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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.
     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 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 */
     88