Index: src/java/org/apache/lucene/util/StringHelper.java
===================================================================
--- src/java/org/apache/lucene/util/StringHelper.java	(revision 996900)
+++ src/java/org/apache/lucene/util/StringHelper.java	(working copy)
@@ -45,7 +45,7 @@
    * @return The number of common elements.
    */
   public static final int bytesDifference(byte[] bytes1, int len1, byte[] bytes2, int len2) {
-    int len = len1 < len2 ? len1 : len2;
+    final int len = len1 < len2 ? len1 : len2;
     for (int i = 0; i < len; i++)
       if (bytes1[i] != bytes2[i])
         return i;
@@ -61,13 +61,15 @@
    * @return The first position where the two strings differ.
    */
   public static final int stringDifference(String s1, String s2) {
-    int len1 = s1.length();
-    int len2 = s2.length();
-    int len = len1 < len2 ? len1 : len2;
-    for (int i = 0; i < len; i++) {
-      if (s1.charAt(i) != s2.charAt(i)) {
+    final char[] left = s1.toCharArray();
+    final char[] right = s2.toCharArray();
+    final int len = left.length < right.length ? left.length : right.length;
+    for (int i = 0; i < len; ) {
+      final int cp = Character.codePointAt(left, i);
+      if(cp != Character.codePointAt(right, i)) {
 	      return i;
       }
+      i += Character.charCount(cp);
     }
     return len;
   }
Index: src/test/org/apache/lucene/util/TestStringHelper.java
===================================================================
--- src/test/org/apache/lucene/util/TestStringHelper.java	(revision 996900)
+++ src/test/org/apache/lucene/util/TestStringHelper.java	(working copy)
@@ -36,5 +36,15 @@
     test2 = "test";
     result = StringHelper.stringDifference(test1, test2);
     assertTrue(result == 4);
+    
+    String surrogateString = "Test\ud801\udc1cing";
+    String twoSurrogateString = "Test\ud801\udc1c\ud801\udc1cing";
+
+    String brokenSurrogateString = "Test\ud8011cing";
+    assertEquals(4, StringHelper.stringDifference(surrogateString, brokenSurrogateString));
+    assertEquals(6, StringHelper.stringDifference(surrogateString, twoSurrogateString));
+
   }
+  
+  
 }