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/list_partition.h
26 * @brief _Functionality to split __sequence referenced by only input
27 * iterators.
28 * This file is a GNU parallel extension to the Standard C++ Library.
29 */
30
31 // Written by Leonor Frias Moya and Johannes Singler.
32
33 #ifndef _GLIBCXX_PARALLEL_LIST_PARTITION_H
34 #define _GLIBCXX_PARALLEL_LIST_PARTITION_H 1
35
36 #include <parallel/parallel.h>
37 #include <vector>
38
39 namespace __gnu_parallel
40 {
41 /** @brief Shrinks and doubles the ranges.
42 * @param __os_starts Start positions worked on (oversampled).
43 * @param __count_to_two Counts up to 2.
44 * @param __range_length Current length of a chunk.
45 * @param __make_twice Whether the @c __os_starts is allowed to be
46 * grown or not
47 */
48 template<typename _IIter>
49 void
50 __shrink_and_double(std::vector<_IIter>& __os_starts,
51 size_t& __count_to_two, size_t& __range_length,
52 const bool __make_twice)
53 {
54 ++__count_to_two;
55 if (!__make_twice || __count_to_two < 2)
56 __shrink(__os_starts, __count_to_two, __range_length);
57 else
58 {
59 __os_starts.resize((__os_starts.size() - 1) * 2 + 1);
60 __count_to_two = 0;
61 }
62 }
63
64 /** @brief Combines two ranges into one and thus halves the number of ranges.
65 * @param __os_starts Start positions worked on (oversampled).
66 * @param __count_to_two Counts up to 2.
67 * @param __range_length Current length of a chunk. */
68 template<typename _IIter>
69 void
70 __shrink(std::vector<_IIter>& __os_starts, size_t& __count_to_two,
71 size_t& __range_length)
72 {
73 for (typename std::vector<_IIter>::size_type __i = 0;
74 __i <= (__os_starts.size() / 2); ++__i)
75 __os_starts[__i] = __os_starts[__i * 2];
76 __range_length *= 2;
77 }
78
79 /** @brief Splits a sequence given by input iterators into parts of
80 * almost equal size
81 *
82 * The function needs only one pass over the sequence.
83 * @param __begin Begin iterator of input sequence.
84 * @param __end End iterator of input sequence.
85 * @param __starts Start iterators for the resulting parts, dimension
86 * @c __num_parts+1. For convenience, @c __starts @c [__num_parts]
87 * contains the end iterator of the sequence.
88 * @param __lengths Length of the resulting parts.
89 * @param __num_parts Number of parts to split the sequence into.
90 * @param __f Functor to be applied to each element by traversing __it
91 * @param __oversampling Oversampling factor. If 0, then the
92 * partitions will differ in at most
93 * \f$\sqrt{\mathrm{end} - \mathrm{begin}}\f$
94 * elements. Otherwise, the ratio between the
95 * longest and the shortest part is bounded by
96 * \f$1/(\mathrm{oversampling} \cdot \mathrm{num\_parts})\f$
97 * @return Length of the whole sequence.
98 */
99 template<typename _IIter, typename _FunctorType>
100 size_t
101 list_partition(const _IIter __begin, const _IIter __end,
102 _IIter* __starts, size_t* __lengths, const int __num_parts,
103 _FunctorType& __f, int __oversampling = 0)
104 {
105 bool __make_twice = false;
106
107 // The resizing algorithm is chosen according to the oversampling factor.
108 if (__oversampling == 0)
109 {
110 __make_twice = true;
111 __oversampling = 1;
112 }
113
114 std::vector<_IIter> __os_starts(2 * __oversampling * __num_parts + 1);
115
116 __os_starts[0] = __begin;
117 _IIter __prev = __begin, __it = __begin;
118 size_t __dist_limit = 0, __dist = 0;
119 size_t __cur = 1, __next = 1;
120 size_t __range_length = 1;
121 size_t __count_to_two = 0;
122 while (__it != __end)
123 {
124 __cur = __next;
125 for (; __cur < __os_starts.size() and __it != __end; ++__cur)
126 {
127 for (__dist_limit += __range_length;
128 __dist < __dist_limit and __it != __end; ++__dist)
129 {
130 __f(__it);
131 ++__it;
132 }
133 __os_starts[__cur] = __it;
134 }
135
136 // Must compare for end and not __cur < __os_starts.size() , because
137 // __cur could be == __os_starts.size() as well
138 if (__it == __end)
139 break;
140
141 __shrink_and_double(__os_starts, __count_to_two, __range_length,
142 __make_twice);
143 __next = __os_starts.size() / 2 + 1;
144 }
145
146 // Calculation of the parts (one must be extracted from __current
147 // because the partition beginning at end, consists only of
148 // itself).
149 size_t __size_part = (__cur - 1) / __num_parts;
150 int __size_greater = static_cast<int>((__cur - 1) % __num_parts);
151 __starts[0] = __os_starts[0];
152
153 size_t __index = 0;
154
155 // Smallest partitions.
156 for (int __i = 1; __i < (__num_parts + 1 - __size_greater); ++__i)
157 {
158 __lengths[__i - 1] = __size_part * __range_length;
159 __index += __size_part;
160 __starts[__i] = __os_starts[__index];
161 }
162
163 // Biggest partitions.
164 for (int __i = __num_parts + 1 - __size_greater; __i <= __num_parts;
165 ++__i)
166 {
167 __lengths[__i - 1] = (__size_part+1) * __range_length;
168 __index += (__size_part+1);
169 __starts[__i] = __os_starts[__index];
170 }
171
172 // Correction of the end size (the end iteration has not finished).
173 __lengths[__num_parts - 1] -= (__dist_limit - __dist);
174
175 return __dist;
176 }
177 }
178
179 #endif /* _GLIBCXX_PARALLEL_LIST_PARTITION_H */