1/* -*- Mode: js; js-indent-level: 2; -*- */
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
6 */
7
8exports.GREATEST_LOWER_BOUND = 1;
9exports.LEAST_UPPER_BOUND = 2;
10
11/**
12 * Recursive implementation of binary search.
13 *
14 * @param aLow Indices here and lower do not contain the needle.
15 * @param aHigh Indices here and higher do not contain the needle.
16 * @param aNeedle The element being searched for.
17 * @param aHaystack The non-empty array being searched.
18 * @param aCompare Function which takes two elements and returns -1, 0, or 1.
19 * @param aBias Either 'binarySearch.GREATEST_LOWER_BOUND' or
20 *     'binarySearch.LEAST_UPPER_BOUND'. Specifies whether to return the
21 *     closest element that is smaller than or greater than the one we are
22 *     searching for, respectively, if the exact element cannot be found.
23 */
24function recursiveSearch(aLow, aHigh, aNeedle, aHaystack, aCompare, aBias) {
25  // This function terminates when one of the following is true:
26  //
27  //   1. We find the exact element we are looking for.
28  //
29  //   2. We did not find the exact element, but we can return the index of
30  //      the next-closest element.
31  //
32  //   3. We did not find the exact element, and there is no next-closest
33  //      element than the one we are searching for, so we return -1.
34  var mid = Math.floor((aHigh - aLow) / 2) + aLow;
35  var cmp = aCompare(aNeedle, aHaystack[mid], true);
36  if (cmp === 0) {
37    // Found the element we are looking for.
38    return mid;
39  }
40  else if (cmp > 0) {
41    // Our needle is greater than aHaystack[mid].
42    if (aHigh - mid > 1) {
43      // The element is in the upper half.
44      return recursiveSearch(mid, aHigh, aNeedle, aHaystack, aCompare, aBias);
45    }
46
47    // The exact needle element was not found in this haystack. Determine if
48    // we are in termination case (3) or (2) and return the appropriate thing.
49    if (aBias == exports.LEAST_UPPER_BOUND) {
50      return aHigh < aHaystack.length ? aHigh : -1;
51    } else {
52      return mid;
53    }
54  }
55  else {
56    // Our needle is less than aHaystack[mid].
57    if (mid - aLow > 1) {
58      // The element is in the lower half.
59      return recursiveSearch(aLow, mid, aNeedle, aHaystack, aCompare, aBias);
60    }
61
62    // we are in termination case (3) or (2) and return the appropriate thing.
63    if (aBias == exports.LEAST_UPPER_BOUND) {
64      return mid;
65    } else {
66      return aLow < 0 ? -1 : aLow;
67    }
68  }
69}
70
71/**
72 * This is an implementation of binary search which will always try and return
73 * the index of the closest element if there is no exact hit. This is because
74 * mappings between original and generated line/col pairs are single points,
75 * and there is an implicit region between each of them, so a miss just means
76 * that you aren't on the very start of a region.
77 *
78 * @param aNeedle The element you are looking for.
79 * @param aHaystack The array that is being searched.
80 * @param aCompare A function which takes the needle and an element in the
81 *     array and returns -1, 0, or 1 depending on whether the needle is less
82 *     than, equal to, or greater than the element, respectively.
83 * @param aBias Either 'binarySearch.GREATEST_LOWER_BOUND' or
84 *     'binarySearch.LEAST_UPPER_BOUND'. Specifies whether to return the
85 *     closest element that is smaller than or greater than the one we are
86 *     searching for, respectively, if the exact element cannot be found.
87 *     Defaults to 'binarySearch.GREATEST_LOWER_BOUND'.
88 */
89exports.search = function search(aNeedle, aHaystack, aCompare, aBias) {
90  if (aHaystack.length === 0) {
91    return -1;
92  }
93
94  var index = recursiveSearch(-1, aHaystack.length, aNeedle, aHaystack,
95                              aCompare, aBias || exports.GREATEST_LOWER_BOUND);
96  if (index < 0) {
97    return -1;
98  }
99
100  // We have found either the exact element, or the next-closest element than
101  // the one we are searching for. However, there may be more than one such
102  // element. Make sure we always return the smallest of these.
103  while (index - 1 >= 0) {
104    if (aCompare(aHaystack[index], aHaystack[index - 1], true) !== 0) {
105      break;
106    }
107    --index;
108  }
109
110  return index;
111};
112