util: expose diff function used by the assertion errors

benchmark/util/diff.js

@@ -0,0 +1,43 @@
+'use strict';
+
+const util = require('util');
+const common = require('../common');
+
+const bench = common.createBenchmark(main, {
+  n: [1e3],
+  length: [1e3, 2e3],
+  scenario: ['identical', 'small-diff', 'medium-diff', 'large-diff'],
+});
+
+function main({ n, length, scenario }) {
+  const actual = Array.from({ length }, (_, i) => `${i}`);
+  let expected;
+
+  switch (scenario) {
+    case 'identical': // 0% difference
+      expected = Array.from({ length }, (_, i) => `${i}`);
+      break;
+
+    case 'small-diff': // ~5% difference
+      expected = Array.from({ length }, (_, i) => {
+        return Math.random() < 0.05 ? `modified-${i}` : `${i}`;
+      });
+      break;
+
+    case 'medium-diff': // ~25% difference
+      expected = Array.from({ length }, (_, i) => {
+        return Math.random() < 0.25 ? `modified-${i}` : `${i}`;
+      });
+      break;
+
+    case 'large-diff': // ~100% difference
+      expected = Array.from({ length }, (_, i) => `modified-${i}`);
+      break;
+  }
+
+  bench.start();
+  for (let i = 0; i < n; i++) {
+    util.diff(actual, expected);
+  }
+  bench.end(n);
+}

doc/api/util.md

@@ -325,6 +325,70 @@ The `--throw-deprecation` command-line flag and `process.throwDeprecation`
 property take precedence over `--trace-deprecation` and
 `process.traceDeprecation`.
 
+## `util.diff(actual, expected)`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+* `actual` {Array|string} The first value to compare
+
+* `expected` {Array|string} The second value to compare
+
+* Returns: {Array} An array of difference entries. Each entry is an array with two elements:
+  * Index 0: {number} Operation code: `-1` for delete, `0` for no-op/unchanged, `1` for insert
+  * Index 1: {string} The value associated with the operation
+
+* Algorithm complexity: O(N\*D), where:
+
+* N is the total length of the two sequences combined (N = actual.length + expected.length)
+
+* D is the edit distance (the minimum number of operations required to transform one sequence into the other).
+
+[`util.diff()`][] compares two string or array values and returns an array of difference entries.
+It uses the Myers diff algorithm to compute minimal differences, which is the same algorithm
+used internally by assertion error messages.
+
+If the values are equal, an empty array is returned.
+
+```js
+const { diff } = require('node:util');
+
+// Comparing strings
+const actualString = '12345678';
+const expectedString = '12!!5!7!';
+console.log(diff(actualString, expectedString));
+// [
+//   [0, '1'],
+//   [0, '2'],
+//   [1, '3'],
+//   [1, '4'],
+//   [-1, '!'],
+//   [-1, '!'],
+//   [0, '5'],
+//   [1, '6'],
+//   [-1, '!'],
+//   [0, '7'],
+//   [1, '8'],
+//   [-1, '!'],
+// ]
+// Comparing arrays
+const actualArray = ['1', '2', '3'];
+const expectedArray = ['1', '3', '4'];
+console.log(diff(actualArray, expectedArray));
+// [
+//   [0, '1'],
+//   [1, '2'],
+//   [0, '3'],
+//   [-1, '4'],
+// ]
+// Equal values return empty array
+console.log(diff('same', 'same'));
+// []
+```
+
 ## `util.format(format[, ...args])`
 
 <!-- YAML
@@ -3622,6 +3686,7 @@ util.isArray({});
 [`napi_create_external()`]: n-api.md#napi_create_external
 [`target` and `handler`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy#Terminology
 [`tty.hasColors()`]: tty.md#writestreamhascolorscount-env
+[`util.diff()`]: #utildiffactual-expected
 [`util.format()`]: #utilformatformat-args
 [`util.inspect()`]: #utilinspectobject-options
 [`util.promisify()`]: #utilpromisifyoriginal

lib/internal/assert/myers_diff.js

@@ -9,6 +9,11 @@ const {
 const colors = require('internal/util/colors');
 
 const kNopLinesToCollapse = 5;
+const kOperations = {
+  DELETE: -1,
+  NOP: 0,
+  INSERT: 1,
+};
 
 function areLinesEqual(actual, expected, checkCommaDisparity) {
   if (actual === expected) {
@@ -87,16 +92,16 @@ function backtrack(trace, actual, expected, checkCommaDisparity) {
     while (x > prevX && y > prevY) {
       const actualItem = actual[x - 1];
       const value = checkCommaDisparity && !StringPrototypeEndsWith(actualItem, ',') ? expected[y - 1] : actualItem;
-      ArrayPrototypePush(result, { __proto__: null, type: 'nop', value });
+      ArrayPrototypePush(result, [ kOperations.NOP, value ]);
       x--;
       y--;
     }
 
     if (diffLevel > 0) {
       if (x > prevX) {
-        ArrayPrototypePush(result, { __proto__: null, type: 'insert', value: actual[--x] });
+        ArrayPrototypePush(result, [ kOperations.INSERT, actual[--x] ]);
       } else {
-        ArrayPrototypePush(result, { __proto__: null, type: 'delete', value: expected[--y] });
+        ArrayPrototypePush(result, [ kOperations.DELETE, expected[--y] ]);
       }
     }
   }
@@ -108,12 +113,12 @@ function printSimpleMyersDiff(diff) {
   let message = '';
 
   for (let diffIdx = diff.length - 1; diffIdx >= 0; diffIdx--) {
-    const { type, value } = diff[diffIdx];
+    const { 0: operation, 1: value } = diff[diffIdx];
     let color = colors.white;
 
-    if (type === 'insert') {
+    if (operation === kOperations.INSERT) {
       color = colors.green;
-    } else if (type === 'delete') {
+    } else if (operation === kOperations.DELETE) {
       color = colors.red;
     }
 
@@ -129,33 +134,33 @@ function printMyersDiff(diff, operator) {
   let nopCount = 0;
 
   for (let diffIdx = diff.length - 1; diffIdx >= 0; diffIdx--) {
-    const { type, value } = diff[diffIdx];
-    const previousType = diffIdx < diff.length - 1 ? diff[diffIdx + 1].type : null;
+    const { 0: operation, 1: value } = diff[diffIdx];
+    const previousOperation = diffIdx < diff.length - 1 ? diff[diffIdx + 1][0] : null;
 
     // Avoid grouping if only one line would have been grouped otherwise
-    if (previousType === 'nop' && type !== previousType) {
+    if (previousOperation === kOperations.NOP && operation !== previousOperation) {
       if (nopCount === kNopLinesToCollapse + 1) {
-        message += `${colors.white}  ${diff[diffIdx + 1].value}\n`;
+        message += `${colors.white}  ${diff[diffIdx + 1][1]}\n`;
       } else if (nopCount === kNopLinesToCollapse + 2) {
-        message += `${colors.white}  ${diff[diffIdx + 2].value}\n`;
-        message += `${colors.white}  ${diff[diffIdx + 1].value}\n`;
+        message += `${colors.white}  ${diff[diffIdx + 2][1]}\n`;
+        message += `${colors.white}  ${diff[diffIdx + 1][1]}\n`;
       } else if (nopCount >= kNopLinesToCollapse + 3) {
         message += `${colors.blue}...${colors.white}\n`;
-        message += `${colors.white}  ${diff[diffIdx + 1].value}\n`;
+        message += `${colors.white}  ${diff[diffIdx + 1][1]}\n`;
         skipped = true;
       }
       nopCount = 0;
     }
 
-    if (type === 'insert') {
+    if (operation === kOperations.INSERT) {
       if (operator === 'partialDeepStrictEqual') {
         message += `${colors.gray}${colors.hasColors ? ' ' : '+'} ${value}${colors.white}\n`;
       } else {
         message += `${colors.green}+${colors.white} ${value}\n`;
       }
-    } else if (type === 'delete') {
+    } else if (operation === kOperations.DELETE) {
       message += `${colors.red}-${colors.white} ${value}\n`;
-    } else if (type === 'nop') {
+    } else if (operation === kOperations.NOP) {
       if (nopCount < kNopLinesToCollapse) {
         message += `${colors.white}  ${value}\n`;
       }

lib/internal/util/diff.js

@@ -0,0 +1,42 @@
+'use strict';
+
+const {
+  ArrayIsArray,
+  ArrayPrototypeReverse,
+} = primordials;
+
+const { validateStringArray, validateString } = require('internal/validators');
+const { myersDiff } = require('internal/assert/myers_diff');
+
+function validateInput(value, name) {
+  if (!ArrayIsArray(value)) {
+    validateString(value, name);
+    return;
+  }
+
+  validateStringArray(value, name);
+}
+
+/**
+ * Generate a difference report between two values
+ * @param {Array | string} actual - The first value to compare
+ * @param {Array | string} expected - The second value to compare
+ * @returns {Array} - An array of differences between the two values.
+ * The returned data is an array of arrays, where each sub-array has two elements:
+ * 1. The operation to perform: -1 for delete, 0 for no-op, 1 for insert
+ * 2. The value to perform the operation on
+ */
+function diff(actual, expected) {
+  if (actual === expected) {
+    return [];
+  }
+
+  validateInput(actual, 'actual');
+  validateInput(expected, 'expected');
+
+  return ArrayPrototypeReverse(myersDiff(actual, expected));
+}
+
+module.exports = {
+  diff,
+};

lib/util.js

@@ -479,3 +479,9 @@ defineLazyProperties(
   'internal/mime',
   ['MIMEType', 'MIMEParams'],
 );
+
+defineLazyProperties(
+  module.exports,
+  'internal/util/diff',
+  ['diff'],
+);

test/parallel/test-diff.js

@@ -0,0 +1,80 @@
+'use strict';
+require('../common');
+
+const { describe, it } = require('node:test');
+const { deepStrictEqual, throws } = require('node:assert');
+
+const { diff } = require('util');
+
+describe('diff', () => {
+  it('throws because actual is nor an array nor a string', () => {
+    const actual = {};
+    const expected = 'foo';
+
+    throws(() => diff(actual, expected), {
+      message: 'The "actual" argument must be of type string. Received an instance of Object'
+    });
+  });
+
+  it('throws because expected is nor an array nor a string', () => {
+    const actual = 'foo';
+    const expected = {};
+
+    throws(() => diff(actual, expected), {
+      message: 'The "expected" argument must be of type string. Received an instance of Object'
+    });
+  });
+
+
+  it('throws because the actual array does not only contain string', () => {
+    const actual = ['1', { b: 2 }];
+    const expected = ['1', '2'];
+
+    throws(() => diff(actual, expected), {
+      message: 'The "actual[1]" argument must be of type string. Received an instance of Object'
+    });
+  });
+
+  it('returns an empty array because actual and expected are the same', () => {
+    const actual = 'foo';
+    const expected = 'foo';
+
+    const result = diff(actual, expected);
+    deepStrictEqual(result, []);
+  });
+
+  it('returns the diff for strings', () => {
+    const actual = '12345678';
+    const expected = '12!!5!7!';
+    const result = diff(actual, expected);
+
+    deepStrictEqual(result, [
+      [0, '1'],
+      [0, '2'],
+      [1, '3'],
+      [1, '4'],
+      [-1, '!'],
+      [-1, '!'],
+      [0, '5'],
+      [1, '6'],
+      [-1, '!'],
+      [0, '7'],
+      [1, '8'],
+      [-1, '!'],
+    ]);
+  });
+
+  it('returns the diff for arrays', () => {
+    const actual = ['1', '2', '3'];
+    const expected = ['1', '3', '4'];
+    const result = diff(actual, expected);
+
+    deepStrictEqual(result, [
+      [0, '1'],
+      [1, '2'],
+      [0, '3'],
+      [-1, '4'],
+    ]
+    );
+  });
+});