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