assert: improve myers diff performance

Giovanni Bucci · aduh95 · commit 2daee76b2679 · 2025-03-09T11:38:42.000+01:00
fix: #57242 PR-URL: #57279 Fixes: #57242 Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de> Reviewed-By: LiviaMedeiros <livia@cirno.name>
diff --git a/benchmark/assert/assertion-error.js b/benchmark/assert/assertion-error.js
@@ -0,0 +1,38 @@
+'use strict';
+const common = require('../common.js');
+const assert = require('assert');
+
+const bench = common.createBenchmark(main, {
+  n: [10, 50, 200, 500],
+  size: [10, 100],
+  datasetName: ['objects'],
+});
+
+const baseObject = {
+  a: 1,
+  b: {
+    c: 2,
+    d: [3, 4, 5],
+    e: 'fghi',
+    j: {
+      k: 6,
+    },
+  },
+};
+
+function createObjects(size) {
+  return Array.from({ length: size }, () => baseObject);
+}
+
+function main({ n, size }) {
+  bench.start();
+  for (let i = 0; i < n; ++i) {
+    new assert.AssertionError({
+      actual: {},
+      expected: createObjects(size),
+      operator: 'partialDeepStrictEqual',
+      stackStartFunction: () => {},
+    });
+  }
+  bench.end(n);
+}
diff --git a/lib/internal/assert/myers_diff.js b/lib/internal/assert/myers_diff.js
@@ -2,7 +2,6 @@
 
 const {
   ArrayPrototypePush,
-  ArrayPrototypeSlice,
   Int32Array,
   StringPrototypeEndsWith,
 } = primordials;
@@ -16,7 +15,7 @@ function areLinesEqual(actual, expected, checkCommaDisparity) {
     return true;
   }
   if (checkCommaDisparity) {
-    return `${actual},` === expected || actual === `${expected},`;
+    return (actual + ',') === expected || actual === (expected + ',');
   }
   return false;
 }
@@ -26,12 +25,10 @@ function myersDiff(actual, expected, checkCommaDisparity = false) {
   const expectedLength = expected.length;
   const max = actualLength + expectedLength;
   const v = new Int32Array(2 * max + 1);
-
   const trace = [];
 
   for (let diffLevel = 0; diffLevel <= max; diffLevel++) {
-    const newTrace = ArrayPrototypeSlice(v);
-    ArrayPrototypePush(trace, newTrace);
+    ArrayPrototypePush(trace, new Int32Array(v)); // Clone the current state of `v`
 
     for (let diagonalIndex = -diffLevel; diagonalIndex <= diffLevel; diagonalIndex += 2) {
       const offset = diagonalIndex + max;
@@ -89,22 +86,17 @@ function backtrack(trace, actual, expected, checkCommaDisparity) {
 
     while (x > prevX && y > prevY) {
       const actualItem = actual[x - 1];
-      const value =
-        !checkCommaDisparity || StringPrototypeEndsWith(actualItem, ',') ?
-          actualItem :
-          expected[y - 1];
+      const value = checkCommaDisparity && !StringPrototypeEndsWith(actualItem, ',') ? expected[y - 1] : actualItem;
       ArrayPrototypePush(result, { __proto__: null, type: 'nop', value });
       x--;
       y--;
     }
 
     if (diffLevel > 0) {
       if (x > prevX) {
-        ArrayPrototypePush(result, { __proto__: null, type: 'insert', value: actual[x - 1] });
-        x--;
+        ArrayPrototypePush(result, { __proto__: null, type: 'insert', value: actual[--x] });
       } else {
-        ArrayPrototypePush(result, { __proto__: null, type: 'delete', value: expected[y - 1] });
-        y--;
+        ArrayPrototypePush(result, { __proto__: null, type: 'delete', value: expected[--y] });
       }
     }
   }
