class/System.Core/System.Linq/QuickSort.cs

   1 //
   2 // QuickSort.cs
   3 //
   4 // Authors:
   5 //   Alejandro Serrano "Serras" (trupill@yahoo.es)
   6 //   Marek Safar  <marek.safar@gmail.com>
   7 //   Jb Evain (jbevain@novell.com)
   8 //
   9 // (C) 2007 - 2008 Novell, Inc. (http://www.novell.com)
  10 //
  11 // Permission is hereby granted, free of charge, to any person obtaining
  12 // a copy of this software and associated documentation files (the
  13 // "Software"), to deal in the Software without restriction, including
  14 // without limitation the rights to use, copy, modify, merge, publish,
  15 // distribute, sublicense, and/or sell copies of the Software, and to
  16 // permit persons to whom the Software is furnished to do so, subject to
  17 // the following conditions:
  18 //
  19 // The above copyright notice and this permission notice shall be
  20 // included in all copies or substantial portions of the Software.
  21 //
  22 // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
  23 // EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
  24 // MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
  25 // NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
  26 // LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
  27 // OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
  28 // WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  29 //
  30
  31 using System;
  32 using System.Collections.Generic;
  33
  34 namespace System.Linq {
  35
  36         class QuickSort<TElement> {
  37
  38                 TElement [] elements;
  39                 int [] indexes;
  40                 SortContext<TElement> context;
  41
  42                 QuickSort (IEnumerable<TElement> source, SortContext<TElement> context)
  43                 {
  44                         this.elements = source.ToArray ();
  45                         this.indexes = CreateIndexes (elements.Length);
  46                         this.context = context;
  47                 }
  48
  49                 static int [] CreateIndexes (int length)
  50                 {
  51                         var indexes = new int [length];
  52                         for (int i = 0; i < length; i++)
  53                                 indexes [i] = i;
  54
  55                         return indexes;
  56                 }
  57
  58                 void PerformSort ()
  59                 {
  60                         // If the source contains just zero or one element, there's no need to sort
  61                         if (elements.Length <= 1)
  62                                 return;
  63
  64                         context.Initialize (elements);
  65
  66                         // Then sorts the elements according to the collected
  67                         // key values and the selected ordering
  68                         Sort (0, indexes.Length - 1);
  69                 }
  70
  71                 int CompareItems (int first_index, int second_index)
  72                 {
  73                         return context.Compare (first_index, second_index);
  74                 }
  75
  76                 // We look at the first, middle, and last items in the subarray.
  77                 // Then we put the largest on the right side, the smallest on
  78                 // the left side, and the median becomes our pivot.
  79                 int MedianOfThree (int left, int right)
  80                 {
  81                         int center = (left + right) / 2;
  82                         if (CompareItems (indexes [center], indexes [left]) < 0)
  83                                 Swap (left, center);
  84                         if (CompareItems (indexes [right], indexes [left]) < 0)
  85                                 Swap (left, right);
  86                         if (CompareItems (indexes [right], indexes [center]) < 0)
  87                                 Swap (center, right);
  88                         Swap (center, right - 1);
  89                         return indexes [right - 1];
  90                 }
  91
  92                 void Sort (int left, int right)
  93                 {
  94                         if (left + 3 <= right) {
  95                                 int l = left, r = right - 1, pivot = MedianOfThree (left, right);
  96                                 while (true) {
  97                                         while (CompareItems (indexes [++l], pivot) < 0) { }
  98                                         while (CompareItems (indexes [--r], pivot) > 0) { }
  99                                         if (l < r)
 100                                                 Swap (l, r);
 101                                         else
 102                                                 break;
 103                                 }
 104
 105                                 // Restore pivot
 106                                 Swap (l, right - 1);
 107                                 // Partition and sort
 108                                 Sort (left, l - 1);
 109                                 Sort (l + 1, right);
 110                         } else
 111                                 // If there are three items in the subarray, insertion sort is better
 112                                 InsertionSort (left, right);
 113                 }
 114
 115                 void InsertionSort (int left, int right)
 116                 {
 117                         for (int i = left + 1; i <= right; i++) {
 118                                 int j, tmp = indexes [i];
 119
 120                                 for (j = i; j > left && CompareItems (tmp, indexes [j - 1]) < 0; j--)
 121                                         indexes [j] = indexes [j - 1];
 122
 123                                 indexes [j] = tmp;
 124                         }
 125                 }
 126
 127                 void Swap (int left, int right)
 128                 {
 129                         int temp = indexes [right];
 130                         indexes [right] = indexes [left];
 131                         indexes [left] = temp;
 132                 }
 133
 134                 public static IEnumerable<TElement> Sort (IEnumerable<TElement> source, SortContext<TElement> context)
 135                 {
 136                         var sorter = new QuickSort<TElement> (source, context);
 137
 138                         sorter.PerformSort ();
 139
 140                         for (int i = 0; i < sorter.indexes.Length; i++)
 141                                 yield return sorter.elements [sorter.indexes [i]];
 142                 }
 143         }
 144 }