diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
index 66f53df0..d4ef91b7 100644
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -29,6 +29,7 @@ phutil_register_library_map(array(
     'ArcanistDiffParser' => 'parser/diff',
     'ArcanistDiffParserTestCase' => 'parser/diff/__tests__',
     'ArcanistDiffUtils' => 'difference',
+    'ArcanistDiffUtilsTestCase' => 'difference/__tests__',
     'ArcanistDiffWorkflow' => 'workflow/diff',
     'ArcanistDifferentialCommitMessage' => 'differential/commitmessage',
     'ArcanistDifferentialCommitMessageParserException' => 'differential/commitmessage',
@@ -110,6 +111,7 @@ phutil_register_library_map(array(
     'ArcanistCommitWorkflow' => 'ArcanistBaseWorkflow',
     'ArcanistCoverWorkflow' => 'ArcanistBaseWorkflow',
     'ArcanistDiffParserTestCase' => 'ArcanistPhutilTestCase',
+    'ArcanistDiffUtilsTestCase' => 'ArcanistPhutilTestCase',
     'ArcanistDiffWorkflow' => 'ArcanistBaseWorkflow',
     'ArcanistDownloadWorkflow' => 'ArcanistBaseWorkflow',
     'ArcanistExportWorkflow' => 'ArcanistBaseWorkflow',
diff --git a/src/difference/ArcanistDiffUtils.php b/src/difference/ArcanistDiffUtils.php
index f41e01dc..dcc7b332 100644
--- a/src/difference/ArcanistDiffUtils.php
+++ b/src/difference/ArcanistDiffUtils.php
@@ -204,7 +204,7 @@ final class ArcanistDiffUtils {
     return array_values($runs);
   }
 
-  private static function buildLevenshteinDifferenceString($o, $n) {
+  public static function buildLevenshteinDifferenceString($o, $n) {
     $olt = strlen($o);
     $nlt = strlen($n);
 
@@ -256,25 +256,27 @@ final class ArcanistDiffUtils {
     $ol = strlen($o);
     $nl = strlen($n);
 
-    $m = array_fill(0, strlen($o) + 1, array_fill(0, strlen($n) + 1, array()));
+    $m = array_fill(0, $ol + 1, array_fill(0, $nl + 1, array()));
 
     $T_D = 'd';
     $T_I = 'i';
     $T_S = 's';
     $T_X = 'x';
 
-    $path = 0;
-    for ($ii = 0; $ii <= $ol; $ii++) {
-      $m[$ii][0][0] = $ii;
-      $m[$ii][0][1] = ($path += $ii);
-      $m[$ii][0][2] = $T_D;
+    $m[0][0] = array(
+      0,
+      null);
+
+    for ($ii = 1; $ii <= $ol; $ii++) {
+      $m[$ii][0] = array(
+        $ii * 1000,
+        $T_D);
     }
 
-    $path = 0;
-    for ($jj = 0; $jj <= $nl; $jj++) {
-      $m[0][$jj][0] = $jj;
-      $m[0][$jj][1] = ($path += $jj);
-      $m[0][$jj][2] = $T_I;
+    for ($jj = 1; $jj <= $nl; $jj++) {
+      $m[0][$jj] = array(
+        $jj * 1000,
+        $T_I);
     }
 
     $ii = 1;
@@ -285,31 +287,35 @@ final class ArcanistDiffUtils {
           $sub_t_cost = $m[$ii - 1][$jj - 1][0] + 0;
           $sub_t      = $T_S;
         } else {
-          $sub_t_cost = $m[$ii - 1][$jj - 1][0] + 2;
+          $sub_t_cost = $m[$ii - 1][$jj - 1][0] + 2000;
           $sub_t      = $T_X;
         }
-        $sub_p_cost = $m[$ii - 1][$jj - 1][1] + $sub_t_cost;
 
-        $del_t_cost = $m[$ii - 1][$jj][0] + 1;
-        $del_p_cost = $m[$ii - 1][$jj][1] + $del_t_cost;
+        if ($m[$ii - 1][$jj - 1][1] != $sub_t) {
+          $sub_t_cost += 1;
+        }
 
-        $ins_t_cost = $m[$ii][$jj - 1][0] + 1;
-        $ins_p_cost = $m[$ii][$jj - 1][1] + $ins_t_cost;
+        $del_t_cost = $m[$ii - 1][$jj][0] + 1000;
+        if ($m[$ii - 1][$jj][1] != $T_D) {
+          $del_t_cost += 1;
+        }
 
-        if ($sub_p_cost <= $del_p_cost && $sub_p_cost <= $ins_p_cost) {
+        $ins_t_cost = $m[$ii][$jj - 1][0] + 1000;
+        if ($m[$ii][$jj - 1][1] != $T_I) {
+          $ins_t_cost += 1;
+        }
+
+        if ($sub_t_cost <= $del_t_cost && $sub_t_cost <= $ins_t_cost) {
           $m[$ii][$jj] = array(
             $sub_t_cost,
-            $sub_p_cost,
             $sub_t);
-        } else if ($ins_p_cost <= $del_p_cost) {
+        } else if ($ins_t_cost <= $del_t_cost) {
           $m[$ii][$jj] = array(
             $ins_t_cost,
-            $ins_p_cost,
             $T_I);
         } else {
           $m[$ii][$jj] = array(
             $del_t_cost,
-            $del_p_cost,
             $T_D);
         }
       } while ($jj++ < $nl);
@@ -319,18 +325,18 @@ final class ArcanistDiffUtils {
     $ii = $ol;
     $jj = $nl;
     do {
-      $r = $m[$ii][$jj][2];
+      $r = $m[$ii][$jj][1];
       $result .= $r;
       switch ($r) {
-        case 's':
-        case 'x':
+        case $T_S:
+        case $T_X:
           $ii--;
           $jj--;
           break;
-        case 'i':
+        case $T_I:
           $jj--;
           break;
-        case 'd':
+        case $T_D:
           $ii--;
           break;
       }
diff --git a/src/difference/__tests__/ArcanistDiffUtilsTestCase.php b/src/difference/__tests__/ArcanistDiffUtilsTestCase.php
new file mode 100644
index 00000000..0f4e3764
--- /dev/null
+++ b/src/difference/__tests__/ArcanistDiffUtilsTestCase.php
@@ -0,0 +1,96 @@
+<?php
+
+/*
+ * Copyright 2011 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * Test cases for @{class:ArcanistDiffUtils}.
+ *
+ * @group testcase
+ */
+class ArcanistDiffUtilsTestCase extends ArcanistPhutilTestCase {
+  public function testLevenshtein() {
+    $tests = array(
+      array(
+        'a',
+        'b',
+        'x'
+      ),
+      array(
+        'kalrmr(array($b))',
+        'array($b)',
+        'dddddddssssssssds'
+      ),
+      array(
+        'array($b)',
+        'kalrmr(array($b))',
+        'iiiiiiissssssssis'
+      ),
+      array(
+        'zkalrmr(array($b))z',
+        'xarray($b)x',
+        'dddddddxsssssssssdx'
+      ),
+      array(
+        'xarray($b)x',
+        'zkalrmr(array($b))z',
+        'iiiiiiixsssssssssix'
+      ),
+      array(
+        'abcdefghi',
+        'abcdefghi',
+        'sssssssss'
+      ),
+      array(
+        'abcdefghi',
+        'abcdefghijkl',
+        'sssssssssiii'
+      ),
+      array(
+        'abcdefghijkl',
+        'abcdefghi',
+        'sssssssssddd'
+      ),
+      array(
+        'xyzabcdefghi',
+        'abcdefghi',
+        'dddsssssssss'
+      ),
+      array(
+        'abcdefghi',
+        'xyzabcdefghi',
+        'iiisssssssss'
+      ),
+      array(
+        'abcdefg',
+        'abxdxfg',
+        'ssxsxss'
+      ),
+      array(
+        'private function a($a, $b) {',
+        'public function and($b, $c) {',
+        'siixsdddxsssssssssssiissxsssxsss'
+      ),
+    );
+
+    foreach ($tests as $test) {
+      $this->assertEqual(
+        $test[2],
+        ArcanistDiffUtils::buildLevenshteinDifferenceString($test[0], $test[1])
+      );
+    }
+  }
+}
diff --git a/src/difference/__tests__/__init__.php b/src/difference/__tests__/__init__.php
new file mode 100644
index 00000000..29d2e917
--- /dev/null
+++ b/src/difference/__tests__/__init__.php
@@ -0,0 +1,13 @@
+<?php
+/**
+ * This file is automatically generated. Lint this module to rebuild it.
+ * @generated
+ */
+
+
+
+phutil_require_module('arcanist', 'difference');
+phutil_require_module('arcanist', 'unit/engine/phutil/testcase');
+
+
+phutil_require_source('ArcanistDiffUtilsTestCase.php');