short_range_cabana.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 <config/config.hpp>
 
#ifdef ESPRESSO_SHARED_MEMORY_PARALLELISM
 
#include "cell_system/CellStructure.hpp"
 
#include "aosoa_pack.hpp"
#include "custom_verlet_list.hpp"
#include "forces_cabana.hpp"
 
#include <Cabana_Core.hpp>
#include <Cabana_NeighborList.hpp>
 
#ifdef ESPRESSO_CALIPER
#include <caliper/cali.h>
#endif
 
#include <iterator>
#include <span>
#include <utility>
 
template <class KokkosRangePolicy = Kokkos::RangePolicy<>>
ESPRESSO_ATTR_ALWAYS_INLINE inline void
kokkos_parallel_range_for(auto const &name, auto start, auto end,
                          auto const &kernel) {
  if (Kokkos::num_threads() > 1) {
    KokkosRangePolicy policy(start, end);
    Kokkos::parallel_for(name, policy, kernel);
  } else {
    for (auto p_index = start; p_index < end; ++p_index) {
      kernel(p_index);
    }
  }
}
 
ESPRESSO_ATTR_ALWAYS_INLINE inline void
commit_particle(Particle const &p, auto const index,
                CellStructure::AoSoA_pack &aosoa, bool const rebuild) {
  // Always commit: positions, velocities, charges, directors, dipm
  aosoa.set_vector_at(aosoa.position, index, p.pos());
#ifdef ESPRESSO_ELECTROSTATICS
  aosoa.charge(index) = p.q();
#endif
#ifdef ESPRESSO_DPD
  aosoa.set_vector_at(aosoa.velocity, index, p.v());
#endif
#if defined(ESPRESSO_GAY_BERNE) or defined(ESPRESSO_DIPOLES)
  aosoa.set_vector_at(aosoa.director, index,
                      Utils::convert_quaternion_to_director(p.quat()));
#endif
#ifdef ESPRESSO_DIPOLES
  aosoa.dipm(index) = p.dipm();
#endif
 
  // Only commit on rebuild: id, type
  if (rebuild) {
    aosoa.id(index) = p.id();
    aosoa.type(index) = p.type();
  }
 
  // Always update exclusion flags (they can change during simulation)
#ifdef ESPRESSO_EXCLUSIONS
  aosoa.set_has_exclusion(index, !p.exclusions().empty());
#else
  aosoa.flags(index) = 0;
#endif
}
 
ESPRESSO_ATTR_ALWAYS_INLINE inline void
link_cell_kokkos(std::span<Cell *const> cells, BoxGeometry const &box_geo,
                 auto const &verlet_criterion,
                 Kokkos::View<int *> const &id_to_index, int const max_id,
                 auto const &intra_operator, auto const &inter_operator) {
 
  auto const distance_function = detail::MinimalImageDistance{box_geo};
 
  // implementation detail: max_id refers to the max local particle id,
  // but ghost particles from other ranks may have larger particle ids;
  // -1 is used as a sentinel value for particle ids from other threads
 
  auto intra_kernel = [&cells, &distance_function, &verlet_criterion,
                       &id_to_index, &intra_operator, max_id](const int i) {
    auto &local_particles = cells[i]->particles();
    for (auto it = local_particles.begin(); it != local_particles.end(); ++it) {
      auto const &p1 = *it;
      if (p1.id() <= max_id) {
        auto const ii = id_to_index(p1.id());
        if (ii >= 0) {
          // pairs in this cell
          for (auto jt = std::next(it); jt != local_particles.end(); ++jt) {
            if ((*jt).id() <= max_id) {
              if (verlet_criterion(p1, *jt, distance_function(p1, *jt))) {
                auto const jj = id_to_index((*jt).id());
                if (jj >= 0) {
                  intra_operator(ii, jj);
                }
              }
            }
          }
        }
      }
    }
  };
 
  auto inter_kernel = [&cells, &distance_function, &verlet_criterion,
                       &id_to_index, &inter_operator, max_id](const int i) {
    auto &local_particles = cells[i]->particles();
    for (auto const &p1 : local_particles) {
      if (p1.id() <= max_id) {
        auto const ii = id_to_index(p1.id());
        if (ii >= 0) {
          // pairs with neighboring cells
          for (auto &neighbor : cells[i]->neighbors().red()) {
            for (auto const &p2 : neighbor->particles()) {
              if (p2.id() <= max_id) {
                if (verlet_criterion(p1, p2, distance_function(p1, p2))) {
                  auto const jj = id_to_index(p2.id());
                  if (jj >= 0) {
                    inter_operator(ii, jj);
                  }
                }
              }
            }
          }
        }
      }
    }
  };
 
  Kokkos::parallel_for("inter", cells.size(), intra_kernel);
  Kokkos::fence();
 
  Kokkos::parallel_for("intra", cells.size(), inter_kernel);
  Kokkos::fence();
}
 
ESPRESSO_ATTR_ALWAYS_INLINE inline void
update_cabana_state(CellStructure &cell_structure, auto const &verlet_criterion,
                    double const pair_cutoff, auto const integ_switch) {
#ifdef ESPRESSO_CALIPER
  CALI_CXX_MARK_FUNCTION;
#endif
  using execution_space = Kokkos::DefaultExecutionSpace;
  using policy_type = Kokkos::RangePolicy<execution_space>;
  auto const rebuild = cell_structure.prepare_verlet_list_cabana(pair_cutoff);
  auto const &unique_particles = cell_structure.get_unique_particles();
  auto const n_part = unique_particles.size();
  auto const max_id = cell_structure.get_cached_max_local_particle_id();
  auto &aosoa = cell_structure.get_aosoa();
 
  if (rebuild) {
    auto &id_to_index = cell_structure.get_id_to_index();
 
    // ===================================================
    // Fill particle storage (full commit)
    // ===================================================
#ifdef ESPRESSO_CALIPER
    CALI_MARK_BEGIN("AoSoA commit full");
#endif
    kokkos_parallel_range_for<policy_type>(
        "AoSoA write", std::size_t{0}, n_part,
        [&unique_particles, &aosoa, &id_to_index](int const index) {
          auto const &p = *unique_particles.at(index);
          commit_particle(p, index, aosoa, true);
          id_to_index(p.id()) = index;
        });
    Kokkos::fence();
#ifdef ESPRESSO_CALIPER
    CALI_MARK_END("AoSoA commit full");
#endif
 
    // ===================================================
    // Get Verlet pairs and fill Verlet list
    // ===================================================
    bool rebuild_vl = (integ_switch != INTEG_METHOD_STEEPEST_DESCENT and
                       cell_structure.use_verlet_list);
#ifdef ESPRESSO_CALIPER
    CALI_MARK_BEGIN("Verlet list creation");
#endif
    cell_structure.rebuild_verlet_list_cabana(
        [&](std::span<Cell *const> cells, BoxGeometry const &box,
            CellStructure::ListType &verlet_list) {
          link_cell_kokkos(
              std::move(cells), box, verlet_criterion, id_to_index, max_id,
              [&](const int i, const int j) {
                // intra cell loop
                verlet_list.addNeighborLB(i, j);
              },
              [&](const int i, const int j) {
                // inter cell loop
                verlet_list.addNeighbor(i, j);
              });
 
          if (verlet_list.hasOverflow()) {
            cell_structure.use_verlet_list = false;
            runtimeWarningMsg()
                << "Verlet list overflow detected: neighbor count exceeded "
                   "max_counts. Falling back to the link cell algorithm. "
                   "Configured max is "
                << Cabana::NeighborList<CellStructure::ListType>::maxNeighbor(
                       verlet_list);
          }
        },
        rebuild_vl);
#ifdef ESPRESSO_CALIPER
    CALI_MARK_END("Verlet list creation");
#endif
  } else {
    // ===================================================
    // Fill particle storage (partial update)
    // ===================================================
#ifdef ESPRESSO_CALIPER
    CALI_MARK_BEGIN("AoSoA commit partial");
#endif
    kokkos_parallel_range_for<policy_type>(
        "AoSoA write", std::size_t{0}, n_part,
        [&unique_particles, &aosoa](int const index) {
          auto const &p = *unique_particles.at(index);
          commit_particle(p, index, aosoa, false);
        });
    Kokkos::fence();
#ifdef ESPRESSO_CALIPER
    CALI_MARK_END("AoSoA commit partial");
#endif
  }
}
 
#ifdef ESPRESSO_ELECTROSTATICS
ESPRESSO_ATTR_ALWAYS_INLINE inline void
update_aosoa_charges(CellStructure &cell_structure) {
  using execution_space = Kokkos::DefaultExecutionSpace;
  using policy_type = Kokkos::RangePolicy<execution_space>;
  auto const &unique_particles = cell_structure.get_unique_particles();
  auto const n_part = unique_particles.size();
  auto &aosoa = cell_structure.get_aosoa();
 
  kokkos_parallel_range_for<policy_type>(
      "Views update charges", std::size_t{0}, n_part,
      [&unique_particles, &aosoa](std::size_t const index) {
        aosoa.charge(index) = unique_particles.at(index)->q();
      });
}
#endif
 
void cabana_short_range(auto const &bond_kernel, auto const &forces_kernel,
                        CellStructure &cell_structure, double pair_cutoff,
                        double bond_cutoff, auto const &verlet_criterion,
                        auto const integ_switch) {
  using execution_space = Kokkos::DefaultExecutionSpace;
  assert(cell_structure.get_resort_particles() == Cells::RESORT_NONE);
 
  if (bond_cutoff >= 0.) {
#ifdef ESPRESSO_CALIPER
    CALI_MARK_BEGIN("cabana_bond_loop");
#endif
    cell_structure.bond_loop(bond_kernel);
#ifdef ESPRESSO_CALIPER
    CALI_MARK_END("cabana_bond_loop");
#endif
  }
 
  // Cabana short range loop
  if (pair_cutoff > 0.) {
#ifdef ESPRESSO_CALIPER
    CALI_MARK_BEGIN("cabana_pair_loop");
#endif
    if (integ_switch != INTEG_METHOD_STEEPEST_DESCENT and
        cell_structure.use_verlet_list) {
      auto const &verlet_list = cell_structure.get_verlet_list_cabana();
      Kokkos::RangePolicy<execution_space> policy(
          std::size_t{0}, cell_structure.get_unique_particles().size());
      Cabana::neighbor_parallel_for(policy, forces_kernel, verlet_list,
                                    Cabana::FirstNeighborsTag(),
                                    Cabana::SerialOpTag());
    } else {
      cell_structure.cell_list_loop(
          [&](std::span<Cell *const> cells, BoxGeometry const &box) {
            link_cell_kokkos(
                std::move(cells), box, verlet_criterion,
                cell_structure.get_id_to_index(),
                cell_structure.get_cached_max_local_particle_id(),
                [&](const int i, const int j) {
                  // intra cell loop
                  forces_kernel(i, j);
                },
                [&](const int i, const int j) {
                  // inter cell loop
                  forces_kernel(i, j);
                });
          });
    }
    Kokkos::fence();
#ifdef ESPRESSO_CALIPER
    CALI_MARK_END("cabana_pair_loop");
#endif
  }
}
 
#endif // ESPRESSO_SHARED_MEMORY_PARALLELISM