Add starts_with to v0 and v1 MultiLocation (#6311)

* add `starts_with` to v0 and v1 MultiLocation

* add tests

* fmt

polkadot/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;
@@ -601,6 +614,24 @@ mod tests {
 		assert_eq!(m.match_and_split(&m), None);
 	}
 
+	#[test]
+	fn starts_with_works() {
+		let full = X3(Parent, Parachain(1000), AccountIndex64 { network: Any, index: 23 });
+		let identity = full.clone();
+		let prefix = X2(Parent, Parachain(1000));
+		let wrong_parachain = X2(Parent, Parachain(1001));
+		let wrong_account = X3(Parent, Parachain(1000), AccountIndex64 { network: Any, index: 24 });
+		let no_parents = X1(Parachain(1000));
+		let too_many_parents = X3(Parent, Parent, Parachain(1000));
+
+		assert!(full.starts_with(&identity));
+		assert!(full.starts_with(&prefix));
+		assert!(!full.starts_with(&wrong_parachain));
+		assert!(!full.starts_with(&wrong_account));
+		assert!(!full.starts_with(&no_parents));
+		assert!(!full.starts_with(&too_many_parents));
+	}
+
 	#[test]
 	fn append_with_works() {
 		let acc = AccountIndex64 { network: Any, index: 23 };

polkadot/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.
@@ -801,15 +819,29 @@ 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)
 	}
 }
 
@@ -929,6 +961,26 @@ mod tests {
 		assert_eq!(m.match_and_split(&m), None);
 	}
 
+	#[test]
+	fn starts_with_works() {
+		let full: MultiLocation =
+			(Parent, Parachain(1000), AccountId32 { network: Any, id: [0; 32] }).into();
+		let identity: MultiLocation = full.clone();
+		let prefix: MultiLocation = (Parent, Parachain(1000)).into();
+		let wrong_parachain: MultiLocation = (Parent, Parachain(1001)).into();
+		let wrong_account: MultiLocation =
+			(Parent, Parachain(1000), AccountId32 { network: Any, id: [1; 32] }).into();
+		let no_parents: MultiLocation = (Parachain(1000)).into();
+		let too_many_parents: MultiLocation = (Parent, Parent, Parachain(1000)).into();
+
+		assert!(full.starts_with(&identity));
+		assert!(full.starts_with(&prefix));
+		assert!(!full.starts_with(&wrong_parachain));
+		assert!(!full.starts_with(&wrong_account));
+		assert!(!full.starts_with(&no_parents));
+		assert!(!full.starts_with(&too_many_parents));
+	}
+
 	#[test]
 	fn append_with_works() {
 		let acc = AccountIndex64 { network: Any, index: 23 };