libstdc++-v3/include/parallel/equally_split.h

   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.
  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/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 _DifferenceType, typename _OutputIterator>
  47   _OutputIterator
  48   equally_split(_DifferenceType __n, _ThreadIndex __num_threads, _OutputIterator __s)
  49   {
  50     _DifferenceType __chunk_length = __n / __num_threads;
  51     _DifferenceType __num_longer_chunks = __n % __num_threads;
  52     _DifferenceType __pos = 0;
  53     for (_ThreadIndex __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 splitting point */
  71 template<typename _DifferenceType>
  72   _DifferenceType
  73   equally_split_point(_DifferenceType __n,
  74                       _ThreadIndex __num_threads,
  75                       _ThreadIndex __thread_no)
  76   {
  77     _DifferenceType __chunk_length = __n / __num_threads;
  78     _DifferenceType __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 */