assert: address large diff review feedback

Author: semimikoh

lib/internal/assert/assertion_error.js

@@ -191,6 +191,12 @@ function getTruncatedDiffValue(lines) {
   return ArrayPrototypeJoin(lines, '\n');
 }
 
+function hasLargeRootMismatch(inspectedActual, inspectedExpected, diffType) {
+  return diffType !== 'full' &&
+         inspectedActual.length + inspectedExpected.length > kMaxDiffLineCount &&
+         inspectedActual[0] !== inspectedExpected[0];
+}
+
 function createErrDiff(actual, expected, operator, customMessage, diffType = 'simple') {
   operator = checkOperator(actual, expected, operator);
 
@@ -222,13 +228,16 @@ function createErrDiff(actual, expected, operator, customMessage, diffType = 'si
       message = ArrayPrototypeJoin(inspectedSplitActual, '\n');
     }
     header = '';
-  } else if (
-    diffType !== 'full' &&
-    inspectedSplitActual.length + inspectedSplitExpected.length > kMaxDiffLineCount
-  ) {
-    message = `\n${colors.green}+${colors.white} ${getTruncatedDiffValue(inspectedSplitActual)}\n` +
+  } else if (hasLargeRootMismatch(inspectedSplitActual, inspectedSplitExpected, diffType)) {
+    const actualPrefix = operator === 'partialDeepStrictEqual' ?
+      `${colors.gray}${colors.hasColors ? ' ' : '+'}` :
+      `${colors.green}+${colors.white}`;
+    message = `\n${actualPrefix} ${getTruncatedDiffValue(inspectedSplitActual)}\n` +
               `${colors.red}-${colors.white} ${getTruncatedDiffValue(inspectedSplitExpected)}`;
     skipped = true;
+    if (operator === 'partialDeepStrictEqual') {
+      header = `${colors.gray}${colors.hasColors ? '' : '+ '}actual${colors.white} ${colors.red}- expected${colors.white}`;
+    }
   } else {
     const checkCommaDisparity = actual != null && typeof actual === 'object';
     const diff = myersDiff(inspectedSplitActual, inspectedSplitExpected, checkCommaDisparity);

test/parallel/test-assert-deep.js

@@ -1193,6 +1193,21 @@ test('Strict equal skips line diff for very large objects', () => {
   );
 });
 
+test('Deep strict equal preserves line diff for large values with the same root', () => {
+  const actual = new Array(1_000).fill(0);
+  const expected = new Array(1_000).fill(0);
+  expected[10] = 1;
+
+  assert.throws(
+    () => assert.deepStrictEqual(actual, expected),
+    {
+      code: 'ERR_ASSERTION',
+      name: 'AssertionError',
+      message: /- {3}1/
+    }
+  );
+});
+
 test('Basic valueOf check', () => {
   const a = new String(1);
   a.valueOf = undefined;