face_polygon.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

#include "libmesh/face_polygon.h"

// Local includes
#include "libmesh/edge_edge2.h"
#include "libmesh/enum_elem_quality.h"
#include "libmesh/hashword.h"

// C++ includes
#include <array>


namespace libMesh
{

// ------------------------------------------------------------
// Polygon class static member initializations

const int Polygon::num_children;

// ------------------------------------------------------------
// Polygon class member functions


Polygon::Polygon (const unsigned int nn,
                  const unsigned int ns,
                  Elem * p) :
  Face(nn, ns, p, nullptr, nullptr),
  _elemlinks_data(ns+2), // neighbors + parent + interior_parent
  _nodelinks_data(nn)
{
  // Do the manual initialization that Elem::Elem couldn't.  No need
  // to manually set nullptr, though, since std::vector does that.
  this->_elemlinks = _elemlinks_data.data();
  this->_nodes = _nodelinks_data.data();
  this->_elemlinks[0] = p;

  // Is this likely to ever be used?  We may do refinement with
  // polygons but it's probably not going to have a hierarchy...
  if (p)
    {
      this->subdomain_id() = p->subdomain_id();
      this->processor_id() = p->processor_id();
      _map_type = p->mapping_type();
      _map_data = p->mapping_data();

#ifdef LIBMESH_ENABLE_AMR
      this->set_p_level(p->p_level());
#endif
    }

  // Make sure the interior parent isn't undefined
  if (LIBMESH_DIM > 2)
    this->set_interior_parent(nullptr);
}



Point Polygon::master_point (const unsigned int i) const
{
  const unsigned int ns = this->n_sides();

  // Non-vertex points need to be handled by a subclass overriding
  // this.
  if (i > ns)
    libmesh_not_implemented();

  // Center-to-midside to center-to-vertex angle
  const Real pi_over_ns = libMesh::pi / ns;

  // Center-to-midside distance
  const Real min_r = 0.5/tan(pi_over_ns);

  // Center-to-vertex distance
  const Real max_r = std::sqrt(min_r*min_r + 0.25);

  const Point center(0.5, min_r);

  libmesh_assert_less(i, this->n_nodes());

  return center + Point(max_r*sin((int(i)*2-1)*pi_over_ns),
                        -max_r*cos((int(i)*2-1)*pi_over_ns));

}



bool Polygon::on_reference_element(const Point & p,
                                   const Real eps) const
{
  const unsigned int ns = this->n_sides();

  // Center-to-midside to center-to-vertex angle
  const Real pi_over_ns = libMesh::pi / ns;

  // Center-to-midside distance
  const Real min_r = 0.5/tan(pi_over_ns);

  // Center-to-vertex distance
  const Real max_r = std::sqrt(min_r*min_r + 0.25);

  const Point center(0.5, min_r);

  // Check that the point is on the same side of all the faces by
  // testing whether:
  //
  // n_i.(p - x_i) <= 0
  //
  // for each i, where:
  //   n_i is the outward normal of face i,
  //   x_i is a point on face i.

  for (auto i : make_range(ns))
    {
      const Point x_i =
        center + Point(max_r*sin((int(i)*2-1)*pi_over_ns),
                       -max_r*cos((int(i)*2-1)*pi_over_ns));
      const Point n_i =
        Point(sin(int(i)*2*pi_over_ns),
              -cos(int(i)*2*pi_over_ns));

      if (n_i * (p - x_i) > eps)
        return false;
    }

  return true;
}



dof_id_type Polygon::key (const unsigned int s) const
{
  const int ns = this->n_sides();
  libmesh_assert_less (s, ns);

  return this->compute_key(this->node_id(s),
                           this->node_id((s+1)%ns));
}



dof_id_type Polygon::low_order_key (const unsigned int s) const
{
  const int ns = this->n_sides();
  libmesh_assert_less (s, ns);

  return this->compute_key(this->node_id(s),
                           this->node_id((s+1)%ns));
}



unsigned int Polygon::local_side_node(unsigned int side,
                                      unsigned int side_node) const
{
  const auto ns = this->n_sides();
  libmesh_assert_less (side, ns);
  libmesh_assert_less (side_node, this->n_nodes_per_side());

  if (!side_node)
    return side;

  if (side_node == 1)
    return (side + 1) % ns;

  return (side_node - 1) * ns + side;
}



unsigned int Polygon::local_edge_node(unsigned int edge,
                                      unsigned int edge_node) const
{
  return local_side_node(edge, edge_node);
}



dof_id_type Polygon::key () const
{
  std::vector<dof_id_type> node_ids;
  for (const auto & n : this->node_ref_range())
    node_ids.push_back(n.id());

  return Utility::hashword(node_ids);
}



std::unique_ptr<Elem> Polygon::side_ptr (const unsigned int i)
{
  const auto ns = this->n_sides();
  libmesh_assert_less (i, ns);

  std::unique_ptr<Elem> edge = std::make_unique<Edge2>();

  edge->set_node(0, this->node_ptr(i));
  edge->set_node(1, this->node_ptr((i+1)%ns));

  return edge;
}



void Polygon::side_ptr (std::unique_ptr<Elem> & side,
                        const unsigned int i)
{
  const auto ns = this->n_sides();
  libmesh_assert_less (i, ns);

  if (!side.get() || side->type() != EDGE2)
    side = std::make_unique<Edge2>();

  side->subdomain_id() = this->subdomain_id();

  side->set_node(0, this->node_ptr(i));
  side->set_node(1, this->node_ptr((i+1)%ns));
}



bool Polygon::is_child_on_side(const unsigned int /*c*/,
                               const unsigned int /*s*/) const
{
  libmesh_not_implemented();
  return false;
}



unsigned int Polygon::opposite_side(const unsigned int side_in) const
{
  const auto ns = this->n_sides();
  if (ns % 2)
    libmesh_error();

  return (ns / 2 + side_in) % ns;
}



bool Polygon::is_flipped() const
{
  // Just check based on vertices, the first ns nodes
  const auto ns = this->n_sides();

#if LIBMESH_DIM > 2
  // Don't bother outside the XY plane
  for (auto n : make_range(ns))
    if (this->point(n)(2))
      return false;
#endif

  Real twice_signed_area = 0;
  for (auto n : make_range(ns))
    {
      const Point & p1 = this->point(n);
      const Point & p2 = this->point((n+1)%ns);
      twice_signed_area += p1(0)*p2(1)-p1(1)*p2(0);
    }

  return (twice_signed_area < 0);
}


std::vector<unsigned int>
Polygon::edges_adjacent_to_node(const unsigned int n) const
{
  libmesh_assert_less(n, this->n_nodes());

  // For mid-face nodes, the subclass had better have overridden this.
  libmesh_assert_less(n, this->n_sides() * (this->n_nodes_per_side() + 1));

  const unsigned int ns = this->n_sides();

  // For vertices, we have two adjacent edges; otherwise each of the
  // mid-edge nodes is adjacent only to the edge it is on.
  if (this->is_vertex(n))
    return {n, (n+ns-1)%ns};

  return {n % ns};
}


std::pair<Real, Real> Polygon::qual_bounds (const ElemQuality q) const
{
  std::pair<Real, Real> bounds;

  switch (q)
    {
    case EDGE_LENGTH_RATIO:
      bounds.first  = 1.;
      bounds.second = 4.;
      break;

    case MIN_ANGLE:
      bounds.first  = 90. - (180./this->n_sides());
      bounds.second = 180. - (360./this->n_sides());
      break;

    case MAX_ANGLE:
      bounds.first  = 180. - (360./this->n_sides());
      bounds.second = 180. - (180./this->n_sides());
      break;

    case JACOBIAN:
    case SCALED_JACOBIAN:
      bounds.first  = 0.5;
      bounds.second = 1.;
      break;

    default:
      libMesh::out << "Warning: Invalid quality measure chosen." << std::endl;
      bounds.first  = -1;
      bounds.second = -1;
    }

  return bounds;
}


std::array<Point, 3> Polygon::master_subtriangle (unsigned int i) const
{
  libmesh_assert_less(i, this->_triangulation.size());

  const auto & tri = this->_triangulation[i];

  return { this->master_point(tri[0]),
           this->master_point(tri[1]),
           this->master_point(tri[2]) };
}


std::tuple<unsigned int, Real, Real>
Polygon::subtriangle_coordinates (const Point & p, Real tol) const
{
  std::tuple<unsigned int, Real, Real> returnval = {libMesh::invalid_uint, -1, -1};

  Real best_bad_coord = -1;

  for (auto s : make_range(this->n_subtriangles()))
    {
      const std::array<Point, 3> subtri =
        this->master_subtriangle (s);

      // Find barycentric coordinates in subtri.
      // Hand coding these since we don't need a full 3D cross product
      // in 2D
      const Real twice_area = (- subtri[1](1) * subtri[2](0)
                               - subtri[0](1) * subtri[1](0)
                               + subtri[0](1) * subtri[2](0)
                               + subtri[0](0) * subtri[1](1)
                               - subtri[0](0) * subtri[2](1)
                               + subtri[1](0) * subtri[2](1));

      const Real xi =  (  subtri[0](1)*subtri[2](0)
                        - subtri[0](0)*subtri[2](1)
                        + (subtri[2](1)-subtri[0](1))*p(0)
                        + (subtri[0](0)-subtri[2](0))*p(1)) / twice_area;

      const Real eta = (  subtri[0](0)*subtri[1](1)
                        - subtri[0](1)*subtri[1](0)
                        + (subtri[0](1)-subtri[1](1))*p(0)
                        + (subtri[1](0)-subtri[0](0))*p(1)) / twice_area;

      if (xi>=0 && eta>=0 && xi+eta<=1)
        return { s, xi, eta };

      Real my_best_bad_coord = std::min(std::min(xi, eta), 1-xi-eta);

      if (my_best_bad_coord > best_bad_coord)
        {
          best_bad_coord = my_best_bad_coord;
          returnval = { s, xi, eta };
        }
    }

  if (best_bad_coord > -tol)
    return returnval;

  return {libMesh::invalid_uint, -1, -1};
}


} // namespace libMesh