diff -r 000000000000 -r 6474c204b198 toolkit/modules/BinarySearch.jsm
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/modules/BinarySearch.jsm	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,74 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = [
+  "BinarySearch",
+];
+
+this.BinarySearch = Object.freeze({
+
+  /**
+   * Returns the index of the given target in the given array or -1 if the
+   * target is not found.
+   *
+   * See search() for a description of this function's parameters.
+   *
+   * @return The index of `target` in `array` or -1 if `target` is not found.
+   */
+  indexOf: function (array, target, comparator) {
+    let [found, idx] = this.search(array, target, comparator);
+    return found ? idx : -1;
+  },
+
+  /**
+   * Returns the index within the given array where the given target may be
+   * inserted to keep the array ordered.
+   *
+   * See search() for a description of this function's parameters.
+   *
+   * @return The index in `array` where `target` may be inserted to keep `array`
+   *         ordered.
+   */
+  insertionIndexOf: function (array, target, comparator) {
+    return this.search(array, target, comparator)[1];
+  },
+
+  /**
+   * Searches for the given target in the given array.
+   *
+   * @param  array
+   *         An array whose elements are ordered by `comparator`.
+   * @param  target
+   *         The value to search for.
+   * @param  comparator
+   *         A function that takes two arguments and compares them, returning a
+   *         negative number if the first should be ordered before the second,
+   *         zero if the first and second have the same ordering, or a positive
+   *         number if the second should be ordered before the first.  The first
+   *         argument is always `target`, and the second argument is a value
+   *         from the array.
+   * @return An array with two elements.  If `target` is found, the first
+   *         element is true, and the second element is its index in the array.
+   *         If `target` is not found, the first element is false, and the
+   *         second element is the index where it may be inserted to keep the
+   *         array ordered.
+   */
+  search: function (array, target, comparator) {
+    let low = 0;
+    let high = array.length - 1;
+    while (low <= high) {
+      let mid = Math.floor((low + high) / 2);
+      let cmp = comparator(target, array[mid]);
+      if (cmp == 0)
+        return [true, mid];
+      if (cmp < 0)
+        high = mid - 1;
+      else
+        low = mid + 1;
+    }
+    return [false, low];
+  },
+});