size_and_offset.hpp

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/*
 * Copyright (C) 2017-2026 The ESPResSo project
 *   Max-Planck-Institute for Polymer Research, Theory Group
 *
 * This file is part of ESPResSo.
 *
 * ESPResSo is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * ESPResSo is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
#pragma once
 
#include <algorithm>
#include <cstddef>
#include <numeric>
#include <vector>
 
#include <boost/mpi/collectives/gather.hpp>
#include <boost/mpi/communicator.hpp>
 
namespace Utils::Mpi::detail {
 
template <typename T>
int size_and_offset(std::vector<int> &sizes, std::vector<int> &displ,
                    int n_elem, const boost::mpi::communicator &comm,
                    int root = 0) {
  auto const world_size = static_cast<unsigned int>(comm.size());
  sizes.resize(world_size);
  displ.resize(world_size);
 
  /* Gather sizes */
  boost::mpi::gather(comm, n_elem, sizes, root);
 
  auto const total_size = std::accumulate(sizes.begin(), sizes.end(), 0);
 
  int offset = 0;
  for (std::size_t i = 0; i < sizes.size(); i++) {
    displ[i] = offset;
    offset += sizes[i];
  }
 
  return total_size;
}
 
inline void size_and_offset(int n_elem, const boost::mpi::communicator &comm,
                            int root = 0) {
  /* Send local size */
  boost::mpi::gather(comm, n_elem, root);
}
 
} // namespace Utils::Mpi::detail