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