1 // -*- C++ -*-
2
3 // Copyright (C) 2007-2023 Free Software Foundation, Inc.
4 //
5 // This file is part of the GNU ISO C++ Library. This library is free
6 // software; you can redistribute it and/or modify it under the terms
7 // of the GNU General Public License as published by the Free Software
8 // Foundation; either version 3, or (at your option) any later
9 // version.
10
11 // This library is distributed in the hope that it will be useful, but
12 // WITHOUT ANY WARRANTY; without even the implied warranty of
13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 // General Public License for more details.
15
16 // Under Section 7 of GPL version 3, you are granted additional
17 // permissions described in the GCC Runtime Library Exception, version
18 // 3.1, as published by the Free Software Foundation.
19
20 // You should have received a copy of the GNU General Public License and
21 // a copy of the GCC Runtime Library Exception along with this program;
22 // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
23 // <http://www.gnu.org/licenses/>.
24
25 /** @file parallel/quicksort.h
26 * @brief Implementation of a unbalanced parallel quicksort (in-place).
27 * This file is a GNU parallel extension to the Standard C++ Library.
28 */
29
30 // Written by Johannes Singler.
31
32 #ifndef _GLIBCXX_PARALLEL_QUICKSORT_H
33 #define _GLIBCXX_PARALLEL_QUICKSORT_H 1
34
35 #include <parallel/parallel.h>
36 #include <parallel/partition.h>
37
38 namespace __gnu_parallel
39 {
40 /** @brief Unbalanced quicksort divide step.
41 * @param __begin Begin iterator of subsequence.
42 * @param __end End iterator of subsequence.
43 * @param __comp Comparator.
44 * @param __pivot_rank Desired __rank of the pivot.
45 * @param __num_samples Choose pivot from that many samples.
46 * @param __num_threads Number of threads that are allowed to work on
47 * this part.
48 */
49 template<typename _RAIter, typename _Compare>
50 typename std::iterator_traits<_RAIter>::difference_type
51 __parallel_sort_qs_divide(_RAIter __begin, _RAIter __end,
52 _Compare __comp, typename std::iterator_traits
53 <_RAIter>::difference_type __pivot_rank,
54 typename std::iterator_traits
55 <_RAIter>::difference_type
56 __num_samples, _ThreadIndex __num_threads)
57 {
58 typedef std::iterator_traits<_RAIter> _TraitsType;
59 typedef typename _TraitsType::value_type _ValueType;
60 typedef typename _TraitsType::difference_type _DifferenceType;
61
62 _DifferenceType __n = __end - __begin;
63 __num_samples = std::min(__num_samples, __n);
64
65 // Allocate uninitialized, to avoid default constructor.
66 _ValueType* __samples = static_cast<_ValueType*>
67 (::operator new(__num_samples * sizeof(_ValueType)));
68
69 for (_DifferenceType __s = 0; __s < __num_samples; ++__s)
70 {
71 const unsigned long long __index = static_cast<unsigned long long>
72 (__s) * __n / __num_samples;
73 ::new(&(__samples[__s])) _ValueType(__begin[__index]);
74 }
75
76 __gnu_sequential::sort(__samples, __samples + __num_samples, __comp);
77
78 _ValueType& __pivot = __samples[__pivot_rank * __num_samples / __n];
79
80 __gnu_parallel::__binder2nd<_Compare, _ValueType, _ValueType, bool>
81 __pred(__comp, __pivot);
82 _DifferenceType __split = __parallel_partition(__begin, __end,
83 __pred, __num_threads);
84
85 for (_DifferenceType __s = 0; __s < __num_samples; ++__s)
86 __samples[__s].~_ValueType();
87 ::operator delete(__samples);
88
89 return __split;
90 }
91
92 /** @brief Unbalanced quicksort conquer step.
93 * @param __begin Begin iterator of subsequence.
94 * @param __end End iterator of subsequence.
95 * @param __comp Comparator.
96 * @param __num_threads Number of threads that are allowed to work on
97 * this part.
98 */
99 template<typename _RAIter, typename _Compare>
100 void
101 __parallel_sort_qs_conquer(_RAIter __begin, _RAIter __end,
102 _Compare __comp,
103 _ThreadIndex __num_threads)
104 {
105 typedef std::iterator_traits<_RAIter> _TraitsType;
106 typedef typename _TraitsType::value_type _ValueType;
107 typedef typename _TraitsType::difference_type _DifferenceType;
108
109 if (__num_threads <= 1)
110 {
111 __gnu_sequential::sort(__begin, __end, __comp);
112 return;
113 }
114
115 _DifferenceType __n = __end - __begin, __pivot_rank;
116
117 if (__n <= 1)
118 return;
119
120 _ThreadIndex __num_threads_left;
121
122 if ((__num_threads % 2) == 1)
123 __num_threads_left = __num_threads / 2 + 1;
124 else
125 __num_threads_left = __num_threads / 2;
126
127 __pivot_rank = __n * __num_threads_left / __num_threads;
128
129 _DifferenceType __split = __parallel_sort_qs_divide
130 (__begin, __end, __comp, __pivot_rank,
131 _Settings::get().sort_qs_num_samples_preset, __num_threads);
132
133 #pragma omp parallel sections num_threads(2)
134 {
135 #pragma omp section
136 __parallel_sort_qs_conquer(__begin, __begin + __split,
137 __comp, __num_threads_left);
138 #pragma omp section
139 __parallel_sort_qs_conquer(__begin + __split, __end,
140 __comp, __num_threads - __num_threads_left);
141 }
142 }
143
144
145 /** @brief Unbalanced quicksort main call.
146 * @param __begin Begin iterator of input sequence.
147 * @param __end End iterator input sequence, ignored.
148 * @param __comp Comparator.
149 * @param __num_threads Number of threads that are allowed to work on
150 * this part.
151 */
152 template<typename _RAIter, typename _Compare>
153 void
154 __parallel_sort_qs(_RAIter __begin, _RAIter __end,
155 _Compare __comp,
156 _ThreadIndex __num_threads)
157 {
158 _GLIBCXX_CALL(__n)
159
160 typedef std::iterator_traits<_RAIter> _TraitsType;
161 typedef typename _TraitsType::value_type _ValueType;
162 typedef typename _TraitsType::difference_type _DifferenceType;
163
164 _DifferenceType __n = __end - __begin;
165
166 // At least one element per processor.
167 if (__num_threads > __n)
168 __num_threads = static_cast<_ThreadIndex>(__n);
169
170 __parallel_sort_qs_conquer(
171 __begin, __begin + __n, __comp, __num_threads);
172 }
173
174 } //namespace __gnu_parallel
175
176 #endif /* _GLIBCXX_PARALLEL_QUICKSORT_H */