binary-search.js
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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
*/
if (typeof define !== 'function') {
var define = require('amdefine')(module, require);
}
define(function (require, exports, module) {
/**
* Recursive implementation of binary search.
*
* @param aLow Indices here and lower do not contain the needle.
* @param aHigh Indices here and higher do not contain the needle.
* @param aNeedle The element being searched for.
* @param aHaystack The non-empty array being searched.
* @param aCompare Function which takes two elements and returns -1, 0, or 1.
*/
function recursiveSearch(aLow, aHigh, aNeedle, aHaystack, aCompare) {
// This function terminates when one of the following is true:
//
// 1. We find the exact element we are looking for.
//
// 2. We did not find the exact element, but we can return the index of
// the next closest element that is less than that element.
//
// 3. We did not find the exact element, and there is no next-closest
// element which is less than the one we are searching for, so we
// return -1.
var mid = Math.floor((aHigh - aLow) / 2) + aLow;
var cmp = aCompare(aNeedle, aHaystack[mid], true);
if (cmp === 0) {
// Found the element we are looking for.
return mid;
}
else if (cmp > 0) {
// aHaystack[mid] is greater than our needle.
if (aHigh - mid > 1) {
// The element is in the upper half.
return recursiveSearch(mid, aHigh, aNeedle, aHaystack, aCompare);
}
// We did not find an exact match, return the next closest one
// (termination case 2).
return mid;
}
else {
// aHaystack[mid] is less than our needle.
if (mid - aLow > 1) {
// The element is in the lower half.
return recursiveSearch(aLow, mid, aNeedle, aHaystack, aCompare);
}
// The exact needle element was not found in this haystack. Determine if
// we are in termination case (2) or (3) and return the appropriate thing.
return aLow < 0 ? -1 : aLow;
}
}
/**
* This is an implementation of binary search which will always try and return
* the index of next lowest value checked if there is no exact hit. This is
* because mappings between original and generated line/col pairs are single
* points, and there is an implicit region between each of them, so a miss
* just means that you aren't on the very start of a region.
*
* @param aNeedle The element you are looking for.
* @param aHaystack The array that is being searched.
* @param aCompare A function which takes the needle and an element in the
* array and returns -1, 0, or 1 depending on whether the needle is less
* than, equal to, or greater than the element, respectively.
*/
exports.search = function search(aNeedle, aHaystack, aCompare) {
if (aHaystack.length === 0) {
return -1;
}
return recursiveSearch(-1, aHaystack.length, aNeedle, aHaystack, aCompare)
};
});