core/observables/PairwiseDistances.hpp

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/*
 * Copyright (C) 2025-2026 The ESPResSo project
 *
 * 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 "BoxGeometry.hpp"
#include "PidPairwiseDistancesObservable.hpp"
#include "cell_system/CellStructure.hpp"
#include "cells.hpp"
#include "system/System.hpp"
 
#include <utils/Vector.hpp>
 
#include <boost/mpi/collectives/gather.hpp>
 
#include <algorithm>
#include <cstddef>
#include <set>
#include <stdexcept>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
 
namespace Observables {
 
/** @brief Track pairwise distances between two sets of particles. */
class PairwiseDistances : public PidPairwiseDistancesObservable {
public:
  using PidPairwiseDistancesObservable::PidPairwiseDistancesObservable;
  explicit PairwiseDistances(std::vector<int> const &ids,
                             std::vector<int> const &target_ids)
      : PidPairwiseDistancesObservable(ids, target_ids),
        m_pairs{get_unique_pairs(ids, target_ids)} {}
 
  /** @brief Evaluate pairwise distances, gathering positions from all ranks. */
  std::vector<double>
  evaluate(boost::mpi::communicator const &comm, ParticleReferenceRange const &,
           ParticleObservables::traits<Particle> const &) const override {
 
    auto const &system = System::get_system();
    auto const &box_geo = *system.box_geo;
    auto &cell_structure = *system.cell_structure;
 
    // Collect local (pid, position) pairs for all particles in m_pairs
    std::unordered_set<int> visited_pids;
    std::vector<std::pair<int, Utils::Vector3d>> local_pid_pos;
    for (auto const &[pid1, pid2] : m_pairs) {
      for (auto const pid : {pid1, pid2}) {
        if (not visited_pids.contains(pid)) {
          auto const *p = cell_structure.get_local_particle(pid);
          if (p and not p->is_ghost()) {
            local_pid_pos.emplace_back(pid, p->pos());
          }
          visited_pids.emplace(pid);
        }
      }
    }
 
    // Gather from all ranks to rank 0
    std::vector<std::vector<std::pair<int, Utils::Vector3d>>> all_pid_pos;
    boost::mpi::gather(comm, local_pid_pos, all_pid_pos, 0);
 
    if (comm.rank() != 0) {
      return {};
    }
 
    // Build position map on rank 0
    std::unordered_map<int, Utils::Vector3d> pos_map;
    for (auto const &rank_data : all_pid_pos) {
      for (auto const &[pid, pos] : rank_data) {
        pos_map.emplace(pid, pos);
      }
    }
 
    std::vector<double> pairwise_distances;
    pairwise_distances.reserve(m_pairs.size());
    for (auto const &[pid1, pid2] : m_pairs) {
      auto const dist =
          box_geo.get_mi_vector(pos_map.at(pid1), pos_map.at(pid2)).norm();
      pairwise_distances.emplace_back(dist);
    }
    return pairwise_distances;
  }
 
  std::vector<std::size_t> shape() const override { return {m_pairs.size()}; }
 
private:
  std::vector<std::pair<int, int>> m_pairs;
 
  std::vector<std::pair<int, int>>
  get_unique_pairs(std::vector<int> const &ids1, std::vector<int> const &ids2) {
    std::set<std::pair<int, int>> unique_pairs;
    for (int id1 : ids1) {
      for (int id2 : ids2) {
        if (id1 != id2) {
          unique_pairs.emplace(std::minmax(id1, id2));
        }
      }
    }
    return {unique_pairs.begin(), unique_pairs.end()};
  }
};
 
} // namespace Observables