perf(bytes): improve performance of equals()

bytes/equals.ts

@@ -27,18 +27,38 @@ function equalsNaive(a: Uint8Array, b: Uint8Array): boolean {
  */
 function equals32Bit(a: Uint8Array, b: Uint8Array): boolean {
   const len = a.length;
-  const compressible = Math.floor(len / 4);
-  const compressedA = new Uint32Array(a, 0, compressible);
-  const compressedB = new Uint32Array(b, 0, compressible);
-  for (let i = compressible * 4; i < len; i++) {
+  const compactOffset = 3 - ((a.byteOffset + 3) % 4);
+  const compactLen = Math.floor((len - compactOffset) / 4);
+  const compactA = new Uint32Array(
+    a.buffer,
+    a.byteOffset + compactOffset,
+    compactLen,
+  );
+  const compactB = new Uint32Array(
+    b.buffer,
+    b.byteOffset + compactOffset,
+    compactLen,
+  );
+  for (let i = 0; i < compactOffset; i++) {
     if (a[i] !== b[i]) return false;
   }
-  for (let i = 0; i < compressedA.length; i++) {
-    if (compressedA[i] !== compressedB[i]) return false;
+  for (let i = 0; i < compactA.length; i++) {
+    if (compactA[i] !== compactB[i]) return false;
+  }
+  for (let i = compactOffset + compactLen * 4; i < len; i++) {
+    if (a[i] !== b[i]) return false;
   }
   return true;
 }
 
+/**
+ * Byte length threshold for when to use 32-bit comparisons, based on
+ * benchmarks.
+ *
+ * @see {@link https://github.com/denoland/deno_std/pull/4635}
+ */
+const THRESHOLD_32_BIT = 160;
+
 /**
  * Check whether byte slices are equal to each other.
  *
@@ -62,5 +82,8 @@ export function equals(a: Uint8Array, b: Uint8Array): boolean {
   if (a.length !== b.length) {
     return false;
   }
-  return a.length < 1000 ? equalsNaive(a, b) : equals32Bit(a, b);
+  return a.length >= THRESHOLD_32_BIT &&
+      (a.byteOffset % 4) === (b.byteOffset % 4)
+    ? equals32Bit(a, b)
+    : equalsNaive(a, b);
 }

bytes/equals_test.ts

@@ -11,9 +11,11 @@ Deno.test("equals()", () => {
   assert(!v3);
 });
 
+const THRESHOLD_32_BIT = 160;
+
 Deno.test("equals() handles randomized testing", () => {
   // run tests before and after cutoff
-  for (let len = 995; len <= 1005; len++) {
+  for (let len = THRESHOLD_32_BIT - 10; len <= THRESHOLD_32_BIT + 10; len++) {
     const arr1 = crypto.getRandomValues(new Uint8Array(len));
     const arr2 = crypto.getRandomValues(new Uint8Array(len));
     const arr3 = arr1.slice(0);
@@ -47,4 +49,20 @@ Deno.test("equals() works with .subarray()", () => {
   c[999] = 123;
   assertNotEquals(c, d); // ok
   assert(!equals(c, d));
+
+  // Test every length/offset combination (modulo 4) to ensure that every byte is checked.
+  for (let offsetA = 0; offsetA < 4; offsetA++) {
+    for (let offsetB = 0; offsetB < 4; offsetB++) {
+      for (let len = THRESHOLD_32_BIT; len < THRESHOLD_32_BIT + 4; len++) {
+        const x = new Uint8Array(new ArrayBuffer(len + offsetA), offsetA);
+        const y = new Uint8Array(new ArrayBuffer(len + offsetB), offsetB);
+        for (let i = 0; i < len; i++) {
+          assert(equals(x, y));
+          x[i] = 1;
+          assert(!equals(x, y));
+          y[i] = 1;
+        }
+      }
+    }
+  }
 });