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diff --git a/src/edit_distance.cc b/src/edit_distance.cc
new file mode 100644
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+++ b/src/edit_distance.cc
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+// Copyright 2011 Google Inc. All Rights Reserved.
+//
+// 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.
+
+#include "edit_distance.h"
+
+#include <vector>
+
+#include "string_piece.h"
+
+int EditDistance(const StringPiece& s1,
+                 const StringPiece& s2,
+                 bool allow_replacements,
+                 int max_edit_distance) {
+  // The algorithm implemented below is the "classic"
+  // dynamic-programming algorithm for computing the Levenshtein
+  // distance, which is described here:
+  //
+  //   http://en.wikipedia.org/wiki/Levenshtein_distance
+  //
+  // Although the algorithm is typically described using an m x n
+  // array, only two rows are used at a time, so this implemenation
+  // just keeps two separate vectors for those two rows.
+  int m = s1.len_;
+  int n = s2.len_;
+
+  std::vector<int> previous(n + 1);
+  std::vector<int> current(n + 1);
+
+  for (int i = 0; i <= n; ++i)
+    previous[i] = i;
+
+  for (int y = 1; y <= m; ++y) {
+    current[0] = y;
+    int best_this_row = current[0];
+
+    for (int x = 1; x <= n; ++x) {
+      if (allow_replacements) {
+        current[x] = min(previous[x-1] + (s1.str_[y-1] == s2.str_[x-1] ? 0 : 1),
+                         min(current[x-1], previous[x])+1);
+      }
+      else {
+        if (s1.str_[y-1] == s2.str_[x-1])
+          current[x] = previous[x-1];
+        else
+          current[x] = min(current[x-1], previous[x]) + 1;
+      }
+      best_this_row = min(best_this_row, current[x]);
+    }
+
+    if (max_edit_distance && best_this_row > max_edit_distance)
+      return max_edit_distance + 1;
+
+    current.swap(previous);
+  }
+
+  return previous[n];
+}