libstdc++
equally_split.h
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1 // -*- C++ -*-
2 
3 // Copyright (C) 2007, 2009 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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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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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 
34 namespace __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 splitting
39  * positions when splitting the range [0,n) into parts of almost
40  * equal size (plus minus 1). The first entry is 0, the last one
41  * 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 difference_type, typename OutputIterator>
47  OutputIterator
48  equally_split(difference_type n, thread_index_t num_threads, OutputIterator s)
49  {
50  difference_type chunk_length = n / num_threads;
51  difference_type num_longer_chunks = n % num_threads;
52  difference_type pos = 0;
53  for (thread_index_t i = 0; i < num_threads; ++i)
54  {
55  *s++ = pos;
56  pos += (i < num_longer_chunks) ? (chunk_length + 1) : chunk_length;
57  }
58  *s++ = n;
59  return s;
60  }
61 
62 
63 /** @brief Function to split a sequence into parts of almost equal size.
64  *
65  * Returns the position of the splitting point between
66  * thread number thread_no (included) and
67  * thread number thread_no+1 (excluded).
68  * @param n Number of elements
69  * @param num_threads Number of parts
70  * @returns _SplittingAlgorithm point */
71 template<typename difference_type>
72  difference_type
73  equally_split_point(difference_type n,
74  thread_index_t num_threads,
75  thread_index_t thread_no)
76  {
77  difference_type chunk_length = n / num_threads;
78  difference_type num_longer_chunks = n % num_threads;
79  if (thread_no < num_longer_chunks)
80  return thread_no * (chunk_length + 1);
81  else
82  return num_longer_chunks * (chunk_length + 1)
83  + (thread_no - num_longer_chunks) * chunk_length;
84  }
85 }
86 
87 #endif /* _GLIBCXX_PARALLEL_EQUALLY_SPLIT_H */