GRASS GIS 7 Programmer's Manual  7.5.svn(2017)-r71933
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quicksort.h
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1 /****************************************************************************
2  *
3  * MODULE: iostream
4  *
5 
6  * COPYRIGHT (C) 2007 Laura Toma
7  *
8  *
9 
10  * Iostream is a library that implements streams, external memory
11  * sorting on streams, and an external memory priority queue on
12  * streams. These are the fundamental components used in external
13  * memory algorithms.
14 
15  * Credits: The library was developed by Laura Toma. The kernel of
16  * class STREAM is based on the similar class existent in the GPL TPIE
17  * project developed at Duke University. The sorting and priority
18  * queue have been developed by Laura Toma based on communications
19  * with Rajiv Wickremesinghe. The library was developed as part of
20  * porting Terraflow to GRASS in 2001. PEARL upgrades in 2003 by
21  * Rajiv Wickremesinghe as part of the Terracost project.
22 
23  *
24  * This program is free software; you can redistribute it and/or modify
25  * it under the terms of the GNU General Public License as published by
26  * the Free Software Foundation; either version 2 of the License, or
27  * (at your option) any later version.
28  *
29 
30  * This program is distributed in the hope that it will be useful,
31  * but WITHOUT ANY WARRANTY; without even the implied warranty of
32  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
33  * General Public License for more details. *
34  * **************************************************************************/
35 
36 
37 
38 #ifndef _QUICKSORT_H
39 #define _QUICKSORT_H
40 
41 #include <stdlib.h> //for random()
42 
43 
44 // The class represented by CMPR, must have a member function called
45 // "compare" which is used for sorting
46 
47 
48 
49 /* ---------------------------------------------------------------------- */
50 // On return from partition(), everything at or below pivot will be
51 // less that or equal to everything above it. Furthermore, it will
52 // not be 0 since this will leave us to recurse on the whole array
53 // again.
54 template<class T, class CMPR>
55 void partition(T *data, size_t n, size_t &pivot, CMPR &cmp) {
56  T *ptpart, tpart;
57  T *p, *q;
58  T t0;
59 
60  // Try to get a good partition value and avoid being bitten by already
61  // sorted input.
62  //ptpart = data + (random() % n);
63 #ifdef __MINGW32__
64  ptpart = data + (rand() % n);
65 #else
66  ptpart = data + (random() % n);
67 #endif
68 
69  tpart = *ptpart;
70  *ptpart = data[0];
71  data[0] = tpart;
72 
73  // Walk through the array and partition it.
74  for (p = data - 1, q = data + n; ; ) {
75 
76  do {
77  q--;
78  } while (cmp.compare(*q, tpart) > 0);
79  do {
80  p++;
81  } while (cmp.compare(*p, tpart) < 0);
82 
83  if (p < q) {
84  t0 = *p;
85  *p = *q;
86  *q = t0;
87  } else {
88  pivot = q - data;
89  break;
90  }
91  }
92 }
93 
94 
95 
96 
97 /* ---------------------------------------------------------------------- */
98 template<class T, class CMPR>
99 void insertionsort(T *data, size_t n, CMPR &cmp) {
100  T *p, *q, test;
101 
102  for (p = data + 1; p < data + n; p++) {
103  for (q = p - 1, test = *p; (cmp.compare(*q, test) > 0); q--) {
104  *(q+1) = *q;
105  if (q==data) {
106  q--; // to make assignment below correct
107  break;
108  }
109  }
110  *(q+1) = test;
111  }
112 }
113 
114 
115 
116 
117 /* ---------------------------------------------------------------------- */
118 template<class T, class CMPR>
119 void quicksort(T *data, size_t n, CMPR &cmp, size_t min_len = 20) {
120 
121  size_t pivot;
122  if (n < min_len) {
123  insertionsort(data, n, cmp);
124  return;
125  }
126  //else
127  partition(data, n, pivot, cmp);
128  quicksort(data, pivot + 1, cmp, min_len);
129  quicksort(data + pivot + 1, n - pivot - 1, cmp, min_len);
130 }
131 
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133 
134 
135 #endif // _QUICKSORT_H
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void partition(T *data, size_t n, size_t &pivot, CMPR &cmp)
Definition: quicksort.h:55
void insertionsort(T *data, size_t n, CMPR &cmp)
Definition: quicksort.h:99
void quicksort(T *data, size_t n, CMPR &cmp, size_t min_len=20)
Definition: quicksort.h:119