threads.cpp

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/****************************************************************************
 *   This file is part of the aGrUM/pyAgrum library.                        *
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 *   Copyright (c) 2005-2025 by                                             *
 *       - Pierre-Henri WUILLEMIN(_at_LIP6)                                 *
 *       - Christophe GONZALES(_at_AMU)                                     *
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 *   SPDX-FileCopyrightText: Copyright 2005-2025                            *
 *       - Pierre-Henri WUILLEMIN(_at_LIP6)                                 *
 *       - Christophe GONZALES(_at_AMU)                                     *
 *   SPDX-License-Identifier: LGPL-3.0-or-later OR MIT                      *
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 *   Contact  : info_at_agrum_dot_org                                       *
 *   homepage : http://agrum.gitlab.io                                      *
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#include <agrum/agrum.h>
 
#include <agrum/base/core/threads/threads.h>
 
namespace gum {
 
  // Set the number of threads to be used in the next parallel region
  void setNumberOfThreads(unsigned int number) {
    if (number == 0) number = 1;
 
    threadsSTL::setNumberOfThreads(number);
 
#ifdef _OPENMP
    threadsOMP::setNumberOfThreads(number);
#endif
  }
 
  // indicates whether aGrUM uses openMP or STL threads
  bool isOMP() {
#ifdef GUM_THREADS_USE_OMP
    return true;
#else
    return false;
#endif
  }
 
  // returns a vector equally splitting elements of a range among threads
  std::vector< std::pair< Idx, Idx > >
      dispatchRangeToThreads(Idx beg, Idx end, unsigned int nb_threads) {
    // there should always be at least one thread
    const auto real_nb_threads = nb_threads < 1 ? Idx(1) : Idx(nb_threads);
    const Idx  range_size      = end - beg;
    std::vector< std::pair< Idx, Idx > > result;
    result.reserve(real_nb_threads);
 
    // if the number of elements in the range is lower than or equal to the
    // number of threads, assign only one element per thread
    if (range_size <= real_nb_threads) {
      for (Idx i = Idx(0); i < range_size; ++i) {
        result.emplace_back(i, i + 1);
      }
      for (Idx i = range_size; i < real_nb_threads; ++i)
        result.emplace_back(range_size, range_size);
    } else {
      // here there are more elements in the range than threads. So try to balance
      // the number of elements among the threads
      Idx nb_elts_par_thread = range_size / real_nb_threads;
      Idx rest_elts          = range_size - nb_elts_par_thread * real_nb_threads;
 
      Idx begin_index = beg;
      for (Idx i = Idx(0); i < real_nb_threads; ++i) {
        Idx end_index = begin_index + nb_elts_par_thread;
        if (rest_elts != Idx(0)) {
          ++end_index;
          --rest_elts;
        }
        result.emplace_back(begin_index, end_index);
        begin_index = end_index;
      }
    }
 
    return result;
  }
 
}   // namespace gum