[Editing][CodeHealth] Improve result comparing in assert_selection

third_party/WebKit/LayoutTests/editing/assert_selection.js

Index: third_party/WebKit/LayoutTests/editing/assert_selection.js
diff --git a/third_party/WebKit/LayoutTests/editing/assert_selection.js b/third_party/WebKit/LayoutTests/editing/assert_selection.js
index 2ee74145ff63043a2113562c4e58759055572bf4..43a67fa9f3e11f88a7f8e46e749a6ff1f695abc8 100644
--- a/third_party/WebKit/LayoutTests/editing/assert_selection.js
+++ b/third_party/WebKit/LayoutTests/editing/assert_selection.js
@@ -580,8 +580,6 @@ class Sample {
 }
 
 function assembleDescription() {
-  const RE_TEST_FUNCTION =
-      new RegExp('at Object.test \\(.*?([^/]+?):(\\d+):(\\d+)\\)');
   function getStack() {
     let stack;
     try {
@@ -591,11 +589,14 @@ function assembleDescription() {
     }
     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 '';
 }
@@ -652,9 +653,21 @@ function assertSelection(
   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);
+  } else {
+    var comparedLength = Math.min(actualText.length, expectedText.length);

[yosin_UTC9, 2016/07/11 06:48:18]

Could you introduce mismatch(str1, str2) and mismatchLast(str1, str2) to cut
both end?

[yoichio, 2016/07/11 09:04:14]

Done.

+    var i = 0;
+    for (; i < comparedLength; ++i) {
+      if (actualText[i] !== expectedText[i])
+          break;
+    }
+
+    throw new Error(`${description}\n` +
+      `\t expected ${expectedText},\n` +
+      `\t but got  ${actualText},\n` +
+      `\t sameupto ${expectedText.substr(0,i)}`);
+  }
 }
 
 // Export symbols