U4-2635 Skip past replaced word when checking

lctjohn · lctjohn · commit b4127294e51a · 2014-04-29T14:06:05.000+02:00
Avoids infinite loop if search and replacement are equal
diff --git a/src/Umbraco.Core/StringExtensions.cs b/src/Umbraco.Core/StringExtensions.cs
@@ -1191,11 +1191,13 @@ public static string ToSafeFileName(this string text, CultureInfo culture)
         /// <returns>Updated string</returns>
         public static string Replace(this string source, string oldString, string newString, StringComparison stringComparison)
         {
-            // Initialised to zero so it can be used as startIndex on first iteration.
-            int index = 0;
+            // This initialisation ensures the first check starts at index zero of the source. On successive checks for
+            // a match, the source is skipped to immediately after the last replaced occurrence for efficiency
+            // and to avoid infinite loops when oldString and newString compare equal. 
+            int index = -1 * newString.Length;
 
-            // Determine if there are any matches left in source, starting from last found match.
-            while((index = source.IndexOf(oldString, index, stringComparison)) >= 0)
+            // Determine if there are any matches left in source, starting from just after the result of replacing the last match.
+            while((index = source.IndexOf(oldString, index + newString.Length, stringComparison)) >= 0)
             {
                 // Remove the old text.
                 source = source.Remove(index, oldString.Length);
diff --git a/src/Umbraco.Tests/CoreStrings/StringExtensionsTests.cs b/src/Umbraco.Tests/CoreStrings/StringExtensionsTests.cs
@@ -83,6 +83,9 @@ public void TrimStart(string input, string forTrimming, string shouldBe)
         [TestCase("Hello this is my string", "hello", "replaced", "replaced this is my string", StringComparison.CurrentCultureIgnoreCase)]
         [TestCase("Hello this is hello my string", "hello", "replaced", "replaced this is replaced my string", StringComparison.CurrentCultureIgnoreCase)]
         [TestCase("Hello this is my string", "nonexistent", "replaced", "Hello this is my string", StringComparison.CurrentCultureIgnoreCase)]
+        [TestCase("Hellohello this is my string", "hello", "replaced", "replacedreplaced this is my string", StringComparison.CurrentCultureIgnoreCase)]
+        // Ensure replacing with the same string doesn't cause infinite loop.
+        [TestCase("Hello this is my string", "hello", "hello", "hello this is my string", StringComparison.CurrentCultureIgnoreCase)]
         public void ReplaceWithStringComparison(string input, string oldString, string newString, string shouldBe, StringComparison stringComparison)
         {
             var replaced = input.Replace(oldString, newString, stringComparison);
