libstdc++: equally_split.h Source File

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equally_split.h
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1// -*- C++ -*-
2
3// Copyright (C) 2007-2026 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.
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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.
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16// Under Section 7 of GPL version 3, you are granted additional
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18// 3.1, as published by the Free Software Foundation.
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22// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
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24
25/** @file parallel/equally_split.h
26 * This file is a GNU parallel extension to the Standard C++ Library.
27 */
28
29// Written by Johannes Singler.
30
31#ifndef _GLIBCXX_PARALLEL_EQUALLY_SPLIT_H
32#define _GLIBCXX_PARALLEL_EQUALLY_SPLIT_H 1
33
34namespace __gnu_parallel
35{
36 /** @brief function to split a sequence into parts of almost equal size.
37 *
38 * The resulting sequence __s of length __num_threads+1 contains the
39 * splitting positions when splitting the range [0,__n) into parts of
40 * almost equal size (plus minus 1). The first entry is 0, the last
41 * one n. There may result empty parts.
42 * @param __n Number of elements
43 * @param __num_threads Number of parts
44 * @param __s Splitters
45 * @returns End of __splitter sequence, i.e. @c __s+__num_threads+1 */
46 template<typename _DifferenceType, typename _OutputIterator>
47 _OutputIterator
48 __equally_split(_DifferenceType __n, _ThreadIndex __num_threads,
49 _OutputIterator __s)
50 {
51 _DifferenceType __chunk_length = __n / __num_threads;
52 _DifferenceType __num_longer_chunks = __n % __num_threads;
53 _DifferenceType __pos = 0;
54 for (_ThreadIndex __i = 0; __i < __num_threads; ++__i)
55 {
56 *__s++ = __pos;
57 __pos += ((__i < __num_longer_chunks)
58 ? (__chunk_length + 1) : __chunk_length);
59 }
60 *__s++ = __n;
61 return __s;
62 }
63
64 /** @brief function to split a sequence into parts of almost equal size.
65 *
66 * Returns the position of the splitting point between
67 * thread number __thread_no (included) and
68 * thread number __thread_no+1 (excluded).
69 * @param __n Number of elements
70 * @param __num_threads Number of parts
71 * @param __thread_no Number of threads
72 * @returns splitting point */
73 template<typename _DifferenceType>
74 _DifferenceType
75 __equally_split_point(_DifferenceType __n,
76 _ThreadIndex __num_threads,
77 _ThreadIndex __thread_no)
78 {
79 _DifferenceType __chunk_length = __n / __num_threads;
80 _DifferenceType __num_longer_chunks = __n % __num_threads;
81 if (__thread_no < __num_longer_chunks)
82 return __thread_no * (__chunk_length + 1);
83 else
84 return __num_longer_chunks * (__chunk_length + 1)
85 + (__thread_no - __num_longer_chunks) * __chunk_length;
86 }
87}
88
89#endif /* _GLIBCXX_PARALLEL_EQUALLY_SPLIT_H */
__gnu_parallel
GNU parallel code for public use.
Definition balanced_quicksort.h:62
__gnu_parallel::_ThreadIndex
uint16_t _ThreadIndex
Unsigned integer to index a thread number. The maximum thread number (for each processor) must fit in...
Definition types.h:123
__gnu_parallel::__equally_split_point
_DifferenceType __equally_split_point(_DifferenceType __n, _ThreadIndex __num_threads, _ThreadIndex __thread_no)
function to split a sequence into parts of almost equal size.
Definition equally_split.h:75
__gnu_parallel::__equally_split
_OutputIterator __equally_split(_DifferenceType __n, _ThreadIndex __num_threads, _OutputIterator __s)
function to split a sequence into parts of almost equal size.
Definition equally_split.h:48