, prefix: &Junctions) -> Option<&Junction> {
-		if prefix.len() + 1 != self.len() {
+		if prefix.len() + 1 != self.len() || !self.starts_with(prefix) {
 			return None
 		}
-		for i in 0..prefix.len() {
-			if prefix.at(i) != self.at(i) {
-				return None
-			}
+		self.at(prefix.len())
+	}
+
+	/// Returns whether `self` begins with or is equal to `prefix`.
+	///
+	/// # Example
+	/// ```rust
+	/// # use xcm::v1::{Junctions::*, Junction::*};
+	/// let mut j = X3(Parachain(2), PalletInstance(3), OnlyChild);
+	/// assert!(j.starts_with(&X2(Parachain(2), PalletInstance(3))));
+	/// assert!(j.starts_with(&j));
+	/// assert!(j.starts_with(&X1(Parachain(2))));
+	/// assert!(!j.starts_with(&X1(Parachain(999))));
+	/// assert!(!j.starts_with(&X4(Parachain(2), PalletInstance(3), OnlyChild, OnlyChild)));
+	/// ```
+	pub fn starts_with(&self, prefix: &Junctions) -> bool {
+		if self.len() < prefix.len() {
+			return false
 		}
-		return self.at(prefix.len())
+		prefix.iter().zip(self.iter()).all(|(l, r)| l == r)
 	}
 }