fro_io.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
#include <fstream>
#include <iomanip>
#include <iostream>
#include <deque>
#include <map>

// Local includes
#include "libmesh/libmesh_config.h"
#include "libmesh/fro_io.h"
#include "libmesh/mesh_base.h"
#include "libmesh/boundary_info.h"
#include "libmesh/elem.h"

namespace libMesh
{



// ------------------------------------------------------------
// FroIO  members
void FroIO::write (const std::string & fname)
{
  // We may need to gather a DistributedMesh to output it, making that
  // const qualifier in our constructor a dirty lie
  MeshSerializer serialize(const_cast<MeshBase &>(this->mesh()), !_is_parallel_format);

  if (this->mesh().processor_id() == 0)
    {
      // Open the output file stream
      std::ofstream out_stream (fname.c_str());
      libmesh_assert (out_stream.good());

      // Make sure it opened correctly
      if (!out_stream.good())
        libmesh_file_error(fname.c_str());

      // Get a reference to the mesh
      const MeshBase & the_mesh = MeshOutput<MeshBase>::mesh();

      // Write the header
      out_stream << the_mesh.n_elem()  << " "
                 << the_mesh.n_nodes() << " "
                 << "0 0 "
                 << the_mesh.get_boundary_info().n_boundary_ids()  << " 1\n";

      // Write the nodes -- 1-based!
      for (auto n : make_range(the_mesh.n_nodes()))
        out_stream << n+1 << " \t"
                   << std::scientific
                   << std::setprecision(this->ascii_precision())
                   << the_mesh.point(n)(0) << " \t"
                   << the_mesh.point(n)(1) << " \t"
                   << 0. << '\n';

      // Write the elements -- 1-based!
      unsigned int e = 0;
      for (const auto & elem : the_mesh.active_element_ptr_range())
        {
          // .fro likes TRI3's
          libmesh_error_msg_if(elem->type() != TRI3,
                               "ERROR:  .fro format only valid for triangles!\n"
                               << "  writing of " << fname << " aborted.");

          out_stream << ++e << " \t";

          for (const Node & node : elem->node_ref_range())
            out_stream << node.id()+1 << " \t";

          //   // LHS -> RHS Mapping, for inverted triangles
          //   out_stream << elem->node_id(0)+1 << " \t";
          //   out_stream << elem->node_id(2)+1 << " \t";
          //   out_stream << elem->node_id(1)+1 << " \t";

          out_stream << "1\n";
        }

      // Write BCs.
      {
        const std::set<boundary_id_type> & bc_ids =
          the_mesh.get_boundary_info().get_boundary_ids();

        // Build a list of (elem, side, bc) tuples.
        auto bc_triples = the_mesh.get_boundary_info().build_side_list();

        // Map the boundary ids into [1,n_bc_ids],
        // treat them one at a time.
        boundary_id_type bc_id=0;
        for (const auto & id : bc_ids)
          {
            std::deque<dof_id_type> node_list;

            std::map<dof_id_type, dof_id_type>
              forward_edges, backward_edges;

            // Get all sides on this element with the relevant BC id.
            for (const auto & t : bc_triples)
              if (std::get<2>(t) == id)
                {
                  // need to build up node_list as a sorted array of edge nodes...
                  // for the following:
                  // a---b---c---d---e
                  // node_list [ a b c d e];
                  //
                  // the issue is just how to get this out of the elem/side based data structure.
                  // the approach is to build up 'chain links' like this:
                  // a---b b---c c---d d---e
                  // and piece them together.
                  //
                  // so, for an arbitrary edge n0---n1, we build the
                  // "forward_edges"  map n0-->n1
                  // "backward_edges" map n1-->n0
                  // and then start with one chain link, and add on...
                  //
                  std::unique_ptr<const Elem> side =
                    the_mesh.elem_ref(std::get<0>(t)).build_side_ptr(std::get<1>(t));

                  const dof_id_type
                    n0 = side->node_id(0),
                    n1 = side->node_id(1);

                  // insert into forward-edge set
                  forward_edges.emplace(n0, n1);

                  // insert into backward-edge set
                  backward_edges.emplace(n1, n0);

                  // go ahead and add one edge to the list -- this will give us the beginning of a
                  // chain to work from!
                  if (node_list.empty())
                    {
                      node_list.push_front(n0);
                      node_list.push_back (n1);
                    }
                }

            // we now have the node_list with one edge, the forward_edges, and the backward_edges
            // the node_list will be filled when (node_list.size() == (n_edges+1))
            // until that is the case simply add on to the beginning and end of the node_list,
            // building up a chain of ordered nodes...
            const std::size_t n_edges = forward_edges.size();

            while (node_list.size() != (n_edges+1))
              {
                const dof_id_type
                  front_node = node_list.front(),
                  back_node  = node_list.back();

                // look for front_pair in the backward_edges list
                if (const auto pos = backward_edges.find(front_node);
                    pos != backward_edges.end())
                  {
                    node_list.push_front(pos->second);
                    backward_edges.erase(pos);
                  }

                // look for back_pair in the forward_edges list
                if (const auto pos = forward_edges.find(back_node);
                    pos != forward_edges.end())
                  {
                    node_list.push_back(pos->second);
                    forward_edges.erase(pos);
                  }
              }

            out_stream << ++bc_id << " " << node_list.size() << '\n';

            for (const auto & node_id : node_list)
              out_stream << node_id + 1 << " \t0\n";
          }
      }
    }
}

} // namespace libMesh