1 /* -*- Mode: js; js-indent-level: 2; -*- */
3 * Copyright 2011 Mozilla Foundation and contributors
4 * Licensed under the New BSD license. See LICENSE or:
5 * http://opensource.org/licenses/BSD-3-Clause
8 // It turns out that some (most?) JavaScript engines don't self-host
9 // `Array.prototype.sort`. This makes sense because C++ will likely remain
10 // faster than JS when doing raw CPU-intensive sorting. However, when using a
11 // custom comparator function, calling back and forth between the VM's C++ and
12 // JIT'd JS is rather slow *and* loses JIT type information, resulting in
13 // worse generated code for the comparator function than would be optimal. In
14 // fact, when sorting with a comparator, these costs outweigh the benefits of
15 // sorting in C++. By using our own JS-implemented Quick Sort (below), we get
16 // a ~3500ms mean speed-up in `bench/bench.html`.
19 * Swap the elements indexed by `x` and `y` in the array `ary`.
24 * The index of the first item.
26 * The index of the second item.
28 function swap(ary, x, y) {
35 * Returns a random integer within the range `low .. high` inclusive.
38 * The lower bound on the range.
39 * @param {Number} high
40 * The upper bound on the range.
42 function randomIntInRange(low, high) {
43 return Math.round(low + (Math.random() * (high - low)));
47 * The Quick Sort algorithm.
51 * @param {function} comparator
52 * Function to use to compare two items.
54 * Start index of the array
56 * End index of the array
58 function doQuickSort(ary, comparator, p, r) {
59 // If our lower bound is less than our upper bound, we (1) partition the
60 // array into two pieces and (2) recurse on each half. If it is not, this is
61 // the empty array and our base case.
66 // The partitioning chooses a pivot between `p` and `r` and moves all
67 // elements that are less than or equal to the pivot to the before it, and
68 // all the elements that are greater than it after it. The effect is that
69 // once partition is done, the pivot is in the exact place it will be when
70 // the array is put in sorted order, and it will not need to be moved
71 // again. This runs in O(n) time.
73 // Always choose a random pivot so that an input array which is reverse
74 // sorted does not cause O(n^2) running time.
75 var pivotIndex = randomIntInRange(p, r);
78 swap(ary, pivotIndex, r);
81 // Immediately after `j` is incremented in this loop, the following hold
84 // * Every element in `ary[p .. i]` is less than or equal to the pivot.
86 // * Every element in `ary[i+1 .. j-1]` is greater than the pivot.
87 for (var j = p; j < r; j++) {
88 if (comparator(ary[j], pivot) <= 0) {
97 // (2) Recurse on each half.
99 doQuickSort(ary, comparator, p, q - 1);
100 doQuickSort(ary, comparator, q + 1, r);
105 * Sort the given array in-place with the given comparator function.
109 * @param {function} comparator
110 * Function to use to compare two items.
112 exports.quickSort = function (ary, comparator) {
113 doQuickSort(ary, comparator, 0, ary.length - 1);