[Editing][CodeHealth] Improve result comparing in assert_selection

third_party/WebKit/LayoutTests/editing/assert_selection.js

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 'use strict';
 
 // This file provides |assert_selection(sample, tester, expectedText)| assertion
 // to W3C test harness to write editing test cases easier.
 //
 // |sample| is an HTML fragment text which is inserted as |innerHTML|. It should
@@ skipping 562 matching lines @@
   serialize() {
     /** @type {!SampleSelection} */
     const selection = SampleSelection.fromDOMSelection(this.selection_);
     /** @type {!Serializer} */
     const serializer = new Serializer(selection);
     return serializer.serialize(this.document_);
   }
 }
 
 function assembleDescription() {
-  const RE_TEST_FUNCTION =
-      new RegExp('at Object.test \\(.*?([^/]+?):(\\d+):(\\d+)\\)');
   function getStack() {
     let stack;
     try {
       throw new Error('get line number');
     } catch (error) {
       stack = error.stack.split('\n').slice(1);
     }
     return stack
   }
+
+  const RE_IN_ASSERT_SELECTION = new RegExp('assert_selection\\.js');
   for (const line of getStack()) {
-    const match = RE_TEST_FUNCTION.exec(line);
-    if (!match)
-      continue;
-    return `${match[1]}(${match[2]})`;
+    const match = RE_IN_ASSERT_SELECTION.exec(line);
+    if (!match) {
+      const RE_LAYOUTTESTS = new RegExp('LayoutTests.*');
+      return RE_LAYOUTTESTS.exec(line);
+    }
   }
   return '';
 }
 
 /**
  * @param {string} expectedText
  */
 function checkExpectedText(expectedText) {
   /** @type {number} */
   const anchorOffset = expectedText.indexOf('^');
@@ skipping 12 matching lines @@
         `You should have a focus marker "|" in "${expectedText}".`);
   }
   if (anchorOffset >= 0 && focusOffset >= 0 &&
      (anchorOffset + 1 === focusOffset || anchorOffset - 1 === focusOffset)) {
     throw new Error(
         `You should have focus marker and should not have anchor marker if and only if selection is a caret in "${expectedText}".`);
   }
 }
 
 /**
+ * @param {string} str1
+ * @param {string} str2
+ * @return {string}
+ */
+function commonPrefixOf(str1, str2) {
+  for (let index = 0; index < str1.length; ++index) {
+    if (str1[index] !== str2[index])
+      return str1.substr(0, index);
+  }
+  return str1;
+}
+
+/**
  * @param {string} inputText
  * @param {function(!Selection)|string}
  * @param {string} expectedText
  * @param {string=} opt_description
  */
 function assertSelection(
     inputText, tester, expectedText, opt_description = '') {
   /** @type {string} */
   const description =
       opt_description === '' ? assembleDescription() : opt_description;
   checkExpectedText(expectedText);
   const sample = new Sample(inputText);
   if (typeof(tester) === 'function') {
     tester.call(window, sample.selection);
   } else if (typeof(tester) === 'string') {
     const strings = tester.split(' ');
     sample.document.execCommand(strings[0], false, strings[1]);
   } else {
     throw new Error(`Invalid tester: ${tester}`);
   }
   /** @type {string} */
   const actualText = sample.serialize();
   // We keep sample HTML when assertion is false for ease of debugging test
   // case.
-  if (actualText === expectedText)
+  if (actualText === expectedText) {
     sample.remove();
-  assert_equals(actualText, expectedText, description);
+    return;
+  }
+  throw new Error(`${description}\n` +
+    `\t expected ${expectedText},\n` +
+    `\t but got  ${actualText},\n` +
+    `\t sameupto ${commonPrefixOf(expectedText, actualText)}`);
 }
 
 // Export symbols
 window.Sample = Sample;
 window.assert_selection = assertSelection;
 })();