Rework computing simple strings difference.

pkg/analysis_server/lib/src/services/correction/strings.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library services.src.correction.strings;
 
 import 'dart:math';
 
 /**
  * "$"
@@ skipping 23 matching lines @@
   }
   if (a == null) {
     return 1;
   }
   if (b == null) {
     return -1;
   }
   return a.compareTo(b);
 }
 
+/**
+ * Return a simple difference between the given [oldStr] and [newStr].
+ */
+SimpleDiff computeSimpleDiff(String oldStr, String newStr) {
+  int prefixLength = findCommonPrefix(oldStr, newStr);
+  int suffixLength = findCommonSuffix(oldStr, newStr);
+  while (prefixLength >= 0) {
+    int oldReplaceLength = oldStr.length - prefixLength - suffixLength;
+    int newReplaceLength = newStr.length - prefixLength - suffixLength;
+    if (oldReplaceLength >= 0 && newReplaceLength >= 0) {
+      return new SimpleDiff(prefixLength, oldReplaceLength,
+          newStr.substring(prefixLength, newStr.length - suffixLength));
+    }
+    prefixLength--;
+  }
+  return new SimpleDiff(0, oldStr.length, newStr);
+}
+
 int countLeadingWhitespaces(String str) {
   int i = 0;
   for (; i < str.length; i++) {
     int c = str.codeUnitAt(i);
     if (!isWhitespace(c)) {
       break;
     }
   }
   return i;
 }
@@ skipping 218 matching lines @@
   }
   if (isEmpty(separator)) {
     return '';
   }
   int pos = str.lastIndexOf(separator);
   if (pos == -1) {
     return str;
   }
   return str.substring(pos + separator.length);
 }
+
+/**
+ * Information about a single replacement that should be made to convert the
+ * "old" string to the "new" one.
+ */
+class SimpleDiff {
+  final int offset;
+  final int length;
+  final String replacement;
+
+  SimpleDiff(this.offset, this.length, this.replacement);
+}