[deal.II] Getting global DoF indices of a face

[Alexander Kiselyov]

Dear deal.II users,

I'm currently developing a way to uniquely identify a boundary face on
a mesh. My idea has been to gather global DoF indices on a face and
hash them together. However, `get_dof_indices` method of a face
accessor doesn't return any useful information, and
`FESystem::max_dofs_per_face()`, surprisingly, returns 0. See the
attachment for a minimal example. The problem persists when running on
a single process. The version of deal.II used is 9.4.0.

What am I doing wrong here? Is there a better way to globally identify
a face?

Best regards,
Alexander Kiselyov

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#include <deal.II/base/mpi.h>

#include <deal.II/distributed/tria.h>

#include <deal.II/dofs/dof_handler.h>
#include <deal.II/dofs/dof_renumbering.h>

#include <deal.II/fe/fe_dgq.h>
#include <deal.II/fe/fe_system.h>

#include <deal.II/grid/grid_generator.h>

#include <mpi.h>
#include <iostream>
#include <vector>


constexpr int dim = 3;


struct BoundarySolverFixture
{
  BoundarySolverFixture()
    : comm(MPI_COMM_WORLD), tria(comm), dof_handler(tria)
    , fe_system(dealii::FE_DGQ<dim>(degree), 1, dealii::FESystem(dealii::FE_DGQ<dim>(degree), dim), 1)
  {
    dealii::GridGenerator::subdivided_hyper_cube(tria, cell_num, 0., 1.);
    dof_handler.distribute_dofs(fe_system);
    dealii::DoFRenumbering::block_wise(dof_handler);
  }

  static const auto cell_num = 5;
  static const auto degree = 1;

  MPI_Comm comm;
  dealii::parallel::distributed::Triangulation<dim> tria;
  dealii::DoFHandler<dim> dof_handler;
  dealii::FESystem<dim> fe_system;
};


int main(int argc, char** argv)
{
  dealii::Utilities::MPI::MPI_InitFinalize _mpi(argc, argv, 1);

  BoundarySolverFixture fixture;

  const auto max_dofs = fixture.fe_system.max_dofs_per_face();
  std::cout << "Max dofs: " << max_dofs << std::endl;
  for (const auto& cell : fixture.dof_handler.active_cell_iterators()) {
    if (!cell->is_locally_owned())
      continue;

    for (const auto& face : cell->face_iterators()) {
      if (!face->at_boundary())
        continue;

      std::vector<dealii::types::global_dof_index> dofs((max_dofs != 0) ? max_dofs : 4);
      face->get_dof_indices(dofs);
      std::cout << "DoF indices for this face: ";
      for (const auto d : dofs)
        std::cout << ' ' << d;
      std::cout << std::endl;
    }
  }

  return 0;
}