parallel_elem.C

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// The libMesh Finite Element Library.
// Copyright (C) 2002-2025 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



// C++ includes

// Local includes
#include "libmesh/boundary_info.h"
#include "libmesh/distributed_mesh.h"
#include "libmesh/elem.h"
#include "libmesh/mesh_base.h"
#include "libmesh/parallel_elem.h"
#include "libmesh/parallel_mesh.h"
#include "libmesh/remote_elem.h"

// Helper functions in anonymous namespace

namespace
{
using namespace libMesh;

#ifdef LIBMESH_ENABLE_UNIQUE_ID
static const unsigned int header_size = 12;
#else
static const unsigned int header_size = 11;
#endif

#ifndef NDEBUG
// Currently this constant is only used for debugging.
static const largest_id_type elem_magic_header = 987654321;
#endif
}


namespace libMesh
{

namespace Parallel
{

template <>
unsigned int
Packing<const Elem *>::packed_size (std::vector<largest_id_type>::const_iterator in)
{
#ifndef NDEBUG
  const largest_id_type packed_header = *in++;
  libmesh_assert_equal_to (packed_header, elem_magic_header);
#endif

  // int 0: level
  const unsigned int level =
    cast_int<unsigned int>(*in);

  // int 4: element type
  const int typeint = cast_int<int>(*(in+4));
  libmesh_assert_greater_equal (typeint, 0);
  libmesh_assert_less (typeint, INVALID_ELEM);
  const ElemType type =
    cast_int<ElemType>(typeint);

  const unsigned int n_nodes =
    Elem::type_to_n_nodes_map[type];

  if (n_nodes == invalid_uint)
    libmesh_not_implemented_msg("Support for Polygons/Polyhedra not yet implemented");

  const unsigned int n_sides =
    Elem::type_to_n_sides_map[type];

  const unsigned int n_edges =
    Elem::type_to_n_edges_map[type];

  const unsigned int pre_indexing_size =
    header_size + n_nodes + n_sides*2;

  const unsigned int indexing_size =
    DofObject::unpackable_indexing_size(in+pre_indexing_size);

  // We communicate if we are on the boundary or not
  unsigned int total_packed_bc_data = 1;
  largest_id_type on_boundary = *(in + pre_indexing_size + indexing_size);

  if (on_boundary)
  {
    // Extracting if the children are allowed on the boundary
    total_packed_bc_data++;
    largest_id_type allow_children_on_boundary = *(in + pre_indexing_size + indexing_size + 1);

    // For now, children are only supported on sides, the nodes and shell faces are
    // treated using the top parents only
    if (level == 0 || allow_children_on_boundary)
    {
      for (unsigned int s = 0; s != n_sides; ++s)
      {
        const int n_bcs = cast_int<int>
          (*(in + pre_indexing_size + indexing_size +
            total_packed_bc_data++));
        libmesh_assert_greater_equal (n_bcs, 0);
        total_packed_bc_data += n_bcs;
      }
    }
  }
  if (level == 0)
    {
      for (unsigned int e = 0; e != n_edges; ++e)
        {
          const int n_bcs = cast_int<int>
            (*(in + pre_indexing_size + indexing_size +
              total_packed_bc_data++));
          libmesh_assert_greater_equal (n_bcs, 0);
          total_packed_bc_data += n_bcs;
        }

      for (unsigned short sf=0; sf != 2; ++sf)
        {
          const int n_bcs = cast_int<int>
            (*(in + pre_indexing_size + indexing_size +
              total_packed_bc_data++));
          libmesh_assert_greater_equal (n_bcs, 0);
          total_packed_bc_data += n_bcs;
        }
    }

  return
#ifndef NDEBUG
    1 + // Account for magic header
#endif
    pre_indexing_size + indexing_size + total_packed_bc_data;
}



template <>
unsigned int
Packing<const Elem *>::packed_size (std::vector<largest_id_type>::iterator in)
{
  return packed_size(std::vector<largest_id_type>::const_iterator(in));
}



template <>
unsigned int
Packing<const Elem *>::packable_size (const Elem * const & elem,
                                      const MeshBase * mesh)
{
  // We always communicate if we are on a boundary or not
  unsigned int total_packed_bcs = 1;
  const unsigned short n_sides = elem->n_sides();

  largest_id_type on_boundary = 0;
  for (auto s : elem->side_index_range())
    if (mesh->get_boundary_info().n_raw_boundary_ids(elem,s))
    {
      on_boundary = 1;
      break;
    }

  if (on_boundary)
  {
    // In this case we need another entry to check if we allow children on the boundary.
    // We only allow children on sides, edges and sheel faces are treated normally using
    // their top parents.
    total_packed_bcs++;
    if (elem->level() == 0 || mesh->get_boundary_info().is_children_on_boundary_side())
    {
      total_packed_bcs += n_sides;
      for (unsigned short s = 0; s != n_sides; ++s)
        total_packed_bcs +=
          mesh->get_boundary_info().n_raw_boundary_ids(elem,s);
    }
  }

  if (elem->level() == 0)
    {
      const unsigned short n_edges = elem->n_edges();
      total_packed_bcs += n_edges;
      for (unsigned short e = 0; e != n_edges; ++e)
        total_packed_bcs +=
          mesh->get_boundary_info().n_edge_boundary_ids(elem,e);

      total_packed_bcs += 2; // shellfaces
      for (unsigned short sf=0; sf != 2; ++sf)
        total_packed_bcs +=
          mesh->get_boundary_info().n_shellface_boundary_ids(elem,sf);
    }

  return
#ifndef NDEBUG
    1 + // add an int for the magic header when testing
#endif
    header_size + elem->n_nodes() + n_sides*2 +
    elem->packed_indexing_size() + total_packed_bcs;
}



template <>
unsigned int
Packing<const Elem *>::packable_size (const Elem * const & elem,
                                      const DistributedMesh * mesh)
{
  return packable_size(elem, static_cast<const MeshBase *>(mesh));
}



template <>
unsigned int
Packing<const Elem *>::packable_size (const Elem * const & elem,
                                      const ParallelMesh * mesh)
{
  return packable_size(elem, static_cast<const MeshBase *>(mesh));
}



template <>
void
Packing<const Elem *>::pack (const Elem * const & elem,
                             std::back_insert_iterator<std::vector<largest_id_type>> data_out,
                             const MeshBase * mesh)
{
  libmesh_assert(elem);

#ifndef NDEBUG
  *data_out++ = elem_magic_header;
#endif

#ifdef LIBMESH_ENABLE_AMR
  *data_out++ = (static_cast<largest_id_type>(elem->level()));
  *data_out++ = (static_cast<largest_id_type>(elem->p_level()));

  // Encode both the refinement flag and whether the element has
  // children together.  This coding is unambiguous because our
  // refinement state encoding starts at 0 and ends at
  // INVALID_REFINEMENTSTATE
  largest_id_type refinement_info =
    static_cast<largest_id_type>(elem->refinement_flag());
  if (elem->has_children())
    refinement_info +=
      static_cast<largest_id_type>(Elem::INVALID_REFINEMENTSTATE) + 1;
  *data_out++ = (refinement_info);

  *data_out++ = (static_cast<largest_id_type>(elem->p_refinement_flag()));
#else
  *data_out++ = (0);
  *data_out++ = (0);
  *data_out++ = (0);
  *data_out++ = (0);
#endif
  *data_out++ = (static_cast<largest_id_type>(elem->type()));
  *data_out++ = (elem->processor_id());
  *data_out++ = (elem->subdomain_id());
  *data_out++ = (elem->id());

#ifdef LIBMESH_ENABLE_UNIQUE_ID
  if (elem->valid_unique_id())
    *data_out++ = (static_cast<largest_id_type>(elem->unique_id()));
  else
    // OK to send invalid unique id, we must not own this DOF
    *data_out++ = (static_cast<largest_id_type>(DofObject::invalid_unique_id));
#endif

#ifdef LIBMESH_ENABLE_AMR
  // use parent_ID of invalid_id to indicate a level 0 element
  if (elem->level() == 0)
    {
      *data_out++ =(DofObject::invalid_id);
      *data_out++ =(DofObject::invalid_id);
    }
  else
    {
      *data_out++ =(elem->parent()->id());
      *data_out++ =(elem->parent()->which_child_am_i(elem));
    }
#else
  *data_out++ = (DofObject::invalid_id);
  *data_out++ = (DofObject::invalid_id);
#endif

  if ((elem->dim() < LIBMESH_DIM) &&
      elem->interior_parent())
    *data_out++ =(elem->interior_parent()->id());
  else
    *data_out++ =(DofObject::invalid_id);

  for (const Node & node : elem->node_ref_range())
    *data_out++ = node.id();

  // Add the id of and the side for any return link from each neighbor
  for (auto neigh : elem->neighbor_ptr_range())
    {
      if (neigh)
      {
        *data_out++ = (neigh->id());
        if (neigh == remote_elem)
          *data_out++ = (DofObject::invalid_id);
        else
          *data_out++ = neigh->which_neighbor_am_i(elem);
      }
      else
      {
        *data_out++ = (DofObject::invalid_id);
        *data_out++ = (DofObject::invalid_id);
      }
    }

  // Add any DofObject indices
  elem->pack_indexing(data_out);

  // We check if this is a boundary cell. We use the raw
  // IDs because we also communicate the parents which
  // will bring their associated IDs
  largest_id_type on_boundary = 0;
  for (auto s : elem->side_index_range())
    if (mesh->get_boundary_info().n_raw_boundary_ids(elem,s))
    {
      on_boundary = 1;
      break;
    }

  *data_out++ = on_boundary;

  if (on_boundary)
  {
    *data_out++ = mesh->get_boundary_info().is_children_on_boundary_side();
    // Again, only do this if we allow children to hold boundary sides, the edges and
    // shell faces are treated normally using their top parents
    if (elem->level() == 0 || mesh->get_boundary_info().is_children_on_boundary_side())
    {
      std::vector<boundary_id_type> bcs;
      for (auto s : elem->side_index_range())
      {
        mesh->get_boundary_info().raw_boundary_ids(elem, s, bcs);

        *data_out++ =(bcs.size());

        for (const auto & bid : bcs)
          *data_out++ = bid;
      }
    }
  }

  // If this is a coarse element,
  // Add any element side boundary condition ids
  if (elem->level() == 0)
  {
    std::vector<boundary_id_type> bcs;
    for (auto e : elem->edge_index_range())
      {
        mesh->get_boundary_info().edge_boundary_ids(elem, e, bcs);

        *data_out++ =(bcs.size());

        for (const auto & bid : bcs)
          *data_out++ = bid;
      }

    for (unsigned short sf=0; sf != 2; ++sf)
      {
        mesh->get_boundary_info().shellface_boundary_ids(elem, sf, bcs);

        *data_out++ =(bcs.size());

        for (const auto & bid : bcs)
          *data_out++ = bid;
      }
  }
}



template <>
void
Packing<const Elem *>::pack (const Elem * const & elem,
                             std::back_insert_iterator<std::vector<largest_id_type>> data_out,
                             const DistributedMesh * mesh)
{
  pack(elem, data_out, static_cast<const MeshBase*>(mesh));
}



template <>
void
Packing<const Elem *>::pack (const Elem * const & elem,
                             std::back_insert_iterator<std::vector<largest_id_type>> data_out,
                             const ParallelMesh * mesh)
{
  pack(elem, data_out, static_cast<const MeshBase*>(mesh));
}



// FIXME - this needs serious work to be 64-bit compatible
template <>
Elem *
Packing<Elem *>::unpack (std::vector<largest_id_type>::const_iterator in,
                         MeshBase * mesh)
{
#ifndef NDEBUG
  const std::vector<largest_id_type>::const_iterator original_in = in;

  const largest_id_type incoming_header = *in++;
  libmesh_assert_equal_to (incoming_header, elem_magic_header);
#endif

  // int 0: level
  const unsigned int level =
    cast_int<unsigned int>(*in++);

#ifdef LIBMESH_ENABLE_AMR
  // int 1: p level
  const unsigned int p_level =
    cast_int<unsigned int>(*in++);

  // int 2: refinement flag and encoded has_children
  const int rflag = cast_int<int>(*in++);
  const int invalid_rflag =
    cast_int<int>(Elem::INVALID_REFINEMENTSTATE);
  libmesh_assert_greater_equal (rflag, 0);

  libmesh_assert_less (rflag, invalid_rflag*2+1);

  const bool has_children = (rflag > invalid_rflag);

  const Elem::RefinementState refinement_flag = has_children ?
    cast_int<Elem::RefinementState>(rflag - invalid_rflag - 1) :
    cast_int<Elem::RefinementState>(rflag);

  // int 3: p refinement flag
  const int pflag = cast_int<int>(*in++);
  libmesh_assert_greater_equal (pflag, 0);
  libmesh_assert_less (pflag, Elem::INVALID_REFINEMENTSTATE);
  const Elem::RefinementState p_refinement_flag =
    cast_int<Elem::RefinementState>(pflag);
#else
  in += 3;
#endif // LIBMESH_ENABLE_AMR

  // int 4: element type
  const int typeint = cast_int<int>(*in++);
  libmesh_assert_greater_equal (typeint, 0);
  libmesh_assert_less (typeint, INVALID_ELEM);
  const ElemType type =
    cast_int<ElemType>(typeint);

  const unsigned int n_nodes =
    Elem::type_to_n_nodes_map[type];

  // int 5: processor id
  const processor_id_type processor_id =
    cast_int<processor_id_type>(*in++);
  libmesh_assert (processor_id < mesh->n_processors() ||
                  processor_id == DofObject::invalid_processor_id);

  // int 6: subdomain id
  const subdomain_id_type subdomain_id =
    cast_int<subdomain_id_type>(*in++);

  // int 7: dof object id
  const dof_id_type id =
    cast_int<dof_id_type>(*in++);
  libmesh_assert_not_equal_to (id, DofObject::invalid_id);

#ifdef LIBMESH_ENABLE_UNIQUE_ID
  // int 8: dof object unique id
  const unique_id_type unique_id =
    cast_int<unique_id_type>(*in++);
#endif

#ifdef LIBMESH_ENABLE_AMR
  // int 9: parent dof object id.
  // Note: If level==0, then (*in) == invalid_id.  In
  // this case, the equality check in cast_int<unsigned>(*in) will
  // never succeed.  Therefore, we should only attempt the more
  // rigorous cast verification in cases where level != 0.
  const dof_id_type parent_id =
    (level == 0)
    ? static_cast<dof_id_type>(*in++)
    : cast_int<dof_id_type>(*in++);
  libmesh_assert (level == 0 || parent_id != DofObject::invalid_id);
  libmesh_assert (level != 0 || parent_id == DofObject::invalid_id);

  // int 10: local child id
  // Note: If level==0, then which_child_am_i is not valid, so don't
  // do the more rigorous cast verification.
  const unsigned int which_child_am_i =
    (level == 0)
    ? static_cast<unsigned int>(*in++)
    : cast_int<unsigned int>(*in++);
#else
  in += 2;
#endif // LIBMESH_ENABLE_AMR

  const dof_id_type interior_parent_id =
    static_cast<dof_id_type>(*in++);

  // Make sure we don't miscount above when adding the "magic" header
  // plus the real data header
  libmesh_assert_equal_to (in - original_in, header_size + 1);

  Elem * elem = mesh->query_elem_ptr(id);

  // if we already have this element, make sure its
  // properties match, and update any missing neighbor
  // links, but then go on
  if (elem)
    {
      libmesh_assert_equal_to (elem->level(), level);
      libmesh_assert_equal_to (elem->id(), id);
      //#ifdef LIBMESH_ENABLE_UNIQUE_ID
      // No check for unique id sanity
      //#endif
      libmesh_assert_equal_to (elem->processor_id(), processor_id);
      libmesh_assert_equal_to (elem->subdomain_id(), subdomain_id);
      libmesh_assert_equal_to (elem->type(), type);
      libmesh_assert_equal_to (elem->n_nodes(), n_nodes);

#ifndef NDEBUG
      // All our nodes should be correct
      for (unsigned int i=0; i != n_nodes; ++i)
        libmesh_assert(elem->node_id(i) ==
                       cast_int<dof_id_type>(*in++));
#else
      in += n_nodes;
#endif

#ifdef LIBMESH_ENABLE_AMR
      libmesh_assert_equal_to (elem->refinement_flag(), refinement_flag);
      libmesh_assert_equal_to (elem->has_children(), has_children);

#ifdef DEBUG
      if (elem->active())
        {
          libmesh_assert_equal_to (elem->p_level(), p_level);
          libmesh_assert_equal_to (elem->p_refinement_flag(), p_refinement_flag);
        }
#endif

      libmesh_assert (!level || elem->parent() != nullptr);
      libmesh_assert (!level || elem->parent()->id() == parent_id);
      libmesh_assert (!level || elem->parent()->child_ptr(which_child_am_i) == elem);
#endif
      // Our interior_parent link should be "close to" correct - we
      // may have to update it, but we can check for some
      // inconsistencies.
      {
        // If the sending processor sees no interior_parent here, we'd
        // better agree.
        if (interior_parent_id == DofObject::invalid_id)
          {
            if (elem->dim() < LIBMESH_DIM)
              libmesh_assert (!(elem->interior_parent()));
          }

        // If the sending processor has a remote_elem interior_parent,
        // then all we know is that we'd better have *some*
        // interior_parent
        else if (interior_parent_id == remote_elem->id())
          {
            libmesh_assert(elem->interior_parent());
          }
        else
          {
            Elem * ip =
              mesh->interior_mesh().query_elem_ptr(interior_parent_id);

            // The sending processor sees an interior parent here, so
            // if we don't have that interior element, then we'd
            // better have a remote_elem signifying that fact.
            if (!ip)
              libmesh_assert_equal_to (elem->interior_parent(), remote_elem);
            else
              {
                // The sending processor has an interior_parent here,
                // and we have that element, but that does *NOT* mean
                // we're already linking to it.  Perhaps we initially
                // received elem from a processor on which the
                // interior_parent link was remote?
                libmesh_assert(elem->interior_parent() == ip ||
                               elem->interior_parent() == remote_elem);

                // If the link was originally remote, update it
                if (elem->interior_parent() == remote_elem)
                  {
                    elem->set_interior_parent(ip);
                  }
              }
          }
      }

      // Our neighbor links should be "close to" correct - we may have
      // to update a remote_elem link, and we can check for possible
      // inconsistencies along the way.
      //
      // For subactive elements, we don't bother keeping neighbor
      // links in good shape, so there's nothing we need to set or can
      // safely assert here.
      if (!elem->subactive())
        for (auto n : elem->side_index_range())
          {
            const dof_id_type neighbor_id =
              cast_int<dof_id_type>(*in++);

            const dof_id_type neighbor_side =
              cast_int<dof_id_type>(*in++);

            // If the sending processor sees a domain boundary here,
            // we'd better agree ... unless all we see is a
            // remote_elem?  In that case maybe we just couldn't keep
            // up with a user's delete_elem.  Let's trust them.
            if (neighbor_id == DofObject::invalid_id)
              {
                const Elem * my_neigh = elem->neighbor_ptr(n);
                if (my_neigh == remote_elem)
                  elem->set_neighbor(n, nullptr);
                else
                  libmesh_assert (!my_neigh);
                continue;
              }

            // If the sending processor has a remote_elem neighbor here,
            // then all we know is that we'd better *not* have a domain
            // boundary ... except that maybe it's the *sending*
            // processor who missed a delete_elem we saw.
            if (neighbor_id == remote_elem->id())
              {
                // At this level of the code we can't even assert in
                // cases where the neighbor should know what they're
                // talking about, so skip it.

                // libmesh_assert(elem->neighbor_ptr(n));
                continue;
              }

            Elem * neigh = mesh->query_elem_ptr(neighbor_id);

            // The sending processor sees a neighbor here, so if we
            // don't have that neighboring element, then we'd better
            // have a remote_elem signifying that fact.
            if (!neigh)
              {
                libmesh_assert_equal_to (elem->neighbor_ptr(n), remote_elem);
                continue;
              }

            // The sending processor has a neighbor here, and we have
            // that element, but that does *NOT* mean we're already
            // linking to it.  Perhaps we initially received both elem
            // and neigh from processors on which their mutual link was
            // remote?
            libmesh_assert(elem->neighbor_ptr(n) == neigh ||
                           elem->neighbor_ptr(n) == remote_elem);

            // If the link was originally remote, we should update it,
            // and make sure the appropriate parts of its family link
            // back to us.
            if (elem->neighbor_ptr(n) == remote_elem)
              {
                elem->set_neighbor(n, neigh);

                elem->make_links_to_me_local(n, neighbor_side);
              }
            else
              libmesh_assert(neigh->level() < elem->level() ||
                             neigh->neighbor_ptr(neighbor_side) == elem);
          }
      else
        // We skip these to go to the boundary information if the element is
        // actually subactive
        in += 2*elem->n_sides();

      // Our p level and refinement flags should be "close to" correct
      // if we're not an active element - we might have a p level
      // increased or decreased by changes in remote_elem children.
      //
      // But if we have remote_elem children, then we shouldn't be
      // doing a projection on this inactive element on this
      // processor, so we won't need correct p settings.  Couldn't
      // hurt to update, though.
#ifdef LIBMESH_ENABLE_AMR
      if (elem->processor_id() != mesh->processor_id())
        {
          // Do this simultaneously; otherwise we can get a false
          // positive when a hack_p_level or set_p_refineemnt_flag
          // assertion sees inconsistency between an old flag and new
          // value or vice-versa
          elem->hack_p_level_and_refinement_flag(p_level, p_refinement_flag);
        }
#endif // LIBMESH_ENABLE_AMR

      // FIXME: We should add some debug mode tests to ensure that the
      // encoded indexing and boundary conditions are consistent.
    }
  else
    {
      // We don't already have the element, so we need to create it.

      // Find the parent if necessary
      Elem * parent = nullptr;
#ifdef LIBMESH_ENABLE_AMR
      // Find a child element's parent
      if (level > 0)
        {
          // Note that we must be very careful to construct the send
          // connectivity so that parents are encountered before
          // children.  If we get here and can't find the parent that
          // is a fatal error.
          parent = mesh->elem_ptr(parent_id);
        }
      // Or assert that the sending processor sees no parent
      else
        libmesh_assert_equal_to (parent_id, DofObject::invalid_id);
#else
      // No non-level-0 elements without AMR
      libmesh_assert_equal_to (level, 0);
#endif

      elem = Elem::build(type,parent).release();
      libmesh_assert (elem);

#ifdef LIBMESH_ENABLE_AMR
      if (level != 0)
        {
          // Since this is a newly created element, the parent must
          // have previously thought of this child as a remote element.
          libmesh_assert_equal_to (parent->child_ptr(which_child_am_i), remote_elem);

          parent->add_child(elem, which_child_am_i);
        }

      // Assign the refinement flags and levels
      elem->set_p_level(p_level);
      elem->set_refinement_flag(refinement_flag);
      elem->set_p_refinement_flag(p_refinement_flag);
      libmesh_assert_equal_to (elem->level(), level);

      // If this element should have children, assign remote_elem to
      // all of them for now, for consistency.  Later unpacked
      // elements may overwrite that.
      if (has_children)
        {
          const unsigned int nc = elem->n_children();
          for (unsigned int c=0; c != nc; ++c)
            elem->add_child(const_cast<RemoteElem *>(remote_elem), c);
        }

#endif // LIBMESH_ENABLE_AMR

      // Assign the IDs
      elem->subdomain_id()  = subdomain_id;
      elem->processor_id()  = processor_id;
      elem->set_id()        = id;
#ifdef LIBMESH_ENABLE_UNIQUE_ID
      elem->set_unique_id(unique_id);
#endif

      // Assign the connectivity
      libmesh_assert_equal_to (elem->n_nodes(), n_nodes);

      for (unsigned int n=0; n != n_nodes; n++)
        elem->set_node (n, mesh->node_ptr
                        (cast_int<dof_id_type>(*in++)));

      // Set interior_parent if found
      {
        // We may be unpacking an element that was a ghost element on the
        // sender, in which case the element's interior_parent may not be
        // known by the packed element.  We'll have to set such
        // interior_parents to remote_elem ourselves and wait for a
        // later packed element to give us better information.
        if (interior_parent_id == remote_elem->id())
          {
            elem->set_interior_parent
              (const_cast<RemoteElem *>(remote_elem));
          }
        else if (interior_parent_id != DofObject::invalid_id)
          {
            // If we don't have the interior parent element, then it's
            // a remote_elem until we get it.
            Elem * ip =
              mesh->interior_mesh().query_elem_ptr(interior_parent_id);
            if (!ip )
              elem->set_interior_parent
                (const_cast<RemoteElem *>(remote_elem));
            else
              elem->set_interior_parent(ip);
          }
      }

      for (auto n : elem->side_index_range())
        {
          const dof_id_type neighbor_id =
            cast_int<dof_id_type>(*in++);

            const dof_id_type neighbor_side =
              cast_int<dof_id_type>(*in++);

          if (neighbor_id == DofObject::invalid_id)
            continue;

          // We may be unpacking an element that was a ghost element on the
          // sender, in which case the element's neighbors may not all be
          // known by the packed element.  We'll have to set such
          // neighbors to remote_elem ourselves and wait for a later
          // packed element to give us better information.
          if (neighbor_id == remote_elem->id())
            {
              elem->set_neighbor(n, const_cast<RemoteElem *>(remote_elem));
              continue;
            }

          // If we don't have the neighbor element, then it's a
          // remote_elem until we get it.
          Elem * neigh = mesh->query_elem_ptr(neighbor_id);
          if (!neigh)
            {
              elem->set_neighbor(n, const_cast<RemoteElem *>(remote_elem));
              continue;
            }

          // If we have the neighbor element, then link to it, and
          // make sure any appropriate parts of its family link back
          // to us.
          elem->set_neighbor(n, neigh);

          elem->make_links_to_me_local(n, neighbor_side);
        }

      elem->unpack_indexing(in);

      mesh->add_elem(elem);
    }

  in += elem->packed_indexing_size();

  // We check if this is cell holds a boundary ID or not
  auto on_boundary = *in++;
  if (on_boundary)
  {
    // Only treat the sides with caution. This is because we might hold boundary IDs
    // on the sides of the children. This is not supported for edges and shell faces, thus
    // they are treated assuming that only top parents can hold the IDs.
    auto children_on_boundary = *in++;
    if (elem->level() == 0 || children_on_boundary)
    {
      for (auto s : elem->side_index_range())
        {
          const boundary_id_type num_bcs =
            cast_int<boundary_id_type>(*in++);

          for (boundary_id_type bc_it=0; bc_it < num_bcs; bc_it++)
            mesh->get_boundary_info().add_side
              (elem, s, cast_int<boundary_id_type>(*in++));
        }
    }
  }

  // If this is a coarse element,
  // add any element side or edge boundary condition ids
  if (level == 0)
    {
      for (auto e : elem->edge_index_range())
        {
          const boundary_id_type num_bcs =
            cast_int<boundary_id_type>(*in++);

          for (boundary_id_type bc_it=0; bc_it < num_bcs; bc_it++)
            mesh->get_boundary_info().add_edge
              (elem, e, cast_int<boundary_id_type>(*in++));
        }

      for (unsigned short sf=0; sf != 2; ++sf)
        {
          const boundary_id_type num_bcs =
            cast_int<boundary_id_type>(*in++);

          for (boundary_id_type bc_it=0; bc_it < num_bcs; bc_it++)
            mesh->get_boundary_info().add_shellface
              (elem, sf, cast_int<boundary_id_type>(*in++));
        }
    }

  // Return the new element
  return elem;
}



template <>
Elem *
Packing<Elem *>::unpack (std::vector<largest_id_type>::const_iterator in,
                         DistributedMesh * mesh)
{
  return unpack(in, static_cast<MeshBase*>(mesh));
}



template <>
Elem *
Packing<Elem *>::unpack (std::vector<largest_id_type>::const_iterator in,
                         ParallelMesh * mesh)
{
  return unpack(in, static_cast<MeshBase*>(mesh));
}

} // namespace Parallel

} // namespace libMesh