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112 lines
4.1 KiB
JavaScript
112 lines
4.1 KiB
JavaScript
/* -*- 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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exports.GREATEST_LOWER_BOUND = 1;
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exports.LEAST_UPPER_BOUND = 2;
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/**
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* Recursive implementation of binary search.
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*
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* @param aLow Indices here and lower do not contain the needle.
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* @param aHigh Indices here and higher do not contain the needle.
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* @param aNeedle The element being searched for.
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* @param aHaystack The non-empty array being searched.
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* @param aCompare Function which takes two elements and returns -1, 0, or 1.
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* @param aBias Either 'binarySearch.GREATEST_LOWER_BOUND' or
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* 'binarySearch.LEAST_UPPER_BOUND'. Specifies whether to return the
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* closest element that is smaller than or greater than the one we are
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* searching for, respectively, if the exact element cannot be found.
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*/
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function recursiveSearch(aLow, aHigh, aNeedle, aHaystack, aCompare, aBias) {
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// This function terminates when one of the following is true:
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//
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// 1. We find the exact element we are looking for.
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//
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// 2. We did not find the exact element, but we can return the index of
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// the next-closest element.
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//
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// 3. We did not find the exact element, and there is no next-closest
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// element than the one we are searching for, so we return -1.
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var mid = Math.floor((aHigh - aLow) / 2) + aLow;
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var cmp = aCompare(aNeedle, aHaystack[mid], true);
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if (cmp === 0) {
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// Found the element we are looking for.
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return mid;
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}
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else if (cmp > 0) {
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// Our needle is greater than aHaystack[mid].
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if (aHigh - mid > 1) {
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// The element is in the upper half.
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return recursiveSearch(mid, aHigh, aNeedle, aHaystack, aCompare, aBias);
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}
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// The exact needle element was not found in this haystack. Determine if
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// we are in termination case (3) or (2) and return the appropriate thing.
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if (aBias == exports.LEAST_UPPER_BOUND) {
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return aHigh < aHaystack.length ? aHigh : -1;
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} else {
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return mid;
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}
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}
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else {
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// Our needle is less than aHaystack[mid].
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if (mid - aLow > 1) {
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// The element is in the lower half.
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return recursiveSearch(aLow, mid, aNeedle, aHaystack, aCompare, aBias);
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}
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// we are in termination case (3) or (2) and return the appropriate thing.
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if (aBias == exports.LEAST_UPPER_BOUND) {
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return mid;
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} else {
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return aLow < 0 ? -1 : aLow;
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}
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}
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}
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/**
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* This is an implementation of binary search which will always try and return
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* the index of the closest element if there is no exact hit. This is because
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* mappings between original and generated line/col pairs are single points,
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* and there is an implicit region between each of them, so a miss just means
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* that you aren't on the very start of a region.
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*
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* @param aNeedle The element you are looking for.
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* @param aHaystack The array that is being searched.
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* @param aCompare A function which takes the needle and an element in the
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* array and returns -1, 0, or 1 depending on whether the needle is less
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* than, equal to, or greater than the element, respectively.
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* @param aBias Either 'binarySearch.GREATEST_LOWER_BOUND' or
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* 'binarySearch.LEAST_UPPER_BOUND'. Specifies whether to return the
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* closest element that is smaller than or greater than the one we are
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* searching for, respectively, if the exact element cannot be found.
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* Defaults to 'binarySearch.GREATEST_LOWER_BOUND'.
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*/
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exports.search = function search(aNeedle, aHaystack, aCompare, aBias) {
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if (aHaystack.length === 0) {
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return -1;
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}
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var index = recursiveSearch(-1, aHaystack.length, aNeedle, aHaystack,
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aCompare, aBias || exports.GREATEST_LOWER_BOUND);
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if (index < 0) {
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return -1;
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}
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// We have found either the exact element, or the next-closest element than
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// the one we are searching for. However, there may be more than one such
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// element. Make sure we always return the smallest of these.
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while (index - 1 >= 0) {
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if (aCompare(aHaystack[index], aHaystack[index - 1], true) !== 0) {
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break;
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}
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--index;
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}
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return index;
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};
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