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

   1 // -*- C++ -*-
   2
   3 // Copyright (C) 2007 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 2, 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 // You should have received a copy of the GNU General Public License
  17 // along with this library; see the file COPYING.  If not, write to
  18 // the Free Software Foundation, 59 Temple Place - Suite 330, Boston,
  19 // MA 02111-1307, USA.
  20
  21 // As a special exception, you may use this file as part of a free
  22 // software library without restriction.  Specifically, if other files
  23 // instantiate templates or use macros or inline functions from this
  24 // file, or you compile this file and link it with other files to
  25 // produce an executable, this file does not by itself cause the
  26 // resulting executable to be covered by the GNU General Public
  27 // License.  This exception does not however invalidate any other
  28 // reasons why the executable file might be covered by the GNU General
  29 // Public License.
  30
  31 /** @file parallel/equally_split.h
  32  *  This file is a GNU parallel extension to the Standard C++ Library.
  33  */
  34
  35 // Written by Johannes Singler.
  36
  37 #ifndef _GLIBCXX_PARALLEL_EQUALLY_SPLIT_H
  38 #define _GLIBCXX_PARALLEL_EQUALLY_SPLIT_H 1
  39
  40 namespace __gnu_parallel
  41 {
  42 /** @brief Function to split a sequence into parts of almost equal size.
  43  *
  44  *  The resulting sequence s of length num_threads+1 contains the splitting
  45  *  positions when splitting the range [0,n) into parts of almost
  46  *  equal size (plus minus 1).  The first entry is 0, the last one
  47  *  n. There may result empty parts.
  48  *  @param n Number of elements
  49  *  @param num_threads Number of parts
  50  *  @param s Splitters
  51  *  @returns End of splitter sequence, i. e. @c s+num_threads+1 */
  52 template<typename difference_type, typename OutputIterator>
  53   OutputIterator
  54   equally_split(difference_type n,
  55                 thread_index_t num_threads,
  56                 OutputIterator s)
  57   {
  58     difference_type chunk_length = n / num_threads,
  59                     num_longer_chunks = n % num_threads,
  60                     pos = 0;
  61     for (thread_index_t i = 0; i < num_threads; ++i)
  62       {
  63         *s++ = pos;
  64         pos += (i < num_longer_chunks) ? (chunk_length + 1) : chunk_length;
  65       }
  66     *s++ = n;
  67     return s;
  68   }
  69
  70
  71 /** @brief Function to split a sequence into parts of almost equal size.
  72  *
  73  *  Returns the position of the splitting point between
  74  *  thread number thread_no (included) and
  75  *  thread number thread_no+1 (excluded).
  76  *  @param n Number of elements
  77  *  @param num_threads Number of parts
  78  *  @returns Splitting point */
  79 template<typename difference_type>
  80   difference_type
  81   equally_split_point(difference_type n,
  82                       thread_index_t num_threads,
  83                       thread_index_t thread_no)
  84   {
  85     difference_type chunk_length = n / num_threads,
  86                     num_longer_chunks = n % num_threads;
  87
  88     if(thread_no < num_longer_chunks)
  89       return thread_no * (chunk_length + 1);
  90     else
  91       return num_longer_chunks * (chunk_length + 1)
  92           + (thread_no - num_longer_chunks) * chunk_length;
  93   }
  94 }
  95
  96 #endif