ghost_point_neighbors.C

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// The libMesh Finite Element Library.
// Copyright (C) 2002-2026 Benjamin S. Kirk, John W. Peterson, Roy H. Stogner

// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.

// This library 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
// Lesser General Public License for more details.

// You should have received a copy of the GNU Lesser General Public
// License along with this library; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA


// Local Includes
#include "libmesh/ghost_point_neighbors.h"

#include "libmesh/elem.h"
#include "libmesh/remote_elem.h"
#ifdef LIBMESH_ENABLE_PERIODIC
#include "libmesh/periodic_boundaries.h"
#include "libmesh/boundary_info.h"
#endif

// C++ Includes
#include <unordered_set>
#include <set>
#include <vector>

namespace libMesh
{

void GhostPointNeighbors::operator()
  (const MeshBase::const_element_iterator & range_begin,
   const MeshBase::const_element_iterator & range_end,
   processor_id_type p,
   GhostPointNeighbors::map_type & coupled_elements)
{
  libmesh_assert(_mesh);

#ifdef LIBMESH_ENABLE_PERIODIC
  const bool check_periodic_bcs =
    (_periodic_bcs && !_periodic_bcs->empty());

  auto * db = _mesh->get_disjoint_neighbor_boundary_pairs();
  const bool check_disjoint_bcs = (db && !db->empty());

  std::unique_ptr<PointLocatorBase> point_locator;
  if (check_periodic_bcs || check_disjoint_bcs)
      point_locator = _mesh->sub_point_locator();

  std::set<const Elem *> periodic_elems_examined;
  const BoundaryInfo & binfo = _mesh->get_boundary_info();
  std::vector<boundary_id_type> appn_bcids;
  std::vector<const Elem *> active_periodic_neighbors;
#endif

  // Using a connected_nodes set rather than point_neighbors() would
  // give us correct results even in corner cases, such as where two
  // elements meet only at a corner.  ;-)
  //
  // std::unordered_set<const Node *> connected_nodes;
  //
  // Unfortunately, it's not a *fast* enough test to use on a
  // ReplicatedMesh (where it's O(Nprocs) times slower), or as a
  // coupling functor (where it's O(Nelem/Nprocs) times slower), or as
  // a coupling functor on a ReplicatedMesh (where you might as well
  // just give up now)

  // Links between boundary and interior elements on mixed
  // dimensional meshes also give us correct ghosting in this way.
  std::unordered_set<const Elem *> interior_parents;

  // We also preserve neighbors and their neighboring children for
  // active local elements - in most cases this is redundant with the
  // node check, but for non-conforming Tet4 meshes and
  // non-level-one-conforming 2D+3D meshes it is possible for an
  // element and its coarse neighbor to not share any vertices.
  //
  // We also preserve interior parents for active pid elements


  // This code is just for geometric coupling, so we use a null
  // CouplingMatrix pointer.  We'll declare that here so as to avoid
  // confusing the insert() calls later.
  CouplingMatrix * nullcm = nullptr;

  for (const auto & elem : as_range(range_begin, range_end))
    {
      libmesh_assert(_mesh->query_elem_ptr(elem->id()) == elem);

      if (elem->processor_id() != p)
        coupled_elements.emplace(elem, nullcm);

      std::set<const Elem *> elem_point_neighbors;
      elem->find_point_neighbors(elem_point_neighbors);

      for (const auto & neigh : elem_point_neighbors)
        if (neigh->processor_id() != p)
          coupled_elements.emplace(neigh, nullcm);

      // An interior_parent isn't on the same manifold so won't be
      // found as a point neighbor, and it may not share nodes so we
      // can't use a connected_nodes test.
      //
      // Trying to preserve interior_parent() only works if it's on
      // the same Mesh, which is *not* guaranteed!  So we'll
      // double-check.
      const Elem * ip = elem->interior_parent();
      if (ip && ip->processor_id() != p &&
          _mesh->query_elem_ptr(ip->id()) == ip)
        coupled_elements.emplace(ip, nullcm);

#ifdef LIBMESH_ENABLE_PERIODIC
      if (check_periodic_bcs)
        {
          for (const auto s : elem->side_index_range())
            {
              if (elem->neighbor_ptr(s))
                continue;

              const Elem * const equal_level_periodic_neigh = elem->topological_neighbor
                (s, *_mesh, *point_locator, _periodic_bcs);

              if (!equal_level_periodic_neigh || equal_level_periodic_neigh == remote_elem)
                continue;

#ifdef LIBMESH_ENABLE_AMR
              equal_level_periodic_neigh->active_family_tree_by_topological_neighbor(
                active_periodic_neighbors,
                elem,
                *_mesh,
                *point_locator,
                _periodic_bcs,
                /*reset=*/true);
#else
              active_periodic_neighbors = { equal_level_periodic_neigh };
#endif

              for (const Elem * const active_periodic_neigh : active_periodic_neighbors)
                {
                  std::set <const Elem *> active_periodic_point_neighbors;

                  // This fills point neighbors *including*
                  // active_periodic_neigh. The documentation for this method
                  // states that this will return *active* point neighbors
                  active_periodic_neigh->find_point_neighbors(active_periodic_point_neighbors);

                  for (const Elem * const appn : active_periodic_point_neighbors)
                    {
                      // Don't need to ghost RemoteElem or an element we already own or an
                      // element we've already examined
                      if (appn == remote_elem || appn->processor_id() == p ||
                          periodic_elems_examined.count(appn))
                        continue;

                      // We only need to keep point neighbors that are along the periodic boundaries
                      bool on_periodic_boundary = false;
                      for (const auto appn_s : appn->side_index_range())
                        {
                          binfo.boundary_ids(appn, appn_s, appn_bcids);
                          for (const auto appn_bcid : appn_bcids)
                            if (_periodic_bcs->find(appn_bcid) != _periodic_bcs->end())
                              {
                                on_periodic_boundary = true;
                                goto jump;
                              }
                        }
                      jump:
                        if (on_periodic_boundary)
                          coupled_elements.emplace(appn, nullcm);

                        periodic_elems_examined.insert(appn);
                    }
                }
            }
        }

      if (check_disjoint_bcs)
        {
          // Also ghost their disjoint neighbors
          for (auto s : elem->side_index_range())
          {
            for (const auto & [id, boundary_ptr] : *db)
            {
              if (!_mesh->get_boundary_info().has_boundary_id(elem, s, id))
                continue;

              unsigned int neigh_side = invalid_uint;
              const Elem * neigh =
                  db->neighbor(id, *point_locator, elem, s, &neigh_side);

              if (!neigh || neigh == remote_elem)
                continue;

              if (neigh->processor_id() != p)
                coupled_elements.emplace(neigh, nullcm);
            }
          }
        }

#endif // LIBMESH_ENABLE_PERIODIC
    }
}

void GhostPointNeighbors::mesh_reinit()
{
  // Unless we have periodic boundary conditions, we don't need
  // anything precomputed.
#ifdef LIBMESH_ENABLE_PERIODIC
  const bool check_periodic_bcs =
    (_periodic_bcs && !_periodic_bcs->empty());

  auto * db = _mesh->get_disjoint_neighbor_boundary_pairs();
  const bool check_disjoint_bcs = (db && !db->empty());

  if (!check_periodic_bcs && !check_disjoint_bcs)
    return;
#endif

  // If we do have periodic boundary conditions, we'll need a master
  // point locator, so we'd better have a mesh to build it on.
  libmesh_assert(_mesh);

  // Make sure an up-to-date master point locator has been
  // constructed; we'll need to grab sub-locators soon.
  _mesh->sub_point_locator();
}

} // namespace libMesh