perf: avoid temporary Vec allocations when computing commitments (#939)

* perf: avoid temporary Vec allocations when computing commitments

* review

* Update crates/ibc/src/core/ics04_channel/commitment.rs

Co-authored-by: Rano | Ranadeep <[email protected]>
Signed-off-by: Michal Nazarewicz <[email protected]>

* Update crates/ibc/src/core/ics04_channel/commitment.rs

Co-authored-by: Rano | Ranadeep <[email protected]>
Signed-off-by: Michal Nazarewicz <[email protected]>

* Update crates/ibc/src/core/ics04_channel/commitment.rs

Co-authored-by: Rano | Ranadeep <[email protected]>
Signed-off-by: Michal Nazarewicz <[email protected]>

* review

* Update .changelog/unreleased/improvements/939-less-alloc.md

Co-authored-by: Rano | Ranadeep <[email protected]>
Signed-off-by: Michal Nazarewicz <[email protected]>

---------

Signed-off-by: Michal Nazarewicz <[email protected]>
Co-authored-by: Rano | Ranadeep <[email protected]>

.changelog/unreleased/improvements/939-less-alloc.md

@@ -0,0 +1,2 @@
+- Reduce vector allocations in Commitment computation. 
+  ([\#939](https://github.com/cosmos/ibc-rs/pull/939))

crates/ibc/src/core/ics04_channel/commitment.rs

@@ -86,31 +86,59 @@ pub(crate) fn compute_packet_commitment(
     timeout_height: &TimeoutHeight,
     timeout_timestamp: &Timestamp,
 ) -> PacketCommitment {
-    let mut hash_input = timeout_timestamp.nanoseconds().to_be_bytes().to_vec();
+    let mut hash_input = [0; 8 * 3 + 32];
 
-    let revision_number = timeout_height.commitment_revision_number().to_be_bytes();
-    hash_input.append(&mut revision_number.to_vec());
+    hash_input[..8].copy_from_slice(&timeout_timestamp.nanoseconds().to_be_bytes());
+    hash_input[8..16].copy_from_slice(&timeout_height.commitment_revision_number().to_be_bytes());
+    hash_input[16..24].copy_from_slice(&timeout_height.commitment_revision_height().to_be_bytes());
+    hash_input[24..].copy_from_slice(&hash(packet_data));
 
-    let revision_height = timeout_height.commitment_revision_height().to_be_bytes();
-    hash_input.append(&mut revision_height.to_vec());
-
-    let packet_data_hash = hash(packet_data);
-    hash_input.append(&mut packet_data_hash.to_vec());
-
-    hash(&hash_input).into()
+    hash(&hash_input).to_vec().into()
 }
 
 /// Compute the commitment for an acknowledgement.
 pub(crate) fn compute_ack_commitment(ack: &Acknowledgement) -> AcknowledgementCommitment {
-    hash(ack.as_ref()).into()
+    hash(ack.as_ref()).to_vec().into()
 }
 
 /// Helper function to hash a byte slice using SHA256.
 ///
 /// Note that computing commitments with anything other than SHA256 will
 /// break the Merkle proofs of the IBC provable store.
-fn hash(data: impl AsRef<[u8]>) -> Vec<u8> {
+fn hash(data: &[u8]) -> [u8; 32] {
     use sha2::Digest;
 
-    sha2::Sha256::digest(&data).to_vec()
+    sha2::Sha256::digest(data).into()
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_compute_packet_commitment() {
+        let expected: [u8; 32] = [
+            0xa9, 0x28, 0xb5, 0x1f, 0x62, 0xbd, 0x54, 0x00, 0x91, 0xec, 0x45, 0x1f, 0x4e, 0xf3,
+            0x45, 0x79, 0x4f, 0x05, 0x9e, 0x65, 0x91, 0x08, 0x16, 0x86, 0x61, 0x26, 0xdc, 0x36,
+            0x4f, 0x84, 0xcc, 0x15,
+        ];
+        let actual = compute_packet_commitment(
+            "packet data".as_bytes(),
+            &TimeoutHeight::At(crate::Height::new(42, 24).unwrap()),
+            &Timestamp::from_nanoseconds(0x42).unwrap(),
+        );
+        assert_eq!(&expected[..], actual.as_ref());
+    }
+
+    #[test]
+    fn test_compute_ack_commitment() {
+        let expected: [u8; 32] = [
+            0x05, 0x4e, 0xde, 0xc1, 0xd0, 0x21, 0x1f, 0x62, 0x4f, 0xed, 0x0c, 0xbc, 0xa9, 0xd4,
+            0xf9, 0x40, 0x0b, 0x0e, 0x49, 0x1c, 0x43, 0x74, 0x2a, 0xf2, 0xc5, 0xb0, 0xab, 0xeb,
+            0xf0, 0xc9, 0x90, 0xd8,
+        ];
+        let ack = Acknowledgement::try_from(vec![0, 1, 2, 3]).unwrap();
+        let actual = compute_ack_commitment(&ack);
+        assert_eq!(&expected[..], actual.as_ref())
+    }
 }