libstdc++
equally_split.h
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1 // -*- C++ -*-
2 
3 // Copyright (C) 2007, 2008, 2009, 2010, 2011 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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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
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 */