// Copyright (c) 2019-2023  GeometryFactory Sarl (France).
// All rights reserved.
//
// This file is part of CGAL (www.cgal.org).
// You can redistribute it and/or modify it under the terms of the GNU
// General Public License as published by the Free Software Foundation,
// either version 3 of the License, or (at your option) any later version.
//
// Licensees holding a valid commercial license may use this file in
// accordance with the commercial license agreement provided with the software.
//
// This file is provided AS IS with NO WARRANTY OF ANY KIND, INCLUDING THE
// WARRANTY OF DESIGN, MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
//
// $URL$
// $Id$
// SPDX-License-Identifier: GPL-3.0-or-later OR LicenseRef-Commercial
//
// Author(s)     : Laurent Rineau

#ifndef CGAL_CONSTRAINED_DELAUNAY_TRIANGULATION_3_H
#define CGAL_CONSTRAINED_DELAUNAY_TRIANGULATION_3_H

#include <CGAL/Triangulation_3/internal/CDT_3_config.h>

#include <CGAL/license/Constrained_triangulation_3.h>

#include <CGAL/Base_with_time_stamp.h>
#include <CGAL/Triangulation_vertex_base_3.h>
#include <CGAL/Triangulation_cell_base_3.h>
#include <CGAL/Triangulation_vertex_base_with_info_2.h>
#include <CGAL/Triangulation_face_base_with_info_2.h>
#include <CGAL/Base_with_time_stamp.h>
#include <CGAL/Constrained_Delaunay_triangulation_2.h>
#include <CGAL/Constrained_triangulation_plus_2.h>
#include <CGAL/Projection_traits_3.h>
#include <CGAL/Union_find.h>
#include <CGAL/intersection_3.h>
#include <CGAL/iterator.h>

#include <CGAL/boost/graph/Dual.h>
#include <CGAL/boost/graph/generators.h>
#include <CGAL/boost/graph/graph_traits_Triangulation_data_structure_2.h>
#include <CGAL/boost/graph/graph_traits_Constrained_Delaunay_triangulation_2.h>

#include <CGAL/Compact_container.h>

#include <CGAL/cdt_debug_io.h>
#include <CGAL/Mesh_3/io_signature.h>

#include <CGAL/Conforming_Delaunay_triangulation_3.h>

#include <boost/graph/filtered_graph.hpp>
#include <boost/graph/breadth_first_search.hpp>
#include <boost/dynamic_bitset.hpp>

#include <boost/container/flat_set.hpp>
#include <boost/unordered_map.hpp>
#include <boost/container/small_vector.hpp>
#include <boost/iterator/function_output_iterator.hpp>

#include <algorithm>
#include <optional>
#include <unordered_map>
#include <ranges>
#if __has_include(<format>)
#  include <format>
#  include <concepts>
#elif CGAL_DEBUG_CDT_3
#  error "Compiler needs <format>"
#endif

#if DOXYGEN_RUNNING

namespace CGAL {

  /*!
   * \brief The class Constrained_Delaunay_triangulation_3 represents a 3D constrained Delaunay triangulation.
   *
   * This class is derived from the Triangulation_3 class and provides additional functionality for handling
   * polygonal constraints during the triangulation process.
   *
   * \todo Explain what is a CDT. Why a given input, like a polyhedron, might not admit a constrained Delaunay triangulation.
   * Explain "conforming to the faces of a polygon mesh".
   *
   * \tparam Traits is the geometric traits class and must be a model of `ConstrainedDelaunayTriangulationTraits_3`.
   * \tparam Triangulation_3 is the base triangulation class.
   *         It must be a an instance of the `Triangulation_3` class template with the same `Traits` template parameter.
   *         Its `Vertex` type must be a model of `ConstrainedDelaunayTriangulationVertexBase_3`,
   *         and its `Cell` type must be a model of `ConstrainedDelaunayTriangulationCellBase_3`.
   *         <br>
   *         The default value is `Triangulation_3<Traits, TDS>` where `TDS` is
   *         `Triangulation_data_structure_3<Constrained_Delaunay_triangulation_vertex_base_3<Traits>, Constrained_Delaunay_triangulation_cell_base_3<Traits>>`.
   */
  template <typename Traits, typename Triangulation_3>
  class Constrained_Delaunay_triangulation_3 : public Triangulation_3 {
  public:
    /*!
     * \brief Create a 3D constrained Delaunay triangulation conforming to the faces of a polygon mesh.
     *
     * This constructor initializes a Constrained_Delaunay_triangulation_3 object using a polygon mesh.
     * The polygon mesh represents the polygonal constraints that will be enforced during the triangulation process.
     *
     * By default, each face of the polygon mesh is considered as a polygonal constraint for the triangulation. The
     * named parameter `face_patch_map` can be used to describe bigger polygonal constraints, possibly with holes. If
     * used, the argument of that parameter must be a property map that maps each face of the polygon mesh to a patch
     * identifier. Faces with the same patch identifier are considered as part of the same surface patch. Each of those
     * surface patches (defined as the union of the mesh faces with a given patch id) is supposed to be a polygon or a
     * polygon with holes, with coplanar vertices (or almost coplanar up to the precision of the number type used).
     *
     * The triangulation will be constrained to conform to the faces of the polygon mesh, or to the surface patches
     * described by the `face_patch_map` property map if provided.
     *
     * \tparam PolygonMesh a model of `FaceListGraph`
     * \tparam NamedParameters a sequence of \ref bgl_namedparameters "Named Parameters"
     *
     * \param mesh The polygon mesh representing the constraints.
     * \param np an optional sequence of \ref bgl_namedparameters "Named Parameters" among the ones listed below
     *
     * \cgalNamedParamsBegin
     *   \cgalParamNBegin{vertex_point_map}
     *     \cgalParamDescription{a property map associating points to the vertices of `mesh`}
     *     \cgalParamType{a class model of `ReadWritePropertyMap` with `boost::graph_traits<PolygonMesh>::%vertex_descriptor`
     *                    as key type and `%Traits::Point_3` as value type}
     *     \cgalParamDefault{`boost::get(CGAL::vertex_point, mesh)`}
     *     \cgalParamExtra{If this parameter is omitted, an internal property map for `CGAL::vertex_point_t`
     *                     must be available in `PolygonMesh`.}
     *   \cgalParamNEnd
     *   \cgalParamNBegin{geom_traits}
     *     \cgalParamDescription{an instance of a geometric traits class}
     *     \cgalParamType{`Traits`}
     *     \cgalParamDefault{the default constructed traits object `Traits{}`}
     *   \cgalParamNEnd
     *   \cgalParamNBegin{face_patch_map}
     *    \cgalParamDescription{a property map associating a patch identifier to each face of `mesh`}
     *    \cgalParamType{a class model of `ReadWritePropertyMap` with `boost::graph_traits<PolygonMesh>::%face_descriptor`
     *                   as key type and with any value type that is a *regular* type}
     *   \cgalParamExtra{If this parameter is omitted, each face of the mesh is considered as a separate patch.}
     *   \cgalParamExtra{Faces with the same patch identifier are considered as part of the same surface patch.}
     *   \cgalParamNEnd
     * \cgalNamedParamsEnd
     *
     * \todo Create a documentation page to describe the concept *regular*, and link it to https://en.cppreference.com/w/cpp/concepts/regular
     */
    template <typename PolygonMesh, typename NamedParams = parameters::Default_named_parameters>
    Constrained_Delaunay_triangulation_3(const PolygonMesh& mesh, const NamedParams& np = parameters::default_values())
    {

      // Implementation of the constructor with variadic named parameters
      // ...
    }

  };

} // end namespace CGAL

#else // not DOXYGEN_RUNNING

namespace CGAL {

template <class K>
typename K::Boolean
does_first_triangle_intersect_second_triangle_interior(const typename K::Triangle_3& t1,
                                                       const typename K::Triangle_3& t2,
                                                       const K& k)
{
  typedef typename K::Point_3 Point_3;

  CGAL_kernel_precondition(!k.is_degenerate_3_object() (t1) );
  CGAL_kernel_precondition(!k.is_degenerate_3_object() (t2) );

  typename K::Construct_vertex_3 vertex_on = k.construct_vertex_3_object();
  typename K::Orientation_3 orientation = k.orientation_3_object();

  const Point_3& p = vertex_on(t1,0);
  const Point_3& q = vertex_on(t1,1);
  const Point_3& r = vertex_on(t1,2);
  const Point_3& a = vertex_on(t2,0);
  const Point_3& b = vertex_on(t2,1);
  const Point_3& c = vertex_on(t2,2);

  // TODO: use boost::containers::static_vector
  std::array<const Point_3*,3> t1_vertices_in_the_line;
  int nb_of_t1_vertices_in_the_line = 0;
  auto push_to_t1_vertices_in_the_line = [&t1_vertices_in_the_line, &nb_of_t1_vertices_in_the_line](const Point_3* p) {
    t1_vertices_in_the_line.at(nb_of_t1_vertices_in_the_line++) = p;
  };

  std::array<const Point_3*,3> t2_vertices_in_the_line;
  int nb_of_t2_vertices_in_the_line = 0;
  auto push_to_t2_vertices_in_the_line = [&t2_vertices_in_the_line, &nb_of_t2_vertices_in_the_line](const Point_3* p) {
    t2_vertices_in_the_line.at(nb_of_t2_vertices_in_the_line++) = p;
  };

  const Point_3* s_min1;
  const Point_3* t_min1;
  const Point_3* s_max1;
  const Point_3* t_max1;

  // Compute distance signs  of p, q and r to the plane of triangle(a,b,c)
  const Orientation dp = make_certain(orientation(a,b,c,p));
  const Orientation dq = make_certain(orientation(a,b,c,q));
  const Orientation dr = make_certain(orientation(a,b,c,r));

  if(dp == COPLANAR) push_to_t1_vertices_in_the_line(&p);
  if(dq == COPLANAR) push_to_t1_vertices_in_the_line(&q);
  if(dr == COPLANAR) push_to_t1_vertices_in_the_line(&r);

  auto comp = k.compare_xyz_3_object();
  auto sort_ptrs = [&comp](const Point_3* p1, const Point_3* p2) { return comp(*p1, *p2) == SMALLER; };
  auto intersection_is_a_vertex_or_a_common_edge = [&]() {
    std::sort(t1_vertices_in_the_line.data(), t1_vertices_in_the_line.data() + nb_of_t1_vertices_in_the_line,
              sort_ptrs);
    std::sort(t2_vertices_in_the_line.data(), t2_vertices_in_the_line.data() + nb_of_t2_vertices_in_the_line,
              sort_ptrs);
    std::size_t nb_of_common_vertices = 0;
    std::set_intersection(
        t1_vertices_in_the_line.data(), t1_vertices_in_the_line.data() + nb_of_t1_vertices_in_the_line,
        t2_vertices_in_the_line.data(), t2_vertices_in_the_line.data() + nb_of_t2_vertices_in_the_line,
        CGAL::Counting_output_iterator(&nb_of_common_vertices), sort_ptrs);
    return nb_of_common_vertices == 1 || nb_of_common_vertices == 2;
  };

  switch(dp)
  {
    case POSITIVE:
      if(dq == POSITIVE)
      {
        if(dr == POSITIVE)
          // pqr lies in the open positive halfspace induced by
          // the plane of triangle(a,b,c)
          return false;
        // r is isolated on the negative side of the plane
        s_min1 = &q; t_min1 = &r; s_max1 = &r; t_max1 = &p;
      }
      else
      {
        if(dr == POSITIVE)
        {
          // q is isolated on the negative side of the plane
          s_min1 =  &p; t_min1 =  &q; s_max1 =  &q; t_max1 =  &r;
        }
        else
        {
          // p is isolated on the positive side of the plane
          s_min1 =  &p; t_min1 =  &q; s_max1 =  &r; t_max1 =  &p;
        }
      }
      break;

    case NEGATIVE:
      if(dq == NEGATIVE)
      {
        if(dr == NEGATIVE)
          // pqr lies in the open negative halfspace induced by
          // the plane of triangle(a,b,c)
          return false;
        // r is isolated on the positive side of the plane
        s_min1 =  &r; t_min1 =  &p; s_max1 =  &q; t_max1 =  &r;

      }
      else
      {
        if(dr == NEGATIVE)
        {
          // q is isolated on the positive side of the plane
          s_min1 =  &q; t_min1 =  &r; s_max1 =  &p; t_max1 =  &q;
        } else {
          // p is isolated on the negative side of the plane
          s_min1 =  &r; t_min1 =  &p; s_max1 =  &p; t_max1 =  &q;
        }
      }
      break;

    case COPLANAR:
      switch(dq)
      {
        case POSITIVE:
          if(dr == POSITIVE )
          {
            // p is isolated on the negative side of the plane
            s_min1 =  &r; t_min1 =  &p; s_max1 =  &p; t_max1 =  &q;
          }
          else
          {
            // q is isolated on the positive side of the plane
            s_min1 =  &q; t_min1 =  &r; s_max1 =  &p; t_max1 =  &q;
          }
          break;

        case NEGATIVE:
          if(dr == NEGATIVE )
          {
            // p is isolated on the positive side of the plane
            s_min1 =  &p; t_min1 =  &q; s_max1 =  &r; t_max1 =  &p;
          }
          else
          {
            // q is isolated on the negative side of the plane
            s_min1 =  &p; t_min1 =  &q; s_max1 =  &q; t_max1 =  &r;
          }
          break;

        case COPLANAR:
          switch(dr)
          {
            case POSITIVE:
              // r is isolated on the positive side of the plane
              s_min1 =  &r; t_min1 =  &p; s_max1 =  &q; t_max1 =  &r;
              break;

            case NEGATIVE:
              // r is isolated on the negative side of the plane
              s_min1 =  &q; t_min1 =  &r; s_max1 =  &r; t_max1 =  &p;
              break;

            case COPLANAR: {
              push_to_t2_vertices_in_the_line(&a);
              push_to_t2_vertices_in_the_line(&b);
              push_to_t2_vertices_in_the_line(&c);
              if(intersection_is_a_vertex_or_a_common_edge()) return false;
              return CGAL::Intersections::internal::do_intersect_coplanar(t1,t2,k);
            }
            default: // should not happen.
              CGAL_kernel_assertion(false);
              return false;
          }
          break;

        default: // should not happen.
          CGAL_kernel_assertion(false);
          return false;
      }
      break;

    default: // should not happen.
      CGAL_kernel_assertion(false);
      return false;
  }

  const Point_3* s_min2;
  const Point_3* t_min2;
  const Point_3* s_max2;
  const Point_3* t_max2;

  // Compute distance signs  of a, b and c to the plane of triangle(p,q,r)
  const Orientation da = make_certain(orientation(p,q,r,a));
  const Orientation db = make_certain(orientation(p,q,r,b));
  const Orientation dc = make_certain(orientation(p,q,r,c));

  if(da == COPLANAR) push_to_t2_vertices_in_the_line(&a);
  if(db == COPLANAR) push_to_t2_vertices_in_the_line(&b);
  if(dc == COPLANAR) push_to_t2_vertices_in_the_line(&c);

  if(intersection_is_a_vertex_or_a_common_edge()) return false;

  switch(da)
  {
    case POSITIVE:
      if(db == POSITIVE)
      {
        if(dc == POSITIVE)
          // abc lies in the open positive halfspace induced by
          // the plane of triangle(p,q,r)
          return false;
        // c is isolated on the negative side of the plane
        s_min2 =  &b; t_min2 =  &c; s_max2 =  &c; t_max2 =  &a;
      }
      else
      {
        if(dc == POSITIVE)
        {
          // b is isolated on the negative side of the plane
          s_min2 =  &a; t_min2 =  &b; s_max2 =  &b; t_max2 =  &c;
        }
        else
        {
          // a is isolated on the positive side of the plane
          s_min2 =  &a; t_min2 =  &b; s_max2 =  &c; t_max2 =  &a;
        }
      }
      break;

    case NEGATIVE:
      if(db == NEGATIVE)
      {
        if(dc == NEGATIVE)
          // abc lies in the open negative halfspace induced by
          // the plane of triangle(p,q,r)
          return false;
        // c is isolated on the positive side of the plane
        s_min2 =  &c; t_min2 =  &a; s_max2 =  &b; t_max2 =  &c;

      }
      else
      {
        if(dc == NEGATIVE)
        {
          // b is isolated on the positive side of the plane
          s_min2 =  &b; t_min2 =  &c; s_max2 =  &a; t_max2 =  &b;
        } else {
          // a is isolated on the negative side of the plane
          s_min2 =  &c; t_min2 =  &a; s_max2 =  &a; t_max2 =  &b;
        }
      }
      break;

    case COPLANAR:
      switch(db)
      {
        case POSITIVE:
          if(dc == POSITIVE)
          {
            // a is isolated on the negative side of the plane
            s_min2 =  &c; t_min2 =  &a; s_max2 =  &a; t_max2 =  &b;
          }
          else
          {
            // b is isolated on the positive side of the plane
            s_min2 =  &b; t_min2 =  &c; s_max2 =  &a; t_max2 =  &b;
          }
          break;

        case NEGATIVE:
          if(dc == NEGATIVE)
          {
            // a is isolated on the positive side of the plane
            s_min2 =  &a; t_min2 =  &b; s_max2 =  &c; t_max2 =  &a;
          }
          else
          {
            // b is isolated on the negative side of the plane
            s_min2 =  &a; t_min2 =  &b; s_max2 =  &b; t_max2 =  &c;
          }
          break;

        case COPLANAR:
          switch(dc)
          {
            case POSITIVE:
              // c is isolated on the positive side of the plane
              s_min2 =  &c; t_min2 =  &a; s_max2 =  &b; t_max2 =  &c;
              break;

            case NEGATIVE:
              // c is isolated on the negative side of the plane
              s_min2 =  &b; t_min2 =  &c; s_max2 =  &c; t_max2 =  &a;
              break;

            case COPLANAR:
              // Supposed unreachable code
              // since the triangles are assumed to be non-flat

              return CGAL::Intersections::internal::do_intersect_coplanar(t1,t2,k);
            default: // should not happen.
              CGAL_kernel_assertion(false);
              return false;
          }
          break;

        default: // should not happen.
          CGAL_kernel_assertion(false);
          return false;
      }
      break;

    default: // should not happen.
      CGAL_kernel_assertion(false);
      return false;
  }

  return orientation(*s_min1, *t_min1, *s_min2, *t_min2) == NEGATIVE &&
         orientation(*s_max1, *t_max1, *t_max2, *s_max2) == NEGATIVE;
}

template <typename Kernel>
bool does_tetrahedron_intersect_triangle_interior(typename Kernel::Tetrahedron_3 tet,
                                                  typename Kernel::Triangle_3 tr,
                                                  const Kernel& k)
{
  CGAL_kernel_precondition(!k.is_degenerate_3_object()(tr));
  CGAL_kernel_precondition(!k.is_degenerate_3_object()(tet));

  for (int i = 0; i < 4; ++i)
  {
    if(does_first_triangle_intersect_second_triangle_interior(
           tr, k.construct_triangle_3_object()(tet[i], tet[(i + 1) % 4], tet[(i + 2) % 4]), k))
      return true;
  }

  // decltype(auto) p = k.construct_vertex_3_object()(tr, 0);
  // if(k.has_on_bounded_side_3_object()(tet, p))
  //   return true;

  return false;
}

} // namespace CGAL

namespace CGAL {

#if __cpp_lib_concepts >= 201806L
template <typename Polygon, typename Kernel>
concept Polygon_3 = std::ranges::common_range<Polygon>
      && (std::is_convertible_v<std::ranges::range_value_t<Polygon>,
                                typename Kernel::Point_3>);
template <typename Polygons, typename Kernel>
concept Range_of_polygon_3 = std::ranges::common_range<Polygons>
      && Polygon_3<std::ranges::range_value_t<Polygons>, Kernel>;
#endif // concepts

template <typename Gt, typename Vb = Triangulation_vertex_base_3<Gt> >
class Constrained_Delaunay_triangulation_vertex_base_3
  : public Conforming_Delaunay_triangulation_vertex_base_3<Gt, Vb>
{
  using Base = Conforming_Delaunay_triangulation_vertex_base_3<Gt, Vb>;
public:
  // To get correct vertex type in TDS
  template < class TDS3 >
  struct Rebind_TDS {
    typedef typename Vb::template Rebind_TDS<TDS3>::Other Vb3;
    typedef Constrained_Delaunay_triangulation_vertex_base_3 <Gt, Vb3> Other;
  };

  using Base::Base;

  static std::string io_signature() {
    return Get_io_signature<Base>()();
  }
};

enum class CDT_3_cell_marker {
  CLEAR = 0,
  IN_REGION = 1,
  VISITED = 1,
  ON_REGION_BOUNDARY = 2,
  nb_of_markers
};

template <typename Gt, typename Cb = Triangulation_cell_base_3<Gt> >
class Constrained_Delaunay_triangulation_cell_base_3
  : public Base_with_time_stamp<Cb>
{
  using Base = Base_with_time_stamp<Cb>;
  std::array<CDT_3_face_index, 4> face_id = { -1, -1, -1, -1 };
  std::array<void*, 4> facet_2d = {nullptr, nullptr, nullptr, nullptr};
  std::bitset<static_cast<unsigned>(CDT_3_cell_marker::nb_of_markers)> markers;

public:
  // To get correct cell type in TDS
  template < class TDS3 >
  struct Rebind_TDS {
    typedef typename Cb::template Rebind_TDS<TDS3>::Other Cb3;
    typedef Constrained_Delaunay_triangulation_cell_base_3 <Gt, Cb3> Other;
  };

  // Constructor
  using Base::Base;

  bool is_marked() const { return markers.any(); }
  bool is_marked(CDT_3_cell_marker m) const { return markers.test(static_cast<unsigned>(m)); }
  void set_mark(CDT_3_cell_marker m) { markers.set(static_cast<unsigned>(m)); }
  void clear_mark(CDT_3_cell_marker m) { markers.reset(static_cast<unsigned>(m)); }
  void clear_marks() { markers.reset(); }

  bool is_facet_constrained(int i) const { return face_id[unsigned(i)] >= 0; }

  template <typename Facet_handle>
  void set_facet_constraint(int i, CDT_3_face_index face_id,
                            Facet_handle facet_2d)
  {
    this->face_id[unsigned(i)] = face_id;
    this->facet_2d[unsigned(i)] = static_cast<void*>(facet_2d == Facet_handle{} ?  nullptr : std::addressof(*facet_2d));
  }

  CDT_3_face_index face_constraint_index(int i) const {
    return face_id[unsigned(i)];
  }

  template <typename CDT_2>
  auto face_2 (const CDT_2& cdt, int i) const {
    using Face = typename CDT_2::Face;
    auto ptr = static_cast<Face*>(facet_2d[unsigned(i)]);
    return cdt.tds().faces().iterator_to(*ptr);
  }

  static std::string io_signature() {
    return Get_io_signature<Base>()() + "+(" + Get_io_signature<int>()()
      + ")[4]";
  }

  friend std::ostream&
  operator<<(std::ostream& os,
             const Constrained_Delaunay_triangulation_cell_base_3& c)
  {
    os << static_cast<const Base&>(c);
    for( unsigned li = 0; li < 4; ++li ) {
      if(IO::is_ascii(os)) {
        os << " " << c.face_id[li];
      } else {
        CGAL::write(os, c.face_id[li]);
      }
    }
    return os;
  }
  friend std::istream&
  operator>>(std::istream& is,
             Constrained_Delaunay_triangulation_cell_base_3& c)
  {
    is >> static_cast<Base&>(c);
    if(!is) return is;
    for( int li = 0; li < 4; ++li ) {
      int i;
      if(IO::is_ascii(is)) {
        is >> i;
      } else {
        CGAL::read(is, i);
      }
      if(!is) return is;
      c.face_id[li] = i;
    }
    return is;
  }
};

template <class DSC, bool Const>
struct Output_rep<CGAL::internal::CC_iterator<DSC, Const>, With_point_and_info_tag>
  : public Output_rep<CGAL::internal::CC_iterator<DSC, Const>>
{
  int offset = 0;
  using Base = Output_rep<CGAL::internal::CC_iterator<DSC, Const>>;
  using CC_iterator = CGAL::internal::CC_iterator<DSC, Const>;
  using Compact_container = typename CC_iterator::CC;
  using Time_stamper = typename Compact_container::Time_stamper;

  using Base::Base;

  Output_rep(const CC_iterator& it, With_point_and_info_tag tag)
    : Base(it), offset(tag.offset)
  {
  }

  std::ostream& operator()(std::ostream& out) const {
    out << Time_stamper::display_id(this->it.operator->(), offset);
    if(this->it.operator->() != nullptr) {
      out << (this->it->is_Steiner_vertex_on_edge() ? "(Steiner)" : "")
          << (this->it->is_Steiner_vertex_in_face() ? "(Steiner in face)" : "")
          << "= " << this->it->point();
      if(this->it->is_marked(CDT_3_vertex_marker::REGION_BORDER)) out << " (region border)";
      if(this->it->is_marked(CDT_3_vertex_marker::REGION_INSIDE)) out << " (inside region)";
      if(this->it->is_marked(CDT_3_vertex_marker::CAVITY)) out << " (cavity vertex)";
      if(this->it->is_marked(CDT_3_vertex_marker::CAVITY_ABOVE)) out << " (vertex above)";
      if(this->it->is_marked(CDT_3_vertex_marker::CAVITY_BELOW)) out << " (vertex below)";
      return out;
    }
    else
      return out;
  }
};


template <typename T_3>
class Constrained_Delaunay_triangulation_3 : public Conforming_Delaunay_triangulation_3<T_3> {
public:
  using Conforming_Dt = Conforming_Delaunay_triangulation_3<T_3>;

  using Vertex_handle = typename T_3::Vertex_handle;
  using Edge = typename T_3::Edge;
  using Facet = typename T_3::Facet;
  using Cell_handle = typename T_3::Cell_handle;

  using Geom_traits = typename T_3::Geom_traits;
  using Point_3 = typename T_3::Point;
  using Segment_3 = typename Geom_traits::Segment_3;
  using Vector_3 = typename Geom_traits::Vector_3;
  using Locate_type = typename T_3::Locate_type;
  using size_type = typename T_3::size_type;

  using Vertex_marker = CDT_3_vertex_marker;
  using Cell_marker = CDT_3_cell_marker;

  using Face_index = CDT_3_face_index;

  static std::string io_signature() {
  return Get_io_signature<Conforming_Dt>()();
}

private:
  struct CDT_2_types {
    struct Projection_traits : public Projection_traits_3<Geom_traits> {
      // struct Side_of_oriented_circle_2 : public Geom_traits::Coplanar_side_of_bounded_circle_3 {
      //   using result_type = Oriented_side;
      //   template <typename... Arg> auto operator()(Arg&&... arg) const
      //   {
      //     return CGAL::enum_cast<Oriented_side>(
      //         Geom_traits::Coplanar_side_of_bounded_circle_3::operator()(std::forward<Arg>(arg)...));
      //   }
      // };
      // Side_of_oriented_circle_2 side_of_oriented_circle_2_object() const { return {}; }

      // using Compare_xy_2 = typename Geom_traits::Compare_xyz_3;
      // Compare_xy_2 compare_xy_2_object() const { return {}; }

      using Projection_traits_3<Geom_traits>::Projection_traits_3; // inherit cstr
    };
    static_assert(std::is_nothrow_move_constructible<Projection_traits>::value,
                  "move cstr is missing");

    struct Vertex_info {
      Vertex_handle vertex_handle_3d = {};
    };

    using Color_value_type = std::int8_t;
    struct Face_info {
      Color_value_type is_outside_the_face = -1;
      Color_value_type is_in_region = 0;
      std::bitset<3> is_edge_also_in_3d_triangulation = 0;
      bool missing_subface = true;
      Facet facet_3d = {};
    };
    using Vb1 = Triangulation_vertex_base_with_info_2<Vertex_info,
                                                      Projection_traits>;
    using Vb = Base_with_time_stamp<Vb1>;
    using Fb1 = Triangulation_face_base_with_info_2<Face_info,
                                                    Projection_traits>;
    using Fb = Constrained_triangulation_face_base_2<Projection_traits, Fb1>;
    using TDS = Triangulation_data_structure_2<Vb,Fb>;
    using Itag = No_constraint_intersection_requiring_constructions_tag;
    using CDT_base =
        Constrained_Delaunay_triangulation_2<Projection_traits, TDS, Itag>;
    using CDT = CDT_base;

    template <Color_value_type Face_info::* member_ptr>
    struct CDT_2_dual_color_map {
      using category = boost::read_write_property_map_tag;
      using reference = Color_value_type&;
      using value_type = Color_value_type;
      using key_type = typename CDT::Face_handle;

      friend reference get(CDT_2_dual_color_map, key_type fh) {
        return fh->info().*member_ptr;
      }
      friend void put(CDT_2_dual_color_map, key_type fh, value_type value) {
        fh->info().*member_ptr = value;
      }
    };
    using Color_map_is_outside_the_face =
        CDT_2_dual_color_map<&Face_info::is_outside_the_face>;
    using Color_map_is_in_region =
        CDT_2_dual_color_map<&Face_info::is_in_region>;
  }; // CDT_2_types
  using CDT_2 = typename CDT_2_types::CDT;
  using CDT_2_traits = typename CDT_2_types::Projection_traits;
  using CDT_2_face_handle = typename CDT_2::Face_handle;
  using CDT_2_edge = typename CDT_2::Edge;
  static_assert(std::is_nothrow_move_constructible<CDT_2>::value,
                "move cstr is missing");
  static_assert(std::is_nothrow_move_assignable<CDT_2>::value,
                "move assignment is missing");

protected:
  struct PLC_error : Error_exception {
    int face_index;
    int region_index;

    PLC_error(std::string msg, std::string file, int line, int face_index, int region_index)
        : Error_exception("CGAL CDT_3", msg, file, line), face_index(face_index), region_index(region_index)
    {
    }
  };
  using Constraint_hierarchy = typename Conforming_Dt::Constraint_hierarchy;
  using Constraint_id = typename Constraint_hierarchy::Constraint_id;
  using Subconstraint = typename Constraint_hierarchy::Subconstraint;

  void register_facet_to_be_constrained(Cell_handle cell, int facet_index) {
    const auto face_id = static_cast<std::size_t>(cell->face_constraint_index(facet_index));
    this->face_constraint_misses_subfaces.set(face_id);
    auto fh_2 = cell->face_2(this->face_cdt_2[face_id], facet_index);
    fh_2->info().facet_3d = {};
    fh_2->info().missing_subface = true;
    this->set_facet_constrained({cell, facet_index}, -1, {});
  }

  void register_facet_to_be_constrained(Facet f) {
    const auto [cell, facet_index] = f;
    register_facet_to_be_constrained(cell, facet_index);
  }

  class Insert_in_conflict_visitor {
    Constrained_Delaunay_triangulation_3<T_3> &self;
    typename Conforming_Dt::Insert_in_conflict_visitor conforming_dt_visitor;

  public:
    Insert_in_conflict_visitor(Constrained_Delaunay_triangulation_3 &self)
        : self(self), conforming_dt_visitor(self) {}

    template <class InputIterator>
    void process_cells_in_conflict(const InputIterator cell_it_begin, const InputIterator end) {
      CGAL_assertion(self.dimension() >= 2);
      if(self.cdt_2_are_initialized) {
        const int first_li = self.dimension() == 2 ? 3 : 0;
        for(auto cell_it = cell_it_begin; cell_it != end; ++cell_it) {
          auto c = *cell_it;
          for(int li = first_li; li < 4; ++li) {
            if(c->is_facet_constrained(li)) {
              self.register_facet_to_be_constrained(c, li);
  #if CGAL_CDT_3_DEBUG_MISSING_TRIANGLES
              std::cerr << "Add missing triangle (from visitor), face #F" << face_id << ": \n";
              self.write_2d_triangle(std::cerr, fh_2);
  #endif // CGAL_CDT_3_DEBUG_MISSING_TRIANGLES
            }
          }
        }
      }
      conforming_dt_visitor.process_cells_in_conflict(cell_it_begin, end);
    }
    void after_insertion(Vertex_handle v) {
      conforming_dt_visitor.after_insertion(v);
    }

    void reinsert_vertices(Vertex_handle v) {
      after_insertion(v);
    }

    Vertex_handle replace_vertex(Cell_handle c, int index,
                                  const Point_3 &) const {
      return c->vertex(index);
    }

    void hide_point(Cell_handle, const Point_3 &) const {}

    void insert_Steiner_point_on_constraint(Constraint_id constraint,
                                            Vertex_handle va,
                                            Vertex_handle vb,
                                            Vertex_handle v_Steiner) const
    {
      const auto point = self.point(v_Steiner);
      if(!self.cdt_2_are_initialized) return;
      for(const auto [_, poly_id] : CGAL::make_range(self.constraint_to_faces.equal_range(constraint))) {
        auto& non_const_cdt_2 = self.face_cdt_2[poly_id];
        const auto& cdt_2 = non_const_cdt_2;

        auto opt_edge = self.edge_of_cdt_2(cdt_2, va, vb);
        CGAL_assume(opt_edge != std::nullopt);
        CGAL_assertion(cdt_2.is_constrained(*opt_edge));
        auto [fh_2d, edge_index]= *opt_edge;
        const auto va_2d = fh_2d->vertex(cdt_2.cw(edge_index));
        const auto vb_2d = fh_2d->vertex(cdt_2.ccw(edge_index));

        const auto [mirror_fh_2d, mirror_edge_index] = cdt_2.mirror_edge(*opt_edge);

        const auto outside_on_right = fh_2d->info().is_outside_the_face;
        const auto outside_on_left = mirror_fh_2d->info().is_outside_the_face;
        const auto in_region_on_right = fh_2d->info().is_in_region;
        const auto in_region_on_left = mirror_fh_2d->info().is_in_region;

        fh_2d->set_constraint(edge_index, false);
        mirror_fh_2d->set_constraint(mirror_edge_index, false);
        const auto v_Steiner_2d = non_const_cdt_2.insert(point, fh_2d);
        v_Steiner_2d->info().vertex_handle_3d = v_Steiner;
        non_const_cdt_2.insert_constraint(va_2d, v_Steiner_2d);
        non_const_cdt_2.insert_constraint(vb_2d, v_Steiner_2d);

        // update the edge {fh_2d, edge_index}
        const bool is_edge = cdt_2.is_edge(va_2d, v_Steiner_2d, fh_2d, edge_index);
        CGAL_assume(is_edge);

        auto fc = cdt_2.incident_faces(v_Steiner_2d, fh_2d), fc_begin(fc);
        // circulators are counter-clockwise, so we start at the right of [va,v]
        do {
          fc->info().is_outside_the_face = outside_on_right;
          fc->info().is_in_region = in_region_on_right;
          ++fc;
        } while ( fc->vertex(cdt_2.ccw(fc->index(v_Steiner_2d))) != vb_2d );

        do {
          fc->info().is_outside_the_face = outside_on_left;
          fc->info().is_in_region = in_region_on_left;
        } while(++fc != fc_begin);
        fc = cdt_2.incident_faces(v_Steiner_2d, fh_2d);
        fc_begin  = fc;
        do {
          fc->info().missing_subface = true;
          const auto v_Steiner_index = fc->index(v_Steiner_2d);
          const auto other_edge = cdt_2.mirror_edge({fc, v_Steiner_index});
          fc->info().is_edge_also_in_3d_triangulation.set(other_edge.first->info().is_edge_also_in_3d_triangulation.test(other_edge.second));
        } while(++fc != fc_begin);

        self.face_constraint_misses_subfaces.set(poly_id);
      }
      conforming_dt_visitor.insert_Steiner_point_on_constraint(constraint, va, vb, v_Steiner);
    }

    Vertex_handle insert_in_triangulation(const Point_3& p, Locate_type lt, Cell_handle c, int li, int lj) {
      if(self.is_Delaunay)
        return self.insert_impl_do_not_split(p, lt, c, li, lj, *this);
      else 
        return self.insert_in_cdt_3(p, lt, c, li, lj, *this);
    }
  };

public:
  Vertex_handle insert(const Point_3 &p, Locate_type lt, Cell_handle c,
                       int li, int lj, bool restore_Delaunay = true)
  {
    this->update_bbox(p);
    auto v = Conforming_Dt::insert_impl(p, lt, c, li, lj, insert_in_conflict_visitor);
    if(restore_Delaunay) {
      Conforming_Dt::restore_Delaunay(insert_in_conflict_visitor);
    }
    return v;
  }

  template <typename Visitor>
  Vertex_handle insert_in_cdt_3(const Point_3& p, [[maybe_unused]] Locate_type lt, Cell_handle ch, int, int,
                                Visitor& visitor)
  {
    CGAL_assertion(lt != Locate_type::VERTEX);
    boost::container::small_vector<Cell_handle,64> cells_of_original_cavity;
    boost::container::small_vector<Facet,64> exterior_border_facets_of_original_cavity;

    auto output_iterator_to_facets = boost::make_function_output_iterator(
        [&](Facet f) { exterior_border_facets_of_original_cavity.push_back(tr.mirror_facet(f)); });

    auto triple_of_output_iterators = make_triple(
        output_iterator_to_facets,
        std::back_inserter(cells_of_original_cavity),
        Emptyset_iterator{});

    switch(tr.dimension()) {
    case 3: {
      typename T_3::Conflict_tester_3 tester(p, this);
      this->find_conflicts(ch, tester, triple_of_output_iterators);
      break;
    } // dim 3
    case 2: {
      typename T_3::Conflict_tester_2 tester(p, this);
      this->find_conflicts(ch, tester, triple_of_output_iterators);
      break;
    } // dim 2
    default: CGAL_error();
    }

    // cleanup of tds_data after T_3::find_conflicts
    for(Cell_handle ch : cells_of_original_cavity) {
      ch->tds_data().clear();
    }
    for(auto [ch, _] : exterior_border_facets_of_original_cavity) {
      ch->tds_data().clear();
    }

    bool the_infinite_vertex_is_in_the_cavity = false;
    std::set<Vertex_handle> vertices_of_original_cavity;
    for(Cell_handle ch : cells_of_original_cavity) {
      for(int i = 0; i < 4; ++i) {
        auto v = ch->vertex(i);
        if(tr.is_infinite(v)) {
          the_infinite_vertex_is_in_the_cavity = true;
        }
        vertices_of_original_cavity.insert(v);
      }
    }
    // add one extra vertex for p:
    auto p_vh = this->tds().create_vertex();
    p_vh->set_point(p);
    vertices_of_original_cavity.insert(p_vh);

    // vertices of the border of the cavity should point to cells outside of it
    for(auto [c, index]: exterior_border_facets_of_original_cavity) {
      for(int i = 0; i < 3; ++i) {
        auto v = c->vertex(tr.vertex_triple_index(index, i));
        v->set_cell(c);
      }
    }

    const auto [cavity_triangulation, vertices_of_cavity, map_cavity_vertices_to_ambient_vertices,
                facets_of_cavity, interior_constrained_faces, cells_of_cavity] =
        triangulate_cavity(cells_of_original_cavity, exterior_border_facets_of_original_cavity,
                           vertices_of_original_cavity);

    for(auto f: interior_constrained_faces) {
      this->register_facet_to_be_constrained(f);
    }

    visitor.process_cells_in_conflict(cells_of_cavity.begin(), cells_of_cavity.end());

    typename T_3::Vertex_triple_Facet_map outer_map;
    for(auto f: facets_of_cavity) {
      typename T_3::Vertex_triple vt = this->make_vertex_triple(f);
      this->make_canonical_oriented_triple(vt);
      outer_map[vt] = f;
    }

    const auto inner_map = tr.create_triangulation_inner_map(
        cavity_triangulation, map_cavity_vertices_to_ambient_vertices, the_infinite_vertex_is_in_the_cavity);

    this->copy_triangulation_into_hole(map_cavity_vertices_to_ambient_vertices,
                                       std::move(outer_map),
                                       inner_map,
                                       this->new_cells_output_iterator());

    for(auto outside_facet : facets_of_cavity) {
      const auto [outside_cell, outside_face_index] = outside_facet;
      const auto mirror_facet = this->mirror_facet(outside_facet);
      if(outside_cell->is_facet_constrained(outside_face_index)) {
        const auto poly_id = outside_cell->face_constraint_index(outside_face_index);
        const CDT_2& cdt_2 = face_cdt_2[poly_id];
        const auto f2d = outside_cell->face_2(cdt_2, outside_face_index);
        set_facet_constrained(mirror_facet, poly_id, f2d);
      }
    }

    for(auto c: cells_of_cavity) {
      this->tds().delete_cell(c);
    }

    this->new_vertex(p_vh);

    CGAL_assume(!this->debug_validity() || this->is_valid(true));

    return p_vh;
  }

  Vertex_handle insert(const Point_3 &p, Cell_handle start = {}, bool restore_Delaunay = true) {
    Locate_type lt;
    int li, lj;

    Cell_handle c = tr.locate(p, lt, li, lj, start);
    return insert(p, lt, c, li, lj, restore_Delaunay);
  }

  Constraint_id insert_constrained_edge(Vertex_handle va, Vertex_handle vb, bool restore_Delaunay = true)
  {
    const auto id = this->insert_constrained_edge_impl(va, vb, insert_in_conflict_visitor);
    if(restore_Delaunay) {
      this->restore_Delaunay();
    }
    return id;
  }

  void restore_Delaunay() {
    Conforming_Dt::restore_Delaunay(insert_in_conflict_visitor);
  }

  bool is_constrained(Facet f) const {
    return f.first->is_facet_constrained(f.second);
  }

  bool same_triangle(Facet f, CDT_2_face_handle fh) const {
    return true;
    const auto [c, facet_index] = f;
    std::array<Vertex_handle, 3> f_vertices{c->vertex(tr.vertex_triple_index(facet_index,0)),
                                            c->vertex(tr.vertex_triple_index(facet_index,1)),
                                            c->vertex(tr.vertex_triple_index(facet_index,2))};
    std::sort(f_vertices.begin(), f_vertices.end());
    std::array<Vertex_handle, 3> fh_vertices{fh->vertex(0)->info().vertex_handle_3d,
                                             fh->vertex(1)->info().vertex_handle_3d,
                                             fh->vertex(2)->info().vertex_handle_3d};
    std::sort(fh_vertices.begin(), fh_vertices.end());
    return (f_vertices == fh_vertices);
  }

  void set_facet_constrained(Facet f, CDT_3_face_index polygon_contraint_id,
                             CDT_2_face_handle fh)
  {
    CGAL_assertion(fh == CDT_2_face_handle{} || same_triangle(f, fh));

    const auto [c, facet_index] = f;
    c->set_facet_constraint(facet_index, polygon_contraint_id, fh);
    if(tr.dimension() > 2) {
      const auto [n, n_index] = tr.mirror_facet({c, facet_index});
      n->set_facet_constraint(n_index, polygon_contraint_id, fh);
    }
    if(fh == CDT_2_face_handle{}) return;

    if(fh != CDT_2_face_handle{}) {
      fh->info().facet_3d = f;
    }
  }

  template <CGAL_TYPE_CONSTRAINT(Polygon_3<Geom_traits>) Polygon>
  std::optional<Face_index> register_new_constrained_polygon(Polygon&& polygon) {
    return insert_constrained_polygon(std::forward<Polygon>(polygon), false);
  }

  template <CGAL_TYPE_CONSTRAINT(Polygon_3<Geom_traits>) Polygon>
  std::optional<Face_index>
  insert_constrained_polygon(const Polygon& polygon, bool restore_Delaunay = true, Face_index face_index = -1)
  {
    std::vector<Vertex_handle> handles;
    handles.reserve(polygon.size());
    std::optional<Cell_handle> hint;
    for(const auto& p : polygon) {
      const auto v = this->insert(p, hint.value_or(Cell_handle{}), restore_Delaunay);
      handles.push_back(v);
      hint = v->cell();
    }
    return insert_constrained_face(std::move(handles), restore_Delaunay, face_index);
  }

  template <typename Vertex_handles>
  CGAL_CPP20_REQUIRE_CLAUSE(std::ranges::common_range<Vertex_handles> &&
                            (std::is_convertible_v<std::ranges::range_value_t<Vertex_handles>, Vertex_handle>))
  std::optional<Face_index> insert_constrained_face(Vertex_handles&& vertex_handles,
                                                      bool restore_Delaunay = true,
                                                      Face_index face_index = -1)
  {
#if CGAL_DEBUG_CDT_3 & 2
    std::cerr << "insert_constrained_face (" << std::size(vertex_handles) << " vertices):\n";
    for(auto v: vertex_handles) {
      std::cerr << "  - " << this->display_vert(v) << '\n';
    }
#endif // CGAL_DEBUG_CDT_3 & 2
    using std::begin;
    using std::endl;
    const auto first_it = begin(vertex_handles);
    const auto vend =  end(vertex_handles);
    const auto size = std::distance(first_it, vend);
    if(size < 2) return {};
    if(size == 2) {
      this->insert_constrained_edge(*first_it, *std::next(first_it));
      return {};
    }
    CGAL::Circulator_from_container<std::remove_reference_t<Vertex_handles>> circ{&vertex_handles};
    const auto circ_end{circ};
    auto& borders = face_index < 0 ? this->face_borders.emplace_back() : this->face_borders[face_index];
    auto& border = borders.emplace_back();
    const auto polygon_contraint_id =
        face_index < 0 ? static_cast<CDT_3_face_index>(this->face_borders.size() - 1) : face_index;
    do {
      const auto va = *circ;
      ++circ;
      const auto vb = *circ;
      const auto c_id = this->constraint_from_extremities(va, vb);
      if(c_id != Constraint_id{}) {
        const bool constraint_c_id_is_reversed = va != (*this->constraint_hierarchy.vertices_in_constraint_begin(c_id));
        border.push_back(Face_edge{c_id, constraint_c_id_is_reversed});
        constraint_to_faces.emplace(c_id, polygon_contraint_id);
      } else {
        const auto c_id = this->insert_constrained_edge(va, vb, restore_Delaunay);
        CGAL_assertion(c_id != Constraint_id{});
        border.push_back(Face_edge{c_id});
        constraint_to_faces.emplace(c_id, polygon_contraint_id);
      }
    } while(circ != circ_end);

    if(face_index < 0) {
      const auto accumulated_normal = std::invoke([&] {
        const auto last_it = std::next(first_it, size - 1);
        const auto &last_point = tr.point(*last_it);

        auto &&traits = tr.geom_traits();
        auto &&cross_product = traits.construct_cross_product_vector_3_object();
        auto &&vector = traits.construct_vector_3_object();
        auto &&sum_vector = traits.construct_sum_of_vectors_3_object();

        Vector_3 accumulated_normal = vector(CGAL::NULL_VECTOR);
        for (auto vit = first_it, next_it = std::next(first_it);
            next_it != last_it; ++vit, ++next_it) {
          accumulated_normal =
              sum_vector(accumulated_normal,
                        cross_product(vector(last_point, tr.point(*vit)),
                                      vector(last_point, tr.point(*next_it))));
        }
        if (accumulated_normal.x() < 0 ||
            (accumulated_normal.x() == 0 && accumulated_normal.y() < 0) ||
            (accumulated_normal.x() == 0 && accumulated_normal.y() == 0 &&
            accumulated_normal.z() < 0)
            )
        {
          accumulated_normal = - accumulated_normal;
        }
        return accumulated_normal;
      });

      face_cdt_2.emplace_back(CDT_2_traits{accumulated_normal});
      face_constraint_misses_subfaces.resize(face_cdt_2.size());
    }
#if CGAL_DEBUG_CDT_3 & 2
    std::cerr << "insert_constrained_face return the polygon_contraint_id: " << polygon_contraint_id << '\n';
#endif
    return polygon_contraint_id;
  }

  std::optional<std::vector<Vertex_handle>>
  sequence_of_Steiner_vertices(Vertex_handle va, Vertex_handle vb) const
  {
    std::vector<Vertex_handle> steiner_vertices;
    const auto c_id = this->constraint_from_extremities(va, vb);
    if(c_id != Constraint_id{}) {
      auto vit = this->constraint_hierarchy.vertices_in_constraint_begin(c_id);
      auto v_end = this->constraint_hierarchy.vertices_in_constraint_end(c_id);
      CGAL_assertion_code(const auto constraint_size = std::distance(vit, v_end);)
      if(vit != v_end)
      {
        const bool constraint_c_id_is_reversed = (*vit != va);
        CGAL_assertion(*vit == (constraint_c_id_is_reversed ? vb : va));
        if(++vit != v_end && vit != --v_end) {
          CGAL_assertion(constraint_size == std::distance(vit, v_end) + 2);
          CGAL_assertion(*v_end == (constraint_c_id_is_reversed ? va : vb));
          if(constraint_c_id_is_reversed) {
            using std::swap;
            swap(vit, v_end);
            --vit;
            --v_end;
          };
          while(vit != v_end) {
            steiner_vertices.push_back(*vit);
            if(constraint_c_id_is_reversed) {
              --vit;
            } else {
              ++vit;
            };
          }
        }
      }
    } else {
      return std::nullopt;
    }
    return {std::move(steiner_vertices)};
  }

private:
  void fill_cdt_2(CDT_2& cdt_2, CDT_3_face_index polygon_contraint_id)
  {
#if CGAL_DEBUG_CDT_3 & 4
    std::cerr << "Polygon #" << polygon_contraint_id << " normal is: " << cdt_2.geom_traits().normal() << '\n';
#endif // CGAL_DEBUG_CDT_3
    const auto vec_of_handles = std::invoke([this, polygon_contraint_id]() {
      std::vector<std::vector<Vertex_handle>> vec_of_handles;
      for(const auto& border : this->face_borders[polygon_contraint_id]) {
        auto& handles = vec_of_handles.emplace_back();
        for(const auto& face_edge : border) {
          const auto c_id = face_edge.constraint_id;
          const bool reversed = face_edge.is_reverse;
          const auto v_begin = this->constraint_hierarchy.vertices_in_constraint_begin(c_id);
          const auto v_end = this->constraint_hierarchy.vertices_in_constraint_end(c_id);
          CGAL_assertion(std::distance(v_begin, v_end) >= 2);
          auto va = *v_begin;
          auto vb = *std::prev(v_end);
          if(reversed) {
            using std::swap;
            swap(va, vb);
          }
          if(handles.empty()) {
            handles.push_back(va);
          } else {
            CGAL_assertion(handles.back() == va);
          }
          handles.push_back(vb);
        }
        CGAL_assertion(handles.front() == handles.back());
      }
      return vec_of_handles;
    });

#if CGAL_DEBUG_CDT_3 & 4
    std::cerr << "  points\n";
    for(const auto& handles : vec_of_handles) {
      for(auto it = handles.begin(), end = std::prev(handles.end()); it != end; ++it) {
        std::cerr << "     " << tr.point(*it) << '\n';
      }
    }
#endif

    // create and fill the 2D triangulation
    {
      for(const auto& handles : vec_of_handles)
      {
        const auto first_2d  = cdt_2.insert(tr.point(handles.front()));
        first_2d->info().vertex_handle_3d = handles.front();
        auto previous_2d = first_2d;
        for(auto it = handles.begin(), end = std::prev(handles.end());
            it != end; /* incremented in the loop */)
        {
          const auto va = *it;
          CGAL_assertion(previous_2d->info().vertex_handle_3d == va);
          ++it;
          const auto vb = *it;
          const auto c_id = this->constraint_from_extremities(va, vb);
          if(c_id != Constraint_id{}) {
            auto vit = this->constraint_hierarchy.vertices_in_constraint_begin(c_id);
            auto v_end = this->constraint_hierarchy.vertices_in_constraint_end(c_id);
            CGAL_assertion_code(const auto constraint_size = std::distance(vit, v_end);)
            if(vit != v_end) {
              const bool constraint_c_id_is_reversed = (*vit != va);
              CGAL_assertion(*vit == (constraint_c_id_is_reversed ? vb : va));
              if(++vit != v_end && vit != --v_end) {
                CGAL_assertion(constraint_size == std::distance(vit, v_end) + 2);
                CGAL_assertion(*v_end == (constraint_c_id_is_reversed ? va : vb));
                if(constraint_c_id_is_reversed) {
                  using std::swap;
                  swap(vit, v_end);
                  --vit;
                  --v_end;
                };
                while(vit != v_end) {
                  auto vh_2d = cdt_2.insert(tr.point(*vit));
                  vh_2d->info().vertex_handle_3d = *vit;
  #if CGAL_DEBUG_CDT_3 & 4
                  std::cerr << "cdt_2.insert_constraint ("
                            << tr.point(previous_2d->info().vertex_handle_3d)
                            << " , "
                            << tr.point(vh_2d->info().vertex_handle_3d)
                            << ")\n";
  #endif // CGAL_DEBUG_CDT_3
                  cdt_2.insert_constraint(previous_2d, vh_2d);
                  previous_2d = vh_2d;
                  if(constraint_c_id_is_reversed) {
                    --vit;
                  } else {
                    ++vit;
                  };
                }
              }
            }
          }

          auto vh_2d = it == end ? first_2d : cdt_2.insert(tr.point(vb));
          if(it != end) {
            vh_2d->info().vertex_handle_3d = vb;
          }
  #if CGAL_DEBUG_CDT_3 & 4
          std::cerr << "cdt_2.insert_constraint ("
                    << tr.point(previous_2d->info().vertex_handle_3d)
                    << " , "
                    << tr.point(vh_2d->info().vertex_handle_3d)
                    << ")\n";
  #endif // CGAL_DEBUG_CDT_3
          cdt_2.insert_constraint(previous_2d, vh_2d);
          previous_2d = vh_2d;
        }
      }
      { // mark all the faces outside/inside, starting from one infinite face
        for(auto fh: cdt_2.all_face_handles())
        {
          fh->info().is_outside_the_face = -1;
        }
        struct Face_handle_and_outside {
          CDT_2_face_handle fh;
          bool outside;
        };
        const auto d = cdt_2.dimension();
        std::stack<Face_handle_and_outside> stack;
        stack.push({cdt_2.infinite_face(), true});
        while(!stack.empty()) {
          const auto [fh, outside] = stack.top();
          stack.pop();
          if(fh->info().is_outside_the_face == -1) {
            fh->info().is_outside_the_face = outside;
            for(int i = 0; i <= d; ++i) {
              const auto neighbor = fh->neighbor(i);
              const auto new_outside = fh->is_constrained(i) ? !outside : outside;
              if(neighbor->info().is_outside_the_face == -1) {
                stack.push({neighbor, new_outside});
              }
            }
          }
        }
      } // end of marking inside/outside
#if CGAL_DEBUG_CDT_3 & 4
      int counter = 0;
      for(const auto fh: cdt_2.finite_face_handles()) {
        if(!fh->info().is_outside_the_face) ++counter;
      }
      std::cerr << counter << " triangles(s) in the face\n";
#endif // CGAL_DEBUG_CDT_3
      if(Algebraic_structure_traits<typename Geom_traits::FT>::Is_exact::value &&
         !std::all_of(cdt_2.finite_face_handles().begin(), cdt_2.finite_face_handles().end(), [=](const auto fh) {
           const auto p0 = cdt_2.point(fh->vertex(0));
           const auto v1 = cdt_2.point(fh->vertex(1)) - p0;
           const auto v2 = cdt_2.point(fh->vertex(2)) - p0;
           return cross_product(cdt_2.geom_traits().normal(), cross_product(v1, v2)) == NULL_VECTOR;
         }))
      {
        std::cerr << std::string("Polygon #") + std::to_string(polygon_contraint_id) +
                         " is not coplanar.\n";
      }
    } // end of the construction of the CDT_2

    if(cdt_2.number_of_vertices() == 4) {
      // for polygons with 4 vertices, 2 triangles
      for(auto [ch, index]: cdt_2.finite_edges()) {
        if(!ch->is_constrained(index)) {
          // here the edge {ch, index} is the diagonal [ac] of the polygon [abcd]
          const auto vb = ch->vertex(index);
          const auto [ch2, index2] = cdt_2.mirror_edge({ch, index});
          const auto vd = ch2->vertex(index2);
          CGAL_assertion(!cdt_2.is_edge(vb, vd));
          const auto vb_3d = vb->info().vertex_handle_3d;
          const auto vd_3d = vd->info().vertex_handle_3d;
          if(this->is_edge(vb_3d, vd_3d)) {
            // let's insert the diagonal [bd] in the CDT_2
            cdt_2.insert_constraint(vb, vd);
#if CGAL_DEBUG_CDT_3 & 64
            std::cerr << "NOTE: CDT_2 has 4 vertices. Flip the diagonal\n";
#endif
          }
        }
        break;
      }
    }
  }

  void search_for_missing_subfaces(CDT_3_face_index polygon_contraint_id)
  {
    const CDT_2& cdt_2 = face_cdt_2[polygon_contraint_id];

    for(const auto fh: cdt_2.all_face_handles())
    {
      if(fh->info().is_outside_the_face) continue;
      const auto v0 = fh->vertex(0)->info().vertex_handle_3d;
      const auto v1 = fh->vertex(1)->info().vertex_handle_3d;
      const auto v2 = fh->vertex(2)->info().vertex_handle_3d;
      Cell_handle c;
      int i, j, k;
      if(!tr.is_facet(v0, v1, v2, c, i, j, k)) {
        fh->info().missing_subface = true;
        face_constraint_misses_subfaces.set(static_cast<std::size_t>(polygon_contraint_id));
#if CGAL_CDT_3_DEBUG_MISSING_TRIANGLES
        std::cerr << std::format("Missing triangle in polygon #{}:\n", polygon_contraint_id);
        write_triangle(std::cerr, v0, v1, v2);
#endif // CGAL_CDT_3_DEBUG_MISSING_TRIANGLES
      } else {
        fh->info().missing_subface = false;
        const int facet_index = 6 - i - j - k;
        set_facet_constrained({c, facet_index}, polygon_contraint_id, fh);
      }
    }
  }

  static auto region(const CDT_2& cdt_2, CDT_2_face_handle fh)
  {
    std::vector<CDT_2_face_handle> fh_region;
    const auto cdt_2_dual_graph = dual(cdt_2.tds());
    const boost::filtered_graph dual(
        cdt_2_dual_graph,
        +[](CDT_2_edge edge) { // the `+` forces conversion of the lambda to a function pointer
          const auto face = edge.first;
          const auto i = unsigned(edge.second);
          return false == face->info().is_edge_also_in_3d_triangulation.test(i);
        },
        +[](CDT_2_face_handle face_handle) { return false == face_handle->info().is_outside_the_face; });
    boost::breadth_first_search(dual, fh,
                                boost::color_map(typename CDT_2_types::Color_map_is_in_region())
                                    .visitor(boost::make_bfs_visitor(boost::write_property(
                                        boost::typed_identity_property_map<CDT_2_face_handle>(),
                                        std::back_inserter(fh_region), boost::on_finish_vertex()))));
    CGAL_assertion(!fh_region.empty());
    CGAL_assertion(fh == fh_region[0]);
    return fh_region;
  }

  auto brute_force_border_3_of_region([[maybe_unused]] CDT_3_face_index face_index,
                                      [[maybe_unused]] int region_index,
                                      [[maybe_unused]] const CDT_2& cdt_2,
                                      const std::vector<CDT_2_face_handle>& fh_region)
  {
    const std::set<CDT_2_face_handle> fh_region_set{fh_region.begin(), fh_region.end()};
    std::vector<Edge> border_edges;
    for(const auto fh: fh_region) {
      for(int index = 0; index < 3; ++index) {
        if(fh_region_set.count(fh->neighbor(index)) > 0) continue;
        // otherwise we have a border edge: fh->neighbor(i) is not in the region
        const auto va = fh->vertex(CDT_2::ccw(index))->info().vertex_handle_3d;
        const auto vb = fh->vertex(CDT_2:: cw(index))->info().vertex_handle_3d;
        Cell_handle c;
        int i, j;
        CGAL_assume_code(bool b =)
        this->tds().is_edge(va, vb, c, i, j);
        CGAL_assume(b);
        border_edges.emplace_back(c, i, j);
      }
    }
    if(this->debug_regions()) {
      std::cerr << "region size is: " << fh_region.size() << "\n";
      std::cerr << "region border size is: " << border_edges.size() << "\n";
    }
    return border_edges;
  }

  struct Intersection_error : public std::runtime_error {
    using Seg = typename Geom_traits::Segment_3;
    using Tri = typename Geom_traits::Triangle_3;
    Intersection_error(Seg s, Tri t, std::string what) : std::runtime_error(what), segment(s), triangle(t) {}

    Seg segment;
    Tri triangle;
  };

  template <typename Fh_region>
  int does_edge_intersect_region(Cell_handle cell, int index_vc, int index_vd,
                                 const CDT_2& cdt_2, const Fh_region& fh_region)
  {
    const auto vc = cell->vertex(index_vd);
    const auto vd = cell->vertex(index_vc);
    const auto pc = this->point(vc);
    const auto pd = this->point(vd);
    const typename Geom_traits::Segment_3 seg{pc, pd};
    for(const auto fh_2d : fh_region) {
      const auto v0 = fh_2d->vertex(0)->info().vertex_handle_3d;
      const auto v1 = fh_2d->vertex(1)->info().vertex_handle_3d;
      const auto v2 = fh_2d->vertex(2)->info().vertex_handle_3d;
      if(vc == v0 || vc == v1 || vc == v2 || vd == v0 || vd == v1 || vd == v2) {
        continue;
      }
      const auto t0 = cdt_2.point(fh_2d->vertex(0));
      const auto t1 = cdt_2.point(fh_2d->vertex(1));
      const auto t2 = cdt_2.point(fh_2d->vertex(2));

      const auto opc = CGAL::orientation(t0, t1, t2, pc);
      const auto opd = CGAL::orientation(t0, t1, t2, pd);
      if(opc == CGAL::COPLANAR || opd == CGAL::COPLANAR || opc == opd) {
        continue;
      } else {
        // otherwise the segment intersects the plane of the triangle
        if(CGAL::orientation(pc, pd, t0, t1) != opc &&
           CGAL::orientation(pc, pd, t1, t2) != opc &&
           CGAL::orientation(pc, pd, t2, t0) != opc)
        {
          return static_cast<int>(opc);
        }
      }
    }
    return 0;
  }

  struct Search_first_intersection_result_type {
    Edge intersecting_edge;
    Edge border_edge;
  };

  // Given a region and a border edge of it, returns an edge in the link of the
  // border edge that intersects the region.
  // The returned edge has its first vertex above the region.
  template <typename Fh_region, typename Edges_container>
  std::optional<Search_first_intersection_result_type>
  search_first_intersection(CDT_3_face_index /*face_index*/,
                            const CDT_2& cdt_2,
                            const Fh_region& fh_region,
                            const Edges_container& border_edges)
  {
    for(const auto border_edge: border_edges) {
      const auto [c, i, j] = border_edge;
      const Vertex_handle va_3d = c->vertex(i);
      const Vertex_handle vb_3d = c->vertex(j);

      // std::ofstream dump_edges_around("dump_edges_around.polylines.txt");
      // dump_edges_around.precision(17);

      auto cell_circ = this->incident_cells(c, i, j), end = cell_circ;
      CGAL_assertion(cell_circ != nullptr);
      do {
        if(this->is_infinite(cell_circ)) {
          continue;
        }
        const auto index_va = cell_circ->index(va_3d);
        const auto index_vb = cell_circ->index(vb_3d);
        const auto index_vc = this->next_around_edge(index_va, index_vb);
        const auto index_vd = this->next_around_edge(index_vb, index_va);

        //write_segment(dump_edges_around, cell_circ->vertex(index_vc), cell_circ->vertex(index_vd));
        if(cell_circ->vertex(index_vc)->is_marked(Vertex_marker::REGION_BORDER)) continue;
        if(cell_circ->vertex(index_vd)->is_marked(Vertex_marker::REGION_BORDER)) continue;
        int cd_intersects_region = does_edge_intersect_region(cell_circ, index_vc, index_vd, cdt_2, fh_region);
        if(cd_intersects_region == 1) {
          return Search_first_intersection_result_type{ Edge{cell_circ, index_vc, index_vd}, border_edge };
        }
        if(cd_intersects_region == -1) {
          return Search_first_intersection_result_type{ Edge{cell_circ, index_vd, index_vc}, border_edge };
        }
      } while(++cell_circ != end);
    }
    return {};
  }

  struct Next_region : std::logic_error {
    using std::logic_error::logic_error;
    CDT_2_face_handle fh_2d;
    // create a new region
    Next_region(const std::string& what, CDT_2_face_handle fh) : std::logic_error(what), fh_2d(fh) {}
  };

  template <typename Fh_region, typename Vertices_container, typename Edges_container>
  auto construct_cavities(CDT_3_face_index face_index,
                          int region_index,
                          const CDT_2& cdt_2,
                          const Fh_region& fh_region,
                          const Vertices_container& region_border_vertices,
                          const Vertices_container& region_vertices,
                          Edge first_intersecting_edge,
                          Edges_container border_edges)
  {
    // outputs
    struct Outputs
    {
      std::vector<Edge> intersecting_edges;
      std::set<Cell_handle> intersecting_cells;
      std::vector<Vertex_handle> vertices_of_upper_cavity;
      std::vector<Vertex_handle> vertices_of_lower_cavity;
      std::vector<Facet> facets_of_upper_cavity;
      std::vector<Facet> facets_of_lower_cavity;
    } outputs{
        {}, {}, {region_vertices.begin(), region_vertices.end()}, {region_vertices.begin(), region_vertices.end()},
        {}, {}};

    auto& [intersecting_edges, intersecting_cells, vertices_of_upper_cavity, vertices_of_lower_cavity,
           facets_of_upper_cavity, facets_of_lower_cavity] = outputs;
    std::set<std::pair<Vertex_handle, Vertex_handle>> non_intersecting_edges_set;

    // marker for already visited elements
    std::set<Vertex_handle> visited_vertices;
    std::map<std::pair<Vertex_handle, Vertex_handle>, bool> visited_edges;
    std::set<Cell_handle> visited_cells;

    auto make__new_element_functor = [](auto& visited_set) {
      return [&visited_set](auto... e) {
        const auto [_, not_already_visited] = visited_set.emplace(e...);
        return not_already_visited;
      };
    };

    auto new_vertex = make__new_element_functor(visited_vertices);
    auto new_cell = make__new_element_functor(visited_cells);
    auto new_edge = [&](Vertex_handle v0, Vertex_handle v1, bool does_intersect) {
      CGAL_assertion(v0 != Vertex_handle{});
      return visited_edges.emplace(CGAL::make_sorted_pair(v0, v1), does_intersect);
    };

    using Mesh = Surface_mesh<Point_3>;
    using Face_index = typename Mesh::Face_index;
    using EK = CGAL::Exact_predicates_exact_constructions_kernel;
    const auto to_exact = CGAL::Cartesian_converter<Geom_traits, EK>();
    const auto from_exact = CGAL::Cartesian_converter<EK, Geom_traits>();
#if CGAL_CDT_3_CAN_USE_CXX20_FORMAT
    if(this->debug_regions()) {

      Mesh tets_intersect_region_mesh;
      auto [color_vpmap, _] = tets_intersect_region_mesh.template add_property_map<Face_index, int>("f:patch_id");

      for(auto ch : tr.finite_cell_handles()) {
        auto tetrahedron = typename Geom_traits::Tetrahedron_3{tr.point(ch->vertex(0)), tr.point(ch->vertex(1)),
                                                              tr.point(ch->vertex(2)), tr.point(ch->vertex(3))};
        if(!std::any_of(fh_region.begin(), fh_region.end(), [&](auto fh) {
            const auto v0 = fh->vertex(0)->info().vertex_handle_3d;
            const auto v1 = fh->vertex(1)->info().vertex_handle_3d;
            const auto v2 = fh->vertex(2)->info().vertex_handle_3d;
            const auto triangle = typename Geom_traits::Triangle_3{tr.point(v0), tr.point(v1), tr.point(v2)};
            return does_tetrahedron_intersect_triangle_interior(tetrahedron, triangle, tr.geom_traits());
          }))
        {
          continue;
        }
        bool intersects = false;
        for(int i = 0; i < 4; ++i) {
          for(int j = i + 1; j < 4; ++j) {
            int intersects_region = does_edge_intersect_region(ch, i, j, cdt_2, fh_region);
            if(intersects_region != 0) {
              intersects = true;
            }
          }
        }
        if(!intersects) {
          std::cerr << "ERROR: tetrahedron #" << ch->time_stamp() << " has no edge intersecting the region\n";
        }
        std::ofstream dump_tetrahedron(
            std::format("dump_intersecting_{}_{}_tetrahedron_{}.off", face_index, region_index, ch->time_stamp()));
        dump_tetrahedron.precision(17);
        Mesh mesh;
        CGAL::make_tetrahedron(tr.point(ch->vertex(0)), tr.point(ch->vertex(1)), tr.point(ch->vertex(2)),
                                tr.point(ch->vertex(3)), mesh);
        dump_tetrahedron << mesh;
        dump_tetrahedron.close();

        auto exact_tetrahedron = to_exact(tetrahedron);
        for(auto fh : fh_region) {
          auto v0 = fh->vertex(0)->info().vertex_handle_3d;
          auto v1 = fh->vertex(1)->info().vertex_handle_3d;
          auto v2 = fh->vertex(2)->info().vertex_handle_3d;
          auto triangle = typename Geom_traits::Triangle_3{tr.point(v0), tr.point(v1), tr.point(v2)};

          auto exact_triangle = to_exact(triangle);
          auto tetrahedron_triangle_intersection_opt = CGAL::intersection(exact_tetrahedron, exact_triangle);
          if(!tetrahedron_triangle_intersection_opt) {
            continue;
          }
          if(const auto* tri = std::get_if<Epeck::Triangle_3>(&tetrahedron_triangle_intersection_opt.value())) {
            exact(*tri);
            auto v0 = tets_intersect_region_mesh.add_vertex(from_exact((*tri)[0]));
            auto v1 = tets_intersect_region_mesh.add_vertex(from_exact((*tri)[1]));
            auto v2 = tets_intersect_region_mesh.add_vertex(from_exact((*tri)[2]));
            std::array arr{v0, v1, v2};
            auto f = CGAL::Euler::add_face(arr, tets_intersect_region_mesh);
            put(color_vpmap, f, static_cast<int>(ch->time_stamp()));
          }
          if(const auto* vec = std::get_if<std::vector<Epeck::Point_3>>(&tetrahedron_triangle_intersection_opt.value()))
          {
            std::vector<typename Mesh::Vertex_index> vec_of_indices;
            for(const auto& p : *vec) {
              exact(p);
              vec_of_indices.push_back(tets_intersect_region_mesh.add_vertex(from_exact(p)));
            }
            CGAL::Euler::add_face(vec_of_indices, tets_intersect_region_mesh);
          }
        }
      }
      std::ofstream tets_intersect_region_out(
          std::format("dump_tets_intersect_region_{}_{}.ply", face_index, region_index));
      tets_intersect_region_out.precision(17);
      CGAL::IO::write_PLY(tets_intersect_region_out, tets_intersect_region_mesh);
      tets_intersect_region_out.close();
    }
#endif // CGAL_CDT_3_CAN_USE_CXX20_FORMAT

    intersecting_edges.push_back(first_intersecting_edge);
    const auto [v0, v1] = tr.vertices(first_intersecting_edge);
    (void)new_edge(v0, v1, true);
    for(std::size_t i = 0; i < intersecting_edges.size(); ++i) {
      const auto intersecting_edge = intersecting_edges[i];
      const auto [v_above, v_below] = tr.vertices(intersecting_edge);
#if CGAL_CDT_3_CAN_USE_CXX20_FORMAT
      if(this->debug_regions()) {
        std::cerr << std::format("restore_subface_region face index: {}, region #{}, intersecting edge #{}: ({}   {})\n",
                                  face_index, region_index, i,
                                  IO::oformat(v_above, with_point_and_info),
                                  IO::oformat(v_below, with_point_and_info));
        dump_region(face_index, region_index, cdt_2);
      }
#endif // CGAL_CDT_3_CAN_USE_CXX20_FORMAT

#if CGAL_CDT_3_CAN_USE_CXX20_FORMAT
      if(this->debug_regions()) {
        const auto p_above = this->point(v_above);
        const auto p_below = this->point(v_below);
        const auto edge_segment = typename Geom_traits::Segment_3{p_above, p_below};
        const auto exact_edge_segment = to_exact(edge_segment);

        std::ofstream intersect_out("dump_edge_region_intersection.xyz");
        intersect_out.precision(17);
        for(auto fh: fh_region) {
            auto v0 = fh->vertex(0)->info().vertex_handle_3d;
            auto v1 = fh->vertex(1)->info().vertex_handle_3d;
            auto v2 = fh->vertex(2)->info().vertex_handle_3d;
            auto triangle = typename Geom_traits::Triangle_3{tr.point(v0), tr.point(v1), tr.point(v2)};
            auto exact_triangle = to_exact(triangle);
            if(auto edge_intersection_opt = CGAL::intersection(exact_edge_segment, exact_triangle)) {
              const auto& edge_intersection = *edge_intersection_opt;
              if(const auto* p = std::get_if<Epeck::Point_3>(&edge_intersection)) {
                exact(*p);
                intersect_out << *p << '\n';
              }
            }
        }
        intersect_out.close();

        auto cells_around_intersecting_edge = Container_from_circulator{this->incident_cells(intersecting_edge)};
        for(const auto& cell: cells_around_intersecting_edge) {
          CGAL_assertion(!cell.has_vertex(tr.infinite_vertex()));
          auto tetrahedron =
              typename Geom_traits::Tetrahedron_3{tr.point(cell.vertex(0)), tr.point(cell.vertex(1)),
                                                  tr.point(cell.vertex(2)), tr.point(cell.vertex(3))};
          for(auto fh: fh_region) {
            auto v0 = fh->vertex(0)->info().vertex_handle_3d;
            auto v1 = fh->vertex(1)->info().vertex_handle_3d;
            auto v2 = fh->vertex(2)->info().vertex_handle_3d;
            auto triangle = typename Geom_traits::Triangle_3{tr.point(v0), tr.point(v1), tr.point(v2)};
            auto exact_triangle = to_exact(triangle);
          }

          std::cerr << std::format("Test tetrahedron (#{}):\n  {}\n  {}\n  {}\n  {}\n",
                                  cell.time_stamp(),
                                  IO::oformat(cell.vertex(0), with_point_and_info),
                                  IO::oformat(cell.vertex(1), with_point_and_info),
                                  IO::oformat(cell.vertex(2), with_point_and_info),
                                  IO::oformat(cell.vertex(3), with_point_and_info));
          if(!std::any_of(fh_region.begin(), fh_region.end(), [&](const auto fh) {
              auto v0 = fh->vertex(0)->info().vertex_handle_3d;
              auto v1 = fh->vertex(1)->info().vertex_handle_3d;
              auto v2 = fh->vertex(2)->info().vertex_handle_3d;
              auto triangle = typename Geom_traits::Triangle_3{tr.point(v0), tr.point(v1), tr.point(v2)};
              bool b = does_tetrahedron_intersect_triangle_interior(tetrahedron, triangle, tr.geom_traits());
              if(b) {
                std::cerr << "  intersects the region\n";
              }
              return b;
            }))
          {
            std::cerr << std::format(
                "ERROR: The following tetrahedron (#{}) does not intersect the region:\n  {}\n  {}\n  {}\n  {}\n",
                cell.time_stamp(),
                IO::oformat(cell.vertex(0), with_point_and_info), IO::oformat(cell.vertex(1), with_point_and_info),
                IO::oformat(cell.vertex(2), with_point_and_info), IO::oformat(cell.vertex(3), with_point_and_info));
          }
        }
      }
#endif // CGAL_CDT_3_CAN_USE_CXX20_FORMAT

      auto test_edge = [&](Cell_handle cell, Vertex_handle v0, int index_v0, Vertex_handle v1, int index_v1,
                           [[maybe_unused]] int expected) {
        auto value_returned = [this](bool b) {
#if CGAL_CDT_3_CAN_USE_CXX20_FORMAT
          if(this->debug_regions()) {
            std::cerr << std::format("   return {}\n", b);
          }
#endif // CGAL_CDT_3_CAN_USE_CXX20_FORMAT
          return b;
        };
#if CGAL_CDT_3_CAN_USE_CXX20_FORMAT
        if(this->debug_regions()) {
          std::cerr << std::format("  test_edge {}   {}  ", IO::oformat(v0, with_point_and_info),
                                   IO::oformat(v1, with_point_and_info));
        }
#endif // CGAL_CDT_3_CAN_USE_CXX20_FORMAT
        auto [cached_value_it, not_visited] = new_edge(v0, v1, false);
        if(!not_visited) return value_returned(cached_value_it->second);
        int v0v1_intersects_region =
            (v0->is_marked(Vertex_marker::REGION_INSIDE) || v1->is_marked(Vertex_marker::REGION_INSIDE))
                ? expected
                : does_edge_intersect_region(cell, index_v0, index_v1, cdt_2, fh_region);
        if(v0v1_intersects_region != 0) {
          if(this->use_older_cavity_algorithm()) {
            if(v0v1_intersects_region != expected) {
              throw PLC_error{"PLC error: v0v1_intersects_region != expected" ,
                    __FILE__, __LINE__, face_index, region_index};
            }
          }
          // report the edge with first vertex above the region
          if(v0v1_intersects_region < 0) {
            std::swap(index_v0, index_v1);
          }
          intersecting_edges.emplace_back(cell, index_v0, index_v1);
          cached_value_it->second = true;
          return value_returned(true);
        } else {
          non_intersecting_edges_set.insert(make_sorted_pair(v0, v1));
          cached_value_it->second = false;
          return value_returned(false);
        }
      };

      if(this->use_older_cavity_algorithm()) {
        CGAL_assertion(0 == region_border_vertices.count(v_above));
        CGAL_assertion(0 == region_border_vertices.count(v_below));
        if(new_vertex(v_above)) {
          vertices_of_upper_cavity.push_back(v_above);
        }
        if(new_vertex(v_below)) {
          vertices_of_lower_cavity.push_back(v_below);
        }
      }
      auto facet_circ = this->incident_facets(intersecting_edge);
      const auto facet_circ_end = facet_circ;
      do { // loop facets around [v_above, v_below]
        CGAL_assertion(false == this->is_infinite(*facet_circ));
        const auto cell = facet_circ->first;
        const auto facet_index = facet_circ->second;
        CGAL_assertion_msg(!cell->is_facet_constrained(facet_index), "intersecting polygons!");
        if(new_cell(cell)) {
          intersecting_cells.insert(cell);
        }
        const auto index_v_above = cell->index(v_above);
        const auto index_v_below = cell->index(v_below);
        const auto index_vc = 6 - index_v_above - index_v_below - facet_index;
        const auto vc = cell->vertex(index_vc);
        if(region_border_vertices.count(vc) > 0) continue; // intersecting edges cannot touch the border

        if(!test_edge(cell, v_above, index_v_above, vc, index_vc, 1) &&
           !test_edge(cell, v_below, index_v_below, vc, index_vc, -1) &&
           this->use_older_cavity_algorithm())
        {
          dump_triangulation();
          dump_region(face_index, region_index, cdt_2);
          {
            std::ofstream out(std::string("dump_two_edges_") + std::to_string(face_index) + ".polylines.txt");
            out.precision(17);
            write_segment(out, Edge{cell, index_v_above, index_vc});
            write_segment(out, Edge{cell, index_v_below, index_vc});
          }
          throw PLC_error{"PLC error: !test_edge(v_above..) && !test_edge(v_below..)" ,
                __FILE__, __LINE__, face_index, region_index};
        }
      } while(++facet_circ != facet_circ_end);
      if(!this->use_older_cavity_algorithm() && i + 1 == intersecting_edges.size()) {
        for(auto ch: intersecting_cells) {
          if(this->debug_regions()) {
            std::cerr << "tetrahedron #" << ch->time_stamp() << " intersects the region\n";
          }
          for(int i = 0; i < 4; ++i) {
            for(int j = i + 1; j < 4; ++j) {
              test_edge(ch, ch->vertex(i), i, ch->vertex(j), j, 1);
            }
          }
          for(int i = 0; i < 4; ++i) {
            auto n_ch = ch->neighbor(i);
            if(tr.is_infinite(n_ch))
              continue;
            if(new_cell(n_ch)) {
              auto tetrahedron =
                  typename Geom_traits::Tetrahedron_3{tr.point(n_ch->vertex(0)), tr.point(n_ch->vertex(1)),
                                                      tr.point(n_ch->vertex(2)), tr.point(n_ch->vertex(3))};
              auto tet_bbox = tetrahedron.bbox();
              if(std::any_of(fh_region.begin(), fh_region.end(), [&](auto fh) {
                   const auto v0 = fh->vertex(0)->info().vertex_handle_3d;
                   const auto v1 = fh->vertex(1)->info().vertex_handle_3d;
                   const auto v2 = fh->vertex(2)->info().vertex_handle_3d;
                   const auto triangle = typename Geom_traits::Triangle_3{tr.point(v0), tr.point(v1), tr.point(v2)};
                   const auto tri_bbox = triangle.bbox();
                   if(CGAL::do_overlap(tet_bbox, tri_bbox)) {
                     return does_tetrahedron_intersect_triangle_interior(tetrahedron, triangle, tr.geom_traits());
                   } else {
                     return false;
                   }
                 }))
              {
                intersecting_cells.insert(n_ch);
                if(this->debug_regions()) {
                  std::cerr << "tetrahedron #" << n_ch->time_stamp() << " intersects the region\n";
                }
              } else if(this->debug_regions()) {
                std::cerr << "NO, tetrahedron #" << n_ch->time_stamp() << " does not intersect the region\n";
              }
              for(int i = 0; i < 4; ++i) {
                for(int j = i + 1; j < 4; ++j) {
                  test_edge(n_ch, n_ch->vertex(i), i, n_ch->vertex(j), j, 1);
                }
              }
            }
          }
        }
      } // last intersecting edge, and new algorithm
    } // end loop on intersecting_edges
    if(this->use_older_cavity_algorithm()) {
      for(auto intersecting_edge: intersecting_edges) {
        const auto [v_above, v_below] = tr.vertices(intersecting_edge);

        auto cell_circ = this->incident_cells(intersecting_edge), end = cell_circ;
        CGAL_assume(cell_circ != nullptr);
        do {
          const Cell_handle cell = cell_circ;
          const auto index_v_above = cell->index(v_above);
          const auto index_v_below = cell->index(v_below);
          const auto cell_above = cell->neighbor(index_v_below);
          const auto cell_below = cell->neighbor(index_v_above);
          if(0 == intersecting_cells.count(cell_above)) {
            facets_of_upper_cavity.emplace_back(cell_above, cell_above->index(cell));
          }
          if(0 == intersecting_cells.count(cell_below)) {
            facets_of_lower_cavity.emplace_back(cell_below, cell_below->index(cell));
          }
        } while(++cell_circ != end);
      }
    } // older algorithm

    std::set<Facet> facets_of_border;
    Union_find<Vertex_handle> vertices_of_cavity_union_find;
    if(!this->use_older_cavity_algorithm()) {
      for(auto c: intersecting_cells) {
        for(int i = 0; i < 4; ++i) {
          auto n = c->neighbor(i);
          if(intersecting_cells.count(n) == 0) {
            facets_of_border.emplace(n, n->index(c));
          }
        }
      }
      Unique_hash_map<Vertex_handle, typename Union_find<Vertex_handle>::handle> vertices_of_cavity_handles;
      for(auto c: intersecting_cells) {
        for(auto v : tr.vertices(c)) {
          if(!v->is_marked()) {
            v->set_mark(Vertex_marker::CAVITY);
            vertices_of_cavity_handles[v] = vertices_of_cavity_union_find.make_set(v);
          }
        }
      }
      for(auto facet: facets_of_border) {
        auto vertices = tr.vertices(facet);
        for(int i = 0; i < 3; ++i) {
          auto v1 = vertices[i];
          auto v2 = vertices[(i + 1) % 3];
          if(v1->is_marked(Vertex_marker::CAVITY) && v2->is_marked(Vertex_marker::CAVITY)) {
            vertices_of_cavity_union_find.unify_sets(vertices_of_cavity_handles[v1],
                                                     vertices_of_cavity_handles[v2]);
          }
        }
      }
      if(vertices_of_cavity_union_find.number_of_sets() > 2) {
        for(auto c : intersecting_cells) {

          for(int i = 0; i < 4; ++i) {
            for(int j = i + 1; j < 4; ++j) {
              const auto v1 = c->vertex(i);
              const auto v2 = c->vertex(j);
              if(v1->is_marked(Vertex_marker::CAVITY) && v2->is_marked(Vertex_marker::CAVITY) &&
                 non_intersecting_edges_set.count(make_sorted_pair(v1, v2)) > 0)
              {
                vertices_of_cavity_union_find.unify_sets(vertices_of_cavity_handles[v1],
                                                         vertices_of_cavity_handles[v2]);
              }
            }
          }
          if(vertices_of_cavity_union_find.number_of_sets() <= 2)
            break;
        }
      }
      Vertex_handle vertex_above{};
      Edges_container all_border_edges{border_edges.begin(), border_edges.end()};
      std::for_each(border_edges.begin(), border_edges.end(), [&](auto edge) {
        all_border_edges.emplace_back(edge.first, edge.third, edge.second);
      });
      for(const auto& border_edge: all_border_edges) {
        const auto [border_edge_va, border_edge_vb] = tr.vertices(border_edge);
        auto circ = tr.incident_cells(border_edge);
        CGAL_assertion(circ != nullptr);
        const auto end = circ;
        do {
          const auto index_va = circ->index(border_edge_va);
          const auto index_vb = circ->index(border_edge_vb);
          const auto face_index = tr.next_around_edge(index_va, index_vb);
          if(facets_of_border.count(Facet{circ, face_index}) > 0) {
            const auto other_vertex_index = 6 - index_va - index_vb - face_index;
            const auto other_vertex = circ->vertex(other_vertex_index);
            if(other_vertex->is_marked(Vertex_marker::CAVITY)) {
              vertex_above = circ->vertex(other_vertex_index);
              break;
            }
          }
        } while(++circ != end);
        if(vertex_above != Vertex_handle{}) break;
      }
      CGAL_assume(vertex_above != Vertex_handle{});

      const auto vertex_above_handle = vertices_of_cavity_handles[vertex_above];
      auto it = vertices_of_cavity_union_find.begin();
      while(it != vertices_of_cavity_union_find.end() &&
            vertices_of_cavity_union_find.same_set(it, vertex_above_handle))
      {
        ++it;
      }
      CGAL_assertion((it == vertices_of_cavity_union_find.end()) ==
                     (vertices_of_cavity_union_find.number_of_sets() == 1));
      const auto vertex_below_handle = it;
      for(auto handle = vertices_of_cavity_union_find.begin(), end = vertices_of_cavity_union_find.end();
          handle != end; ++handle)
      {
        auto v = *handle;
        v->clear_mark(Vertex_marker::CAVITY);
        if(vertices_of_cavity_union_find.same_set(handle, vertex_above_handle)) {
          vertices_of_upper_cavity.push_back(v);
          v->set_mark(Vertex_marker::CAVITY_ABOVE);
        } else if(vertices_of_cavity_union_find.same_set(handle, vertex_below_handle)) {
          vertices_of_lower_cavity.push_back(v);
          v->set_mark(Vertex_marker::CAVITY_BELOW);
        } else {
          CGAL_error();
        }
      }
      while(std::any_of(intersecting_cells.begin(), intersecting_cells.end(), [&](Cell_handle c) {
           const auto vs = tr.vertices(c);
           return std::any_of(vs.begin(), vs.end(), [&](auto v) {
            if(!v->is_marked()) {
              std::cerr << "INFO: Vertex " << IO::oformat(v, with_point_and_info) << " is not marked\n";
              return true;
            }
            return false;
          });
         }))
      {
        std::for_each(intersecting_cells.begin(), intersecting_cells.end(), [&](Cell_handle c) {
          for(int i = 0; i < 4; ++i) {
            for(int j = i + 1; j < 4; ++j) {
              auto v1 = c->vertex(i);
              auto v2 = c->vertex(j);
              if(v1->is_marked() != v2->is_marked()) {
                if(v2->is_marked()) {
                  std::swap(v1, v2);
                } // here v1 is marked and v2 is not
                if(v1->is_marked(Vertex_marker::CAVITY_ABOVE)) {
                  vertices_of_upper_cavity.push_back(v2);
                  v2->set_mark(Vertex_marker::CAVITY_ABOVE);
                } else if(v1->is_marked(Vertex_marker::CAVITY_BELOW)) {
                  vertices_of_lower_cavity.push_back(v2);
                  v2->set_mark(Vertex_marker::CAVITY_BELOW);
                }
              }
            }
          }
        });
      }
    } // new algorithm
    if(this->debug_regions()) {
      std::stringstream ss_filename;
      ss_filename << "dump_facets_of_cavity_region_" << face_index << "_" << region_index << "_border.off";
      std::ofstream out(ss_filename.str());
      out.precision(17);
      write_facets(out, tr, facets_of_border);
    }

    if(this->debug_regions()) {
      for(auto edge : intersecting_edges) {
        auto [v1, v2] = tr.vertices(edge);
        std::cerr << std::format("  edge: {}   {}\n", IO::oformat(v1, with_point_and_info),
                                IO::oformat(v2, with_point_and_info));
      }
    }
    if(!this->use_older_cavity_algorithm()) {
      for(auto facet: facets_of_border) {
        if(this->debug_regions()) {
          std::cerr << "  facet:  ";
          const auto facet_vertices = tr.vertices(facet);
          for(auto v: facet_vertices) {
            std::cerr << IO::oformat(v, with_point_and_info) << "  ";
          }
          // This assertion is wrong, because there might be only one half-cavity and not a full cavity.
          // CGAL_assertion(!std::all_of(facet_vertices.begin(), facet_vertices.end(),
          //                             [](auto v) { return v->is_marked(Vertex_marker::REGION_BORDER); }));
          std::cerr << "\n";
        }
        for(auto v: tr.vertices(facet)) {
          if(v->is_marked(Vertex_marker::CAVITY_ABOVE)) {
            facets_of_upper_cavity.push_back(facet);
            break;
          }
          if(v->is_marked(Vertex_marker::CAVITY_BELOW)) {
            facets_of_lower_cavity.push_back(facet);
            break;
          }
        }
      }
      for(auto v: vertices_of_upper_cavity) {
        v->clear_mark(Vertex_marker::CAVITY_ABOVE);
      }
      for(auto v: vertices_of_lower_cavity) {
        v->clear_mark(Vertex_marker::CAVITY_BELOW);
      }
    }
    if(this->debug_regions()) {
      std::stringstream ss_filename;
      ss_filename << "dump_facets_of_upper_cavity_region_" << face_index << "_" << region_index << "_border.off";
      std::ofstream out(ss_filename.str());
      out.precision(17);
      write_facets(out, tr, facets_of_upper_cavity);
      out.close();
      ss_filename.str("");
      ss_filename << "dump_facets_of_lower_cavity_region_" << face_index << "_" << region_index << "_border.off";
      out.open(ss_filename.str());
      write_facets(out, tr, facets_of_lower_cavity);
      out.close();
    }

    return outputs;
  }

  template <typename Tr, typename Function>
  static void visit_convex_hull_of_triangulation(const Tr& tr, Function f)
  {
    const auto inf_vh = tr.infinite_vertex();
    tr.incident_cells(inf_vh, boost::make_function_output_iterator([&](Cell_handle c) {
                        const auto facet_index = c->index(inf_vh);
                        f(Facet{c, facet_index});
                        return true;
                      }));
  }

  using Conforming_Dt::with_offset;
  using Conforming_Dt::with_point;
  using Conforming_Dt::with_point_and_info;

  template <typename Fh_region>
  void restore_subface_region(CDT_3_face_index face_index, int region_index,
                              CDT_2& non_const_cdt_2, Fh_region& non_const_fh_region)
  {
    if(this->debug_regions()) {
      std::cerr << "restore_subface_region face index: " << face_index << ", region #" << region_index << "\n";
    }
    const auto& cdt_2 = non_const_cdt_2;
    const auto& fh_region = non_const_fh_region;
    const auto border_edges = brute_force_border_3_of_region(face_index, region_index, cdt_2, fh_region);
    const auto region_vertices = std::invoke([&]() {
      std::set<Vertex_handle> vertices;
      for(const auto fh_2d: fh_region) {
        for(int i = 0; i < 3; ++i) {
          vertices.insert(fh_2d->vertex(i)->info().vertex_handle_3d);
        }
      }
      return vertices;
    });
    const auto region_border_vertices = std::invoke([&]() {
      std::set<Vertex_handle> vertices;
      for(const auto& [c, i, j]: border_edges) {
        vertices.insert(c->vertex(i));
        vertices.insert(c->vertex(j));
      }
      return vertices;
    });
#if CGAL_CDT_3_CAN_USE_CXX20_FORMAT
    if(this->debug_regions()) {
      std::cerr << "region_border_vertices.size() = " << region_border_vertices.size() << "\n";
      for(auto v : region_border_vertices) {
        std::cerr << std::format("  {}\n", IO::oformat(v, with_point));
      }
    }
#endif // CGAL_CDT_3_CAN_USE_CXX20_FORMAT
    for(auto v: region_border_vertices) {
      v->set_mark(Vertex_marker::REGION_BORDER);
    }
    const auto found_edge_opt = search_first_intersection(face_index, cdt_2, fh_region, border_edges);
    for(auto v: region_border_vertices) {
      v->clear_mark(Vertex_marker::REGION_BORDER);
    }

    [[maybe_unused]] auto try_flip_region_size_4 = [&] {
      if(region_border_vertices.size() == 4) {
        std::set<Vertex_handle> vertices;
        std::set<Vertex_handle> diagonal;
        for(auto fh : fh_region) {
          for(int i = 0; i < 3; ++i) {
            auto [it, new_vertex] = vertices.insert(fh->vertex(i)->info().vertex_handle_3d);
            if(!new_vertex) {
              diagonal.insert(*it);
            }
          }
        }
        std::set<Vertex_handle> other_diagonal;
        std::set_difference(region_border_vertices.begin(), region_border_vertices.end(),
                            diagonal.begin(), diagonal.end(),
                            std::inserter(other_diagonal, other_diagonal.begin()));
        CGAL_assertion(diagonal.size() == 2);
        CGAL_assertion(other_diagonal.size() == 2);

        const auto diagonal_index = fh_region[0]->index(fh_region[1]);
        CGAL_assertion(diagonal_index >= 0 && diagonal_index < 3);
        const auto v0 = fh_region[0]->vertex(diagonal_index)->info().vertex_handle_3d;
        const auto v1 = fh_region[0]->vertex(cdt_2.ccw(diagonal_index))->info().vertex_handle_3d;
        const auto v2 = fh_region[0]->vertex(cdt_2.cw(diagonal_index))->info().vertex_handle_3d;
        const auto v3 = fh_region[1]->vertex(fh_region[1]->index(fh_region[0]))->info().vertex_handle_3d;
        if(tr.is_facet(v0, v1, v3) && tr.is_facet(v0, v3, v2))
        {
          if(cdt_2.orientation(v0->point(), v1->point(), v3->point()) == CGAL::POSITIVE &&
             cdt_2.orientation(v0->point(), v3->point(), v2->point()) == CGAL::POSITIVE)
          {
            std::cerr << "NOTE: the other diagonal is in the 3D triangulation: flip the edge\n";
            non_const_cdt_2.flip(non_const_fh_region[0], diagonal_index);
            for(auto fh : fh_region) {
              for(int i = 0; i < 3; ++i) {
                const auto mirror_edge = cdt_2.mirror_edge({fh, i});
                fh->set_constraint(i, mirror_edge.first->is_constrained(mirror_edge.second));
              }
              int i, j, k;
              Cell_handle c;
              [[maybe_unused]] bool fh_is_3d_facet = tr.is_facet(fh->vertex(0)->info().vertex_handle_3d,
                                                                fh->vertex(1)->info().vertex_handle_3d,
                                                                fh->vertex(2)->info().vertex_handle_3d,
                                                                c, i, j, k);
              CGAL_assertion(fh_is_3d_facet);
              set_facet_constrained({c, 6-i-j-k}, face_index, fh);
              fh->info().missing_subface = false;
            }
            return true;
          } else {
            std::cerr << "NOTE: the other diagonal is in the 3D triangulation BUT the edge is not flippable!\n";
            std::cerr << "  The region " << region_index << " of face #F" << face_index << " has four points:\n";
            std::cerr << "    v0: " << v0->point() << '\n';
            std::cerr << "    v1: " << v1->point() << '\n';
            std::cerr << "    v2: " << v2->point() << '\n';
            std::cerr << "    v3: " << v3->point() << '\n';
          }
        }

#if CGAL_CAN_USE_CXX20_FORMAT
        std::cerr << std::format
            ("NOTE: diagonal: {:.6} {:.6}  {} in tr\n",
            IO::oformat(*diagonal.begin(), with_point),
            IO::oformat(*std::next(diagonal.begin()), with_point),
            this->is_edge(*diagonal.begin(), *std::next(diagonal.begin())) ? "IS" : "is NOT");
        std::cerr << std::format(
            "NOTE: the other diagonal: {:.6} {:.6}  {} in tr\n",
            IO::oformat(*other_diagonal.begin(), with_point),
            IO::oformat(*std::next(other_diagonal.begin()), with_point),
            this->is_edge(*other_diagonal.begin(), *std::next(other_diagonal.begin())) ? "IS" : "is NOT");
        if(cdt_2.geom_traits().side_of_oriented_circle_2_object()(
               (*region_border_vertices.begin())->point(), (*std::next(region_border_vertices.begin()))->point(),
               (*std::next(region_border_vertices.begin(), 2))->point(),
               (*std::next(region_border_vertices.begin(), 3))->point()) == CGAL::ZERO)
        {
          std::cerr << std::format(
              "NOTE: In polygon #{}, region {}, the 4 vertices are co-circular in the 2D triangulation\n",
              face_index, region_index);
        }
        if(CGAL::coplanar(
               (*region_border_vertices.begin())->point(),
               (*std::next(region_border_vertices.begin()))->point(),
               (*std::next(region_border_vertices.begin(), 2))->point(),
               (*std::next(region_border_vertices.begin(), 3))->point()))
        {
          std::cerr << std::format("NOTE: In polygon #{}, region {}, the 4 vertices are coplanar\n",
                                   face_index, region_index);
          if(CGAL::coplanar_side_of_bounded_circle(
               (*region_border_vertices.begin())->point(),
               (*std::next(region_border_vertices.begin()))->point(),
               (*std::next(region_border_vertices.begin(), 2))->point(),
               (*std::next(region_border_vertices.begin(), 3))->point()) == CGAL::ON_BOUNDARY)
          {
            std::cerr << std::format(
                "NOTE: In polygon #{}, region {}, the 4 vertices are co-circular in the 3D triangulation\n",
                face_index, region_index);
          }
        }
#endif // CGAL_CAN_USE_CXX20_FORMAT
      }
      return false;
    };
    if(!found_edge_opt) {
      if(try_flip_region_size_4()) {
        return;
      }
      // {
      //   Constrained_Delaunay_triangulation_3 new_tr;
      //   for(const auto v : region_border_vertices) {
      //     new_tr.insert(v->point());
      //   }
      //   std::cerr << "new_tr.dimension() = " << new_tr.dimension() << '\n';
      //   std::ofstream out(std::string("dump_polygon_") + std::to_string(face_index) + "_tr.off");
      //   out.precision(17);
      //   if(new_tr.dimension() == 2) {
      //     write_facets(out, new_tr, new_tr.finite_facets());
      //   }
      //   else {
      //     write_facets(out, new_tr, std::views::filter(new_tr.finite_facets(), [&](auto f) {
      //                    return new_tr.is_infinite(f.first) || new_tr.is_infinite(f.first->neighbor(f.second));
      //                  }));
      //   }
      // }
      // {
      //   dump_edge_link(std::string("dump_around_edge_") + std::to_string(face_index) + "_" +
      //                  std::to_string(region_index) + ".polylines.txt", border_edges[0]);
      //   std::ofstream dump(std::string("dump_no_segment_found_") + std::to_string(face_index) + "_" +
      //                      std::to_string(region_index) + ".binary.cgal");
      //   CGAL::IO::save_binary_file(dump, *this);
        dump_region(face_index, region_index, cdt_2);
      // }
      throw Next_region{"No segment found", fh_region[0]};
    }
    CGAL_assertion(found_edge_opt != std::nullopt);

    for(auto v : region_border_vertices) {
      v->set_mark(Vertex_marker::REGION_BORDER);
    }
    for(auto v : region_vertices) {
      if(v->is_marked(Vertex_marker::REGION_BORDER))
        continue;
      v->set_mark(Vertex_marker::REGION_INSIDE);
    }

    Scope_exit guard{[&] {
      for(auto v : region_vertices) {
        v->clear_mark(Vertex_marker::REGION_BORDER);
        v->clear_mark(Vertex_marker::REGION_INSIDE);
      }
    }};

    const auto [first_intersecting_edge, _] = *found_edge_opt;
    const auto [intersecting_edges, original_intersecting_cells, original_vertices_of_upper_cavity,
                original_vertices_of_lower_cavity, original_facets_of_upper_cavity, original_facets_of_lower_cavity] =
        construct_cavities(face_index, region_index, cdt_2, fh_region, region_border_vertices, region_vertices,
                           first_intersecting_edge, border_edges);

    const std::set<Point_3> polygon_points = std::invoke([&](){
      std::set<Point_3> polygon_points;
      for(auto vh : region_vertices) {
        polygon_points.insert(this->point(vh));
      }
      return polygon_points;
    });

    auto is_facet_of_polygon = [&](const auto& tr, Facet f) {
      const auto [c, facet_index] = f;
      for(int i = 0; i < 3; ++i) {
        const auto vh = c->vertex(T_3::vertex_triple_index(facet_index, i));
        if(0 == polygon_points.count(tr.point(vh))) {
          return false;
        }
      }
      return true;
    };

#if CGAL_CDT_3_CAN_USE_CXX20_FORMAT
    if(this->debug_regions()) {
      std::cerr << std::format("Cavity has {} cells and {} edges, "
                              "{} vertices in upper cavity and {} in lower, "
                              "{} facets in upper cavity and {} in lower\n",
                              original_intersecting_cells.size(),
                              intersecting_edges.size(),
                              original_vertices_of_upper_cavity.size(),
                              original_vertices_of_lower_cavity.size(),
                              original_facets_of_upper_cavity.size(),
                              original_facets_of_lower_cavity.size());
      if(original_intersecting_cells.size() > 3 || intersecting_edges.size() > 1) {
        std::cerr << "!! Interesting case !!\n";
        // dump_region(face_index, region_index, cdt_2);
        // {
        //   std::ofstream out(std::string("dump_intersecting_edges_") + std::to_string(face_index) + "_" +
        //                     std::to_string(region_index) + ".polylines.txt");
        //   out.precision(17);
        //   for(auto edge: intersecting_edges) {
        //     write_segment(out, edge);
        //   }
        // }
        // dump_facets_of_cavity(face_index, region_index, "lower", original_facets_of_lower_cavity);
        // dump_facets_of_cavity(face_index, region_index, "upper", original_facets_of_upper_cavity);
      }
    }
#endif // CGAL_CDT_3_CAN_USE_CXX20_FORMAT
    auto register_internal_constrained_facet = [this](Facet f) { this->register_facet_to_be_constrained(f); };

    if(this->debug_copy_triangulation_into_hole()) {
      std::cerr << "# upper cavity\n";
    }
    [[maybe_unused]] const auto [upper_cavity_triangulation, vertices_of_upper_cavity,
                                 map_upper_cavity_vertices_to_ambient_vertices, facets_of_upper_cavity,
                                 interior_constrained_faces_upper, cells_of_upper_cavity] =
        triangulate_cavity(original_intersecting_cells, original_facets_of_upper_cavity, original_vertices_of_upper_cavity);
    std::for_each(interior_constrained_faces_upper.begin(), interior_constrained_faces_upper.end(),
                  register_internal_constrained_facet);
    if(this->debug_copy_triangulation_into_hole()) {
      std::cerr << "# lower cavity\n";
    }
    [[maybe_unused]] const auto [lower_cavity_triangulation, vertices_of_lower_cavity,
                                 map_lower_cavity_vertices_to_ambient_vertices, facets_of_lower_cavity,
                                 interior_constrained_faces_lower, cells_of_lower_cavity] =
        triangulate_cavity(original_intersecting_cells, original_facets_of_lower_cavity, original_vertices_of_lower_cavity);
    std::for_each(interior_constrained_faces_lower.begin(), interior_constrained_faces_lower.end(),
                  register_internal_constrained_facet);

    // the following transform_reduce is like `std::any_of` but without the fast-exit
    if(std::transform_reduce(fh_region.begin(), fh_region.end(), false, std::logical_or<bool>{}, [&](auto fh) {
      const auto v0 = fh->vertex(0)->info().vertex_handle_3d;
      const auto v1 = fh->vertex(1)->info().vertex_handle_3d;
      const auto v2 = fh->vertex(2)->info().vertex_handle_3d;
      auto is_fh_facet_of = [&](const auto& tr) -> std::optional<Facet> {
        return this->vertex_triple_is_facet_of_other_triangulation(*this, v0, v1, v2, tr);
      };

      const bool fail_upper = !is_fh_facet_of(upper_cavity_triangulation);
      const bool fail_lower = !is_fh_facet_of(lower_cavity_triangulation);
      if(fail_upper || fail_lower) {
        fh->info().is_in_region = 1;
        auto display_face = [&]() {
          std::stringstream s;
          s.precision(std::cerr.precision());
          s << "(" << IO::oformat(v0, this->with_offset) << ", " << IO::oformat(v1, this->with_offset)
            << ", " << IO::oformat(v2, this->with_offset) << ") = ( "
            << tr.point(v0) << "  " << tr.point(v1) << "  " << tr.point(v2)
            << " )";
          return s.str();
        };
        if(fail_upper) {
          std::cerr << "NOTE: Face " << display_face() << " is not a facet of the upper cavity\n";
        }
        if(fail_lower) {
          std::cerr << "NOTE: Face " << display_face() << " is not a facet of the lower cavity\n";
        }
        return true;
      }
      return false;
    })) {
      if(this->debug_missing_region()) {
        // debug_region_size_4();
        dump_region(face_index, region_index, cdt_2);
        std::for_each(fh_region.begin(), fh_region.end(), [](auto fh) { fh->info().is_in_region = 3; });
        // dump_3d_triangulation(face_index, region_index, "lower", lower_cavity_triangulation);
        // dump_3d_triangulation(face_index, region_index, "upper", upper_cavity_triangulation);
        auto dump_facets_of_cavity_border = [&](CDT_3_face_index face_index, int region_index, std::string type,
                                                const auto& cavity_triangulation) {
          std::ofstream out(std::string("dump_plane_facets_of_region_") + std::to_string(face_index) + "_" +
                            std::to_string(region_index) + "_" + type + ".off");
          std::ofstream other_out(std::string("dump_non_plane_facets_of_region_") + std::to_string(face_index) + "_" +
                            std::to_string(region_index) + "_" + type + ".off");
          out.precision(17);
          other_out.precision(17);

          std::vector<Facet> border_faces;
          std::vector<Facet> non_border_faces;
          visit_convex_hull_of_triangulation(cavity_triangulation,
              [&](Facet f) {
                if(is_facet_of_polygon(cavity_triangulation, f))
                  border_faces.push_back(f);
                else
                  non_border_faces.push_back(f);
              });
          CGAL_warning(!border_faces.empty());
          write_facets(out, cavity_triangulation, border_faces);
          write_facets(other_out, cavity_triangulation, non_border_faces);
        };
        dump_facets_of_cavity_border(face_index, region_index, "lower", lower_cavity_triangulation);
        dump_facets_of_cavity_border(face_index, region_index, "upper", upper_cavity_triangulation);
      }
      throw Next_region{"missing facet in polygon", fh_region[0]};
    }

    insert_in_conflict_visitor.process_cells_in_conflict(cells_of_upper_cavity.begin(), cells_of_upper_cavity.end());
    insert_in_conflict_visitor.process_cells_in_conflict(cells_of_lower_cavity.begin(), cells_of_lower_cavity.end());

    if(this->debug_copy_triangulation_into_hole()) {
      std::cerr << "# glu the upper triangulation of the cavity\n";
      if(cells_of_lower_cavity.size() > original_intersecting_cells.size() ||
        cells_of_upper_cavity.size() > original_intersecting_cells.size())
      {
        std::cerr << std::format("!! Cavity has grown and has now "
                                "{} vertices in upper cavity and {} in lower, "
                                "{} facets in upper cavity and {} in lower\n",
                                vertices_of_upper_cavity.size(),
                                vertices_of_lower_cavity.size(),
                                facets_of_upper_cavity.size(),
                                facets_of_lower_cavity.size());
      }
    }

    typename T_3::Vertex_triple_Facet_map outer_map;
    auto add_to_outer_map = [this, &outer_map](typename T_3::Vertex_triple vt, Facet f,
                                               [[maybe_unused]] std::string_view extra = {}) {
      outer_map[vt] = f;
#if CGAL_CDT_3_CAN_USE_CXX20_FORMAT
      if(this->debug_copy_triangulation_into_hole()) {
        CGAL_assertion(vt[0] != vt[1]);
        CGAL_assertion(vt[0] != vt[2]);
        CGAL_assertion(vt[1] != vt[2]);
        std::cerr << std::format("outer map: Adding {}triple ({:.6}, {:.6}, {:.6})\n", extra,
                                IO::oformat(vt[0], with_point),
                                IO::oformat(vt[1], with_point),
                                IO::oformat(vt[2], with_point));
        std::ofstream out("dump_upper_outer_map.off");
        out.precision(17);
        write_facets(out, *this, std::ranges::views::values(outer_map));
        out.close();
      }
#endif // CGAL_CDT_3_CAN_USE_CXX20_FORMAT
    };
    auto fill_outer_map_of_cavity = [&](const auto&, const auto& facets) {
      for(auto f : facets) {
        typename T_3::Vertex_triple vt = this->make_vertex_triple(f);
        this->make_canonical_oriented_triple(vt);
        add_to_outer_map(vt, f);
      }
    };

    fill_outer_map_of_cavity(upper_cavity_triangulation, facets_of_upper_cavity);

    auto add_pseudo_cells_to_outer_map = [&](const auto& tr, const auto& map_cavity_vertices_to_ambient_vertices,
                                             bool is_upper_cavity) { // @TODO: comment this piece of code
                                             // @TODO: this should not be a lambda
      std::vector<std::pair<Cell_handle, CDT_2_face_handle>> pseudo_cells;
      std::vector<Facet> facets_of_polygon;
      for(auto f : tr.finite_facets()) {
        if(!is_facet_of_polygon(tr, f))
          continue;
        const auto is_facet = facet_is_facet_of_cdt_2(tr, f, cdt_2);
        if(!is_facet)
          continue; // we might be in a sliver in the plane of the polygon
        const auto [fh_2d, reverse_orientation] = *is_facet;
        if(this->debug_regions()) facets_of_polygon.push_back(f);
        const auto vt_aux = this->make_vertex_triple(f);
        typename T_3::Vertex_triple vt{map_cavity_vertices_to_ambient_vertices[vt_aux[0]],
                                       map_cavity_vertices_to_ambient_vertices[vt_aux[1]],
                                       map_cavity_vertices_to_ambient_vertices[vt_aux[2]]};
        this->make_canonical_oriented_triple(vt);
        if(reverse_orientation == is_upper_cavity) {
          std::swap(vt[1], vt[2]);
        }
        auto new_cell = this->tds().create_cell();
        pseudo_cells.emplace_back(new_cell, fh_2d);
        new_cell->set_vertices(vt[0], vt[1], vt[2], this->infinite_vertex());
        CGAL_assertion(static_cast<bool>(facet_is_facet_of_cdt_2(*this, {new_cell, 3}, cdt_2)));
        add_to_outer_map(vt, {new_cell, 3}, "extra ");
      }
      if(this->debug_regions()) {
        std::ofstream out(std::format("dump_{}_pseudo_cells_region_{}_{}.off", is_upper_cavity ? "upper" : "lower",
                                      face_index, region_index));
        out.precision(17);
        write_facets(out, tr, facets_of_polygon);
      }
      return pseudo_cells;
    };
    const auto pseudo_cells =
        add_pseudo_cells_to_outer_map(upper_cavity_triangulation, map_upper_cavity_vertices_to_ambient_vertices, true);

    {
// #if CGAL_DEBUG_CDT_3 & 64
//       std::ofstream out("dump_upper_outer_map.off");
//       out.precision(17);
//       write_facets(out, *this, std::ranges::views::values(outer_map));
//       out.close();
// #endif // CGAL_DEBUG_CDT_3
      const auto upper_inner_map = tr.create_triangulation_inner_map(
          upper_cavity_triangulation, map_upper_cavity_vertices_to_ambient_vertices, false);

#if CGAL_CDT_3_CAN_USE_CXX20_FORMAT
      if(this->debug_copy_triangulation_into_hole()) {
        std::cerr << "upper_inner_map:\n";
        for(auto [vt, _] : upper_inner_map) {
          std::cerr << std::format("  {:.6}, {:.6}, {:.6})\n",
                                  IO::oformat(vt[0], with_point),
                                  IO::oformat(vt[1], with_point),
                                  IO::oformat(vt[2], with_point));
        }
      }
#endif // CGAL_CDT_3_CAN_USE_CXX20_FORMAT
      if(this->debug_copy_triangulation_into_hole()) {
        std::cerr << "# glu the lower triangulation of the cavity\n";
      }
      this->copy_triangulation_into_hole(map_upper_cavity_vertices_to_ambient_vertices,
                                         std::move(outer_map),
                                         upper_inner_map,
                                         this->new_cells_output_iterator());
    }
#if CGAL_DEBUG_CDT_3 & 64
    std::cerr << "# glu the lower triangulation of the cavity\n";
#endif // CGAL_DEBUG_CDT_3

    outer_map.clear();
    std::vector<std::pair<Facet, CDT_2_face_handle>> new_constrained_facets;
    new_constrained_facets.reserve(pseudo_cells.size());
    for(const auto& [c, fh_2d] : pseudo_cells) {
      const Facet f = this->mirror_facet({c, 3});
      new_constrained_facets.emplace_back(f, fh_2d);
      CGAL_assertion(static_cast<bool>(facet_is_facet_of_cdt_2(*this, f, cdt_2)));
      auto vt = this->make_vertex_triple(f);
      this->make_canonical_oriented_triple(vt);
      add_to_outer_map(vt, f);
      this->tds().delete_cell(c);
    }
    fill_outer_map_of_cavity(lower_cavity_triangulation, facets_of_lower_cavity);
    {
      const auto lower_inner_map = tr.create_triangulation_inner_map(
          lower_cavity_triangulation, map_lower_cavity_vertices_to_ambient_vertices, false);
#if CGAL_CDT_3_CAN_USE_CXX20_FORMAT
      if(this->debug_copy_triangulation_into_hole()) {
        std::cerr << "outer_map:\n";
        for(auto [vt, _] : outer_map) {
          std::cerr << std::format("  {:.6}, {:.6}, {:.6})\n",
                                  IO::oformat(vt[0], with_point),
                                  IO::oformat(vt[1], with_point),
                                  IO::oformat(vt[2], with_point));
        }
        std::ofstream out("dump_lower_outer_map.off");
        out.precision(17);
        write_facets(out, *this, std::ranges::views::values(outer_map));
        out.close();
      }
#endif // CGAL_CDT_3_CAN_USE_CXX20_FORMAT
      this->copy_triangulation_into_hole(map_lower_cavity_vertices_to_ambient_vertices, std::move(outer_map), lower_inner_map,
                                         this->new_cells_output_iterator());
    }
    std::set<Cell_handle> cells_to_remove{cells_of_lower_cavity.begin(), cells_of_lower_cavity.end()};
    cells_to_remove.insert(cells_of_upper_cavity.begin(), cells_of_upper_cavity.end());
    for(auto c : cells_to_remove) {
      this->tds().delete_cell(c);
    }

    auto restore_markers = [&](Facet outside_facet) {
      const auto [outside_cell, outside_face_index] = outside_facet;
      const auto mirror_facet = this->mirror_facet(outside_facet);
      if(outside_cell->is_facet_constrained(outside_face_index)) {
        const auto poly_id = outside_cell->face_constraint_index(outside_face_index);
        const CDT_2& cdt_2 = face_cdt_2[poly_id];
        const auto f2d = outside_cell->face_2(cdt_2, outside_face_index);
        set_facet_constrained(mirror_facet, poly_id, f2d);
      }
    };

    std::for_each(facets_of_lower_cavity.begin(), facets_of_lower_cavity.end(), restore_markers);
    std::for_each(facets_of_upper_cavity.begin(), facets_of_upper_cavity.end(), restore_markers);

    for(const auto& [f, f2d] : new_constrained_facets) {
      set_facet_constrained(f, face_index, f2d);
      f2d->info().missing_subface = false;
    }
    CGAL_assume(!this->debug_validity() || this->is_valid(true));
  };

  struct Oriented_face_of_cdt_2 {
    CDT_2_face_handle fh;
    bool reversed_orientation = false;
  };

  static auto vertex_of_cdt_2_functor(const CDT_2& cdt_2) {
    return [&, hint = CDT_2_face_handle{}](const auto& p) mutable {
      int i;
      typename CDT_2::Locate_type lt;
      const auto fh = cdt_2.locate(p, lt, i, hint);
      CGAL_assume(lt == CDT_2::VERTEX);
      hint = fh;
      return fh->vertex(i);
    };
  }

  template <typename Tr>
  static auto facet_is_facet_of_cdt_2(const Tr& tr, typename Tr::Facet f, const CDT_2& cdt_2)
      -> std::optional<Oriented_face_of_cdt_2>
  {
    const auto [c, facet_index] = f;
    const auto v0 = c->vertex(Tr::vertex_triple_index(facet_index, 0));
    const auto v1 = c->vertex(Tr::vertex_triple_index(facet_index, 1));
    const auto v2 = c->vertex(Tr::vertex_triple_index(facet_index, 2));

    auto v = vertex_of_cdt_2_functor(cdt_2);

    const auto cdt_2_v0 = v(tr.point(v0));
    const auto cdt_2_v1 = v(tr.point(v1));
    const auto cdt_2_v2 = v(tr.point(v2));

    CDT_2_face_handle fh;
    const bool is_face = cdt_2.is_face(cdt_2_v0, cdt_2_v1, cdt_2_v2, fh);
    if(is_face && fh->info().is_in_region != 0) {
      const int index_v0 = fh->index(cdt_2_v0);
      const bool reverse_orientation = (cdt_2_v2 == fh->vertex(T_3::ccw(index_v0)));
      return Oriented_face_of_cdt_2{fh, reverse_orientation};
    }
    else
      return std::nullopt;
  }

  auto edge_of_cdt_2(const CDT_2& cdt_2, const Vertex_handle va, const Vertex_handle vb) const
      -> std::optional<typename CDT_2::Edge>
  {
    auto v = vertex_of_cdt_2_functor(cdt_2);

    const auto cdt_2_v0 = v(this->point(va));
    const auto cdt_2_v1 = v(this->point(vb));
    CDT_2_face_handle fh;
    int edge_index;
    const bool is_edge = cdt_2.is_edge(cdt_2_v0, cdt_2_v1, fh, edge_index);
    if(is_edge) {
      typename CDT_2::Edge edge{fh, edge_index};
      // if(fh->vertex(cdt_2.cw(edge_index)) != cdt_2_v0) {
      //   edge = cdt_2.mirror_edge(edge);
      // }
      CGAL_assertion(edge.first->vertex(cdt_2.cw(edge.second)) == cdt_2_v0);
      CGAL_assertion(edge.first->vertex(cdt_2.ccw(edge.second)) == cdt_2_v1);
      return edge;
    }
    else
      return std::nullopt;
  }

  template <typename Tr1, typename Tr2, typename Vertex_handle1>
  static auto vertex_triple_is_facet_of_other_triangulation(
      const Tr1& tr, Vertex_handle1 v0, Vertex_handle1 v1, Vertex_handle1 v2, const Tr2& other_tr)
      -> std::optional<typename Tr2::Facet>
  {
    const auto p0 = tr.point(v0);
    const auto p1 = tr.point(v1);
    const auto p2 = tr.point(v2);
    auto v = [&, hint = typename Tr2::Cell_handle{}](const auto& p) mutable {
      int i, j;
      Locate_type lt;
      const auto c = other_tr.locate(p, lt, i, j, hint);
      if(lt != T_3::VERTEX) {
        std::cerr << std::format("vertex_triple_is_facet_of_other_triangulation: point {}  lt = {}\n", IO::oformat(p),
                                 int(lt));
      }
      CGAL_assume(lt == T_3::VERTEX);
      hint = c;
      return c->vertex(i);
    };
    typename Tr2::Cell_handle c;
    int i, j, k;
    const bool ok = other_tr.is_facet(v(p0), v(p1), v(p2), c, i, j, k);
    if(ok)
      return {typename Tr2::Facet(c, 6 - i - j - k)};
    else
      return {std::nullopt};
  };

  template <typename Cell_range, typename Facets_range, typename Vertices_range>
  auto triangulate_cavity(const Cell_range& orig_cells_of_cavity,
                          const Facets_range& orig_facets_of_cavity_border,
                          const Vertices_range& orig_vertices_of_cavity) const ///@TODO: not deterministic, without time stamps
  {
    using Vertex_map = typename T_3::Vertex_handle_unique_hash_map;
    struct {
      T_3 cavity_triangulation;
      std::set<Vertex_handle> vertices;
      Vertex_map vertices_to_ambient_vertices;
      std::set<Facet> facets_of_cavity_border_;
      std::vector<Facet> interior_constrained_faces;
      std::set<Cell_handle> cell_of_cavity_;
    } result{ {},
              {orig_vertices_of_cavity.begin(), orig_vertices_of_cavity.end()},
              {},
              {orig_facets_of_cavity_border.begin(), orig_facets_of_cavity_border.end()},
              {},
              {orig_cells_of_cavity.begin(), orig_cells_of_cavity.end()}
        };
    auto& cavity_triangulation =  result.cavity_triangulation;
    auto& map_cavity_vertices_to_ambient_vertices = result.vertices_to_ambient_vertices;
    auto& vertices_of_cavity = result.vertices;
    auto& facets_of_cavity_border = result.facets_of_cavity_border_;
    auto& cells_of_cavity = result.cell_of_cavity_;
    CGAL::Unique_hash_map<Vertex_handle, Vertex_handle> map_ambient_vertices_to_cavity_vertices;

    auto insert_new_vertex = [&](Vertex_handle v, [[maybe_unused]] std::string_view extra = "") {
      const auto cavity_v =
          tr.is_infinite(v) ? cavity_triangulation.infinite_vertex() : cavity_triangulation.insert(this->point(v));
      map_ambient_vertices_to_cavity_vertices[v] = cavity_v;
      map_cavity_vertices_to_ambient_vertices[cavity_v] = v;
#if CGAL_CDT_3_CAN_USE_CXX20_FORMAT
      if(this->debug_regions()) {
        std::cerr << std::format("inserted {}cavity vertex {:.6} -> {:.6}\n",
                                extra,
                                IO::oformat(cavity_v, with_point_and_info),
                                IO::oformat(v, with_point_and_info));
      }
#endif // CGAL_CDT_3_CAN_USE_CXX20_FORMAT
      return cavity_v;
    };

    for(const auto v : vertices_of_cavity) {
      insert_new_vertex(v);
    }

    boost::container::small_vector<Facet, 32> missing_faces;
    do {
      missing_faces.clear();
      boost::container::small_vector<Facet, 32> internal_facets;
      for(auto f : facets_of_cavity_border) {
        if(cells_of_cavity.count(f.first) > 0) {
          // internal facet, due to cavity growing
          internal_facets.push_back(f);
          continue;
        }
        const auto [v0, v1, v2] = this->make_vertex_triple(f);
        Cell_handle c;
        int i, j, k;
        if(!cavity_triangulation.is_facet(map_ambient_vertices_to_cavity_vertices[v0],
                                          map_ambient_vertices_to_cavity_vertices[v1],
                                          map_ambient_vertices_to_cavity_vertices[v2], c, i, j, k))
        {
          missing_faces.push_back(f);
        }
      }
      for(auto f : internal_facets) {
        facets_of_cavity_border.erase(f);
      }
      for(auto [cell, facet_index] : missing_faces) {
        facets_of_cavity_border.erase({cell, facet_index});
        if(cell->is_facet_constrained(facet_index)) {
          result.interior_constrained_faces.emplace_back(cell, facet_index);
        }
        auto is_new_cell = cells_of_cavity.insert(cell).second;
        if(!is_new_cell)
          continue;
        const auto v3 = cell->vertex(facet_index);
        auto v3_is_new_vertex = vertices_of_cavity.insert(v3).second;
        if(v3_is_new_vertex) {
          insert_new_vertex(v3, "extra ");
        }
        for(int i = 0; i < 3; ++i) {
          Facet other_f{cell, this->vertex_triple_index(facet_index, i)};
          Facet mirror_f = this->mirror_facet(other_f);
          if(cells_of_cavity.count(mirror_f.first) == 0) {
            facets_of_cavity_border.insert(mirror_f);
          }
        }
      }
    } while(!missing_faces.empty());
    CGAL_assertion(std::all_of(facets_of_cavity_border.begin(), facets_of_cavity_border.end(), [&](const auto& f) {
      const auto [v0, v1, v2] = this->make_vertex_triple(f);
      Cell_handle c;
      int i, j, k;
      return cavity_triangulation.is_facet(map_ambient_vertices_to_cavity_vertices[v0],
                                           map_ambient_vertices_to_cavity_vertices[v1],
                                           map_ambient_vertices_to_cavity_vertices[v2], c, i, j, k);
    }));
    return result;
  }

  std::optional<std::pair<Vertex_handle, Vertex_handle>>
  return_encroached_constrained_edge([[maybe_unused]] CDT_3_face_index face_index,
                                      const CDT_2& cdt_2,
                                      Point_3 steiner_pt) const
  {
    for(auto [other_fh, index] : cdt_2.finite_edges()) {
      if(!other_fh->is_constrained(index))
        continue;
      const auto va = other_fh->vertex(cdt_2.cw(index));
      const auto vb = other_fh->vertex(cdt_2.ccw(index));
      const auto a = cdt_2.point(va);
      const auto b = cdt_2.point(vb);
      // std::cerr << std::format("Test candidate Steiner point {} with edge ( {}   {} ), result is: {}", IO::oformat(steiner_pt),
      //                          IO::oformat(a), IO::oformat(b), IO::oformat(CGAL::angle(a, steiner_pt, b)))
      //           << '\n';
      if(CGAL::angle(a, steiner_pt, b) != CGAL::ACUTE) {
        const auto va_3d = va->info().vertex_handle_3d;
        const auto vb_3d = vb->info().vertex_handle_3d;
        return std::make_pair(va_3d, vb_3d);
      }
    }
    return std::nullopt;
  }

  std::optional<std::pair<Vertex_handle, Vertex_handle>>
  try_to_insert_circumcenter_in_face_or_return_encroached_edge(CDT_3_face_index face_index,
                                                               CDT_2& non_const_cdt_2,
                                                               CDT_2_face_handle fh_2d)
  {
    const auto& cdt_2 = non_const_cdt_2;
    auto steiner_pt = CGAL::centroid(cdt_2.triangle(fh_2d));
#if CGAL_DEBUG_CDT_3 & 64 && CGAL_CAN_USE_CXX20_FORMAT
    std::cerr << std::format("Trying to insert Steiner (centroid) point {} in non-coplanar face {}.\n", IO::oformat(steiner_pt),
                             IO::oformat(cdt_2.triangle(fh_2d)));
#endif // CGAL_DEBUG_CDT_3
    auto encroached_edge_opt = return_encroached_constrained_edge(face_index, cdt_2, steiner_pt);
    if(encroached_edge_opt) {
      return encroached_edge_opt;
    }
    if constexpr (cdt_3_can_use_cxx20_format()) if(this->debug_Steiner_points()) {
      std::cerr << std::format("Inserting Steiner (centroid) point {} in non-coplanar face {}: {}.\n",
                               IO::oformat(steiner_pt), face_index, IO::oformat(cdt_2.triangle(fh_2d)));
    }

    Locate_type lt;
    int li, lj;
    const auto ch = this->locate(steiner_pt, lt, li, lj);

    boost::container::small_vector<Cell_handle,32> cells;
    boost::container::small_vector<Facet,32> facets;
    auto cleanup = [&cells, &facets] {
      for(Cell_handle ch : cells) {
        ch->tds_data().clear();
      }

      for(Facet& f : facets) {
        f.first->neighbor(f.second)->tds_data().clear();
      }
    };
    switch(tr.dimension()) {
    case 3: {
      typename T_3::Conflict_tester_3 tester(steiner_pt, this);
      this->find_conflicts(ch,
                           tester,
                           make_triple(
                             std::back_inserter(facets),
                             std::back_inserter(cells),
                             Emptyset_iterator()));
      break;
    } // dim 3
    case 2: {
      typename T_3::Conflict_tester_2 tester(steiner_pt, this);
      this->find_conflicts(ch,
                           tester,
                           make_triple(
                             std::back_inserter(facets),
                             std::back_inserter(cells),
                             Emptyset_iterator()));
      break;
    } // dim 2
    default: CGAL_error();
    }
    std::set<std::pair<Vertex_handle, Vertex_handle>> visited_edges;
    for(auto c : cells) {
      for(int i = 0; i < 4; ++i) {
        for(int j = i + 1; j < 4; ++j) {
          auto pair = make_sorted_pair(c->vertex(i),
                                       c->vertex(j));
          auto is_a_new_edge = visited_edges.insert(pair).second;
          if(!is_a_new_edge)
            continue;
          auto [va, vb] = pair;
          Constraint_id c_id = this->constraint_around(va, vb);
          if(c_id != Constraint_id{}) {
            if(CGAL::angle(this->point(va), steiner_pt, this->point(vb)) != CGAL::ACUTE) {
              cleanup();
              return std::make_pair(va, vb);
            }
          }
        }
      }
    }
    cleanup();

    // assert(is_valid(true));
    // this->study_bug = true;
    const auto v = this->insert_in_cdt_3(steiner_pt, lt, ch, li, lj, insert_in_conflict_visitor);// TODO: use "insert in hole"
    // this->study_bug = false;
    // assert(is_valid(true));
    if constexpr (cdt_3_can_use_cxx20_format()) if(this->debug_Steiner_points()) {
      std::cerr << "  -> " << IO::oformat(v, with_offset) << '\n';
    }
    v->set_Steiner_vertex_in_face(face_index);
    [[maybe_unused]] typename CDT_2::Locate_type lt_2;
    int i;
    auto fh = cdt_2.locate(steiner_pt, lt_2, i, fh_2d);
    CGAL_assertion(!fh->info().is_outside_the_face); CGAL_USE(fh);
    const auto v_2d = non_const_cdt_2.insert(steiner_pt, fh_2d);
    v_2d->info().vertex_handle_3d = v;
    auto f_circ = cdt_2.incident_faces(v_2d);
    const auto end = f_circ;
    do {
      f_circ->info().is_outside_the_face = false;
    } while(++f_circ != end);
    search_for_missing_subfaces(face_index);
    return std::nullopt;
  }

  void insert_mid_point_in_constrained_edge(Vertex_handle va_3d, Vertex_handle vb_3d) {
    const auto a = this->point(va_3d);
    const auto b = this->point(vb_3d);
    const auto mid = CGAL::midpoint(a, b);
    if constexpr (cdt_3_can_use_cxx20_format()) if(this->debug_Steiner_points()) {
      std::cerr << std::format("Inserting Steiner (midpoint) point {} of constrained edge ({:.6} , {:.6})\n",
                              IO::oformat(mid), IO::oformat(va_3d, with_point_and_info),
                              IO::oformat(vb_3d, with_point_and_info));
    }
    auto&& contexts = this->constraint_hierarchy.contexts_range(va_3d, vb_3d);
#if CGAL_DEBUG_CDT_3 & 64 && CGAL_CAN_USE_CXX20_FORMAT
    if(std::next(contexts.begin()) != contexts.end()) {
      std::cerr << "ERROR: Edge is constrained by more than one constraint\n";
      for(const auto& c : contexts) {
        std::cerr << std::format("  - {} with {} vertices\n", IO::oformat(c.id().vl_ptr()),
                                                              c.number_of_vertices());
        for(auto vh_it = c.vertices_begin(), end = c.vertices_end(), current = c.current();
            vh_it != end; ++vh_it)
        {
          std::cerr << std::format("    {} {}\n",
                                   (vh_it == current) ? '>' : '-',
                                   IO::oformat(*vh_it, with_point_and_info));
        }
      }
    }
#endif // CGAL_DEBUG_CDT_3 & 64
    CGAL_assertion(std::next(contexts.begin()) == contexts.end());
    const auto& context = *contexts.begin();
    const auto constraint_id = context.id();
    CGAL_assertion(constraint_id != Constraint_id{});
    // this->study_bug = true;
    Locate_type mid_lt;
    int mid_li, min_lj;
    Cell_handle mid_c = tr.locate(mid, mid_lt, mid_li, min_lj, va_3d->cell());
    CGAL_assertion(mid_lt != Locate_type::VERTEX);
    [[maybe_unused]] auto v =
      this->insert_Steiner_point_on_subconstraint(mid, mid_c, {va_3d, vb_3d},
                                                  constraint_id, insert_in_conflict_visitor);
    if constexpr (cdt_3_can_use_cxx20_format()) if(this->debug_Steiner_points()) {
      std::cerr << "  -> " << IO::oformat(v, with_offset) << '\n';
    }
    // this->study_bug = false;
    // assert(is_valid(true));
  }

  bool restore_face(CDT_3_face_index face_index) {
    CDT_2& non_const_cdt_2 = face_cdt_2[face_index];
    const CDT_2& cdt_2 = non_const_cdt_2;
#if CGAL_CDT_3_CAN_USE_CXX20_FORMAT
    if(this->debug_copy_triangulation_into_hole()) {
      std::cerr << std::format("restore_face({}): CDT_2 has {} vertices\n", face_index, cdt_2.number_of_vertices());
    }
#endif // CGAL_CDT_3_CAN_USE_CXX20_FORMAT
    for(const auto& edge : cdt_2.finite_edges()) {
      const auto fh = edge.first;
      const auto i = edge.second;
      const auto va_3d = fh->vertex(cdt_2.cw(i))->info().vertex_handle_3d;
      const auto vb_3d = fh->vertex(cdt_2.ccw(i))->info().vertex_handle_3d;
      const bool is_3d = this->is_edge(va_3d, vb_3d);
#if CGAL_CDT_3_CAN_USE_CXX20_FORMAT
      if(this->debug_copy_triangulation_into_hole()) {
        std::cerr << std::format("Edge is 3D: {:6}  ({} , {})\n",
                                  is_3d,
                                  IO::oformat(va_3d, with_point_and_info),
                                  IO::oformat(vb_3d, with_point_and_info));
      }
#endif // CGAL_CDT_3_CAN_USE_CXX20_FORMAT
      CGAL_assertion(is_3d || !cdt_2.is_constrained(edge));
      fh->info().is_edge_also_in_3d_triangulation[unsigned(i)] = is_3d;
      const auto reverse_edge = cdt_2.mirror_edge(edge);
      reverse_edge.first->info().is_edge_also_in_3d_triangulation[unsigned(reverse_edge.second)] = is_3d;
    }
    std::set<CDT_2_face_handle> processed_faces;
    auto& region_index = faces_region_numbers[face_index];
    for(const CDT_2_face_handle fh : cdt_2.finite_face_handles()) {
      if(fh->info().is_outside_the_face) continue;
      if(false == fh->info().missing_subface) {
        continue;
      }
      Cell_handle c;
      int i, j, k;
      if(tr.is_facet(fh->vertex(0)->info().vertex_handle_3d,
                     fh->vertex(1)->info().vertex_handle_3d,
                     fh->vertex(2)->info().vertex_handle_3d, c, i, j, k))
      {
        const int facet_index = 6 - i - j - k;
        set_facet_constrained({c, facet_index}, face_index, fh);
        fh->info().missing_subface = false;
        continue;
      }
      if(processed_faces.count(fh)> 0)
        continue;
      auto fh_region = region(cdt_2, fh);
      processed_faces.insert(fh_region.begin(), fh_region.end());

      auto handle_error_with_region = [&](const char* what, CDT_2_face_handle fh_2d) {

              std::cerr << "NOTE: " << what << " in sub-region " << (region_index - 1)
                  << " of face #F" << face_index << '\n';
#if CGAL_DEBUG_CDT_3 & 64 && CGAL_CAN_USE_CXX20_FORMAT
        std::cerr << "  constrained edges are:\n";
        for(auto [c, index]: cdt_2.constrained_edges()) {
          const auto va = c->vertex(cdt_2.cw(index));
          const auto vb = c->vertex(cdt_2.ccw(index));
          const auto va_3d = va->info().vertex_handle_3d;
          const auto vb_3d = vb->info().vertex_handle_3d;
          std::cerr << std::format("    ({:.6} , {:.6})\n",
                                    IO::oformat(va_3d, with_point_and_info),
                                    IO::oformat(vb_3d, with_point_and_info));
        }
#endif // CGAL_DEBUG_CDT_3
        const auto encroach_edge_opt =
            try_to_insert_circumcenter_in_face_or_return_encroached_edge(face_index, non_const_cdt_2, fh_2d);
        if(encroach_edge_opt) {
          const auto [va_3d, vb_3d] = *encroach_edge_opt;
          insert_mid_point_in_constrained_edge(va_3d, vb_3d);
        }
      };
      try {
        restore_subface_region(face_index, region_index++, non_const_cdt_2, fh_region);
      }
      catch(Next_region& e) {
        handle_error_with_region(e.what(), e.fh_2d);
        return false;
      }
      // catch(PLC_error& e) {
      //   handle_error_with_region(e.what(), fh_region[0]);
      //   return false;
      // }
    }
    return true;
  }

public:
  bool is_valid(bool verbose = false, int level = 0) const
  {
    if(!this->tds().is_valid(verbose, level)) {
      if(verbose)
        std::cerr << "invalid data structure" << std::endl;

      CGAL_assertion(false);
      return false;
    }

    if(this->infinite_vertex() == Vertex_handle()) {
      if(verbose)
        std::cerr << "no infinite vertex" << std::endl;

      CGAL_assertion(false);
      return false;
    }

    bool result = true;
    switch(this->dimension()) {
    case 3: {
      for(auto it = this->finite_cells_begin(), end = this->finite_cells_end(); it != end; ++it) {
        result = result && this->is_valid_finite(it, verbose, level);
        for(int i = 0; i < 4; i++) {
          const auto n = it->neighbor(i);
          const auto n_index = n->index(it);
          if(!this->is_infinite(n->vertex(n_index)))
          {
            if(!it->is_facet_constrained(i) && this->side_of_sphere(it, n->vertex(n_index)->point()) == ON_BOUNDED_SIDE) {
              if(verbose) {
                const auto v = tr.vertices(it);
                std::cerr << "non-empty sphere at non-constrained facet (" << IO::oformat(Cell_handle(it))
                          << ", " << i << ") the cell is:\n  "
                          << IO::oformat(v[0], with_point) << "\n  "
                          << IO::oformat(v[1], with_point) << "\n  "
                          << IO::oformat(v[2], with_point) << "\n  "
                          << IO::oformat(v[3], with_point) << "\ncontains:\n  "
                          << IO::oformat(n->vertex(n_index), with_point_and_info) << '\n';
                using EK = CGAL::Exact_predicates_exact_constructions_kernel;
                const auto to_exact = CGAL::Cartesian_converter<Geom_traits, EK>();
                const auto from_exact = CGAL::Cartesian_converter<EK, Geom_traits>();

                const auto exact_circ = CGAL::circumcenter(to_exact(tr.point(v[0])),
                                                           to_exact(tr.point(v[1])),
                                                           to_exact(tr.point(v[2])),
                                                           to_exact(tr.point(v[3])));
                const auto exact_sq_circumradius = CGAL::squared_distance(to_exact(tr.point(v[0])), exact_circ);
                const auto exact_sq_distance =
                    CGAL::squared_distance(exact_circ, to_exact(tr.point(n->vertex(n_index))));
                std::cerr << "exact squared circumradius: " << exact_sq_circumradius << '\n';
                std::cerr << "exact squared distance:     " << exact_sq_distance << '\n';
                std::cerr << "ratio (non-squared):        "
                          << CGAL::sqrt(CGAL::to_double(from_exact(exact_sq_distance / exact_sq_circumradius))) << '\n';
              }
              result = false;
            }
          }
        }
      }
      break;
    }
    case 2: {
      for(auto it = this->finite_facets_begin(), end = this->finite_facets_end(); it != end; ++it) {
        const auto c = it->first;
        result = result && this->is_valid_finite(c, verbose, level);
        for(int i = 0; i < 3; i++) {
          const auto n = c->neighbor(i);
          const auto n_index = n->index(c);
          if(!this->is_infinite(n->vertex(n_index))) {
            if(this->side_of_circle(c, 3, n->vertex(n_index)->point()) == ON_BOUNDED_SIDE) {
              if(verbose)
                std::cerr << "non-empty circle " << std::endl;

              result = false;
            }
          }
        }
      }
      break;
    }
    case 1: {
      for(auto it = this->finite_edges_begin(), end = this->finite_edges_end(); it != end; ++it) {
        result = result && this->is_valid_finite((*it).first, verbose, level);
      }
      break;
    }
    }
    if(this->use_finite_edges_map()) {
      const auto number_of_elements_in_finite_edges_map =
          std::accumulate(this->all_finite_edges.begin(), this->all_finite_edges.end(), size_type(0),
                          [&](size_type res, const auto& hash_map) { return res + hash_map.size(); });
      bool test = number_of_elements_in_finite_edges_map == this->number_of_finite_edges();
      result = result && test;
      if(!test && verbose) {
        std::cerr << "all_finite_edges.size() = " << number_of_elements_in_finite_edges_map
                  << " != number_of_finite_edges() = " << this->number_of_finite_edges() << std::endl;
      }
      for(auto v1: this->finite_vertex_handles()) {
        for(auto v2: this->all_finite_edges[v1->time_stamp()]) {
          test = this->is_edge(v1, v2);
          result = result && test;
          if(!test && verbose) {
            std::cerr << "edge (" << IO::oformat(v1, with_point_and_info) << ", "
                << IO::oformat(v2, with_point_and_info) << ") is not an edge" << std::endl;
          }
        }
      }
      for(auto e : this->finite_edges()) {
        auto [v1, v2] = make_sorted_pair(this->vertices(e));
        auto v1_index = v1->time_stamp();
        test = this->all_finite_edges[v1_index].find(v2)!= this->all_finite_edges[v1_index].end();
        result = result && test;
        if(!test && verbose) {
          std::cerr << "finite edge (" << IO::oformat(v1, with_point_and_info) << ", "
                    << IO::oformat(v2, with_point_and_info) << ") is not in the set all_finite_edges"
                    << std::endl;
        }
      }
    }

    if(result && verbose)
      std::cerr << "valid constrained Delaunay triangulation" << std::endl;

    return result;
  }

  void recheck_constrained_Delaunay() {
    for(int i = 0, end = face_constraint_misses_subfaces.size(); i < end; ++i) {
      search_for_missing_subfaces(i);
    }
  }

  void restore_constrained_Delaunay()
  {
    this->is_Delaunay = false;
    faces_region_numbers.resize(face_constraint_misses_subfaces.size());
    for(int i = 0, end = face_constraint_misses_subfaces.size(); i < end; ++i) {
      CDT_2& cdt_2 = face_cdt_2[i];
      fill_cdt_2(cdt_2, i);
      search_for_missing_subfaces(i);
    }
    cdt_2_are_initialized = true;
    const auto npos = face_constraint_misses_subfaces.npos;
    auto i = face_constraint_misses_subfaces.find_first();
    bool the_process_made_progress = false;
    while(i != npos) {
      try {
        if(restore_face(i)) {
          face_constraint_misses_subfaces.reset(i);
        } else {
          std::cerr << "restore_face(" << i << ") incomplete, back to conforming...\n";
          Conforming_Dt::restore_Delaunay(insert_in_conflict_visitor);
        }
        the_process_made_progress = true;
      }
      catch(PLC_error& e) {
        std::cerr << std::string("ERROR: PLC error with face #F") << std::to_string(e.face_index) + "\n";
        i = face_constraint_misses_subfaces.find_next(i);
        if(i == npos) {
          std::cerr << "ERROR: No more missing face to restore after a PLC error\n";
          dump_region(e.face_index, e.region_index);
          throw;
        }
        std::cerr << "Next face is face #F " << i << '\n';
        continue;
      }
      i = face_constraint_misses_subfaces.find_next(i);

      // If we have made progress, we start again from the beginning.
      // Otherwise, either we are done, or there was a full loop with
      // only PLC errors.
      if(i == npos && true == the_process_made_progress) {
        i = face_constraint_misses_subfaces.find_first();
        the_process_made_progress = false;
      }
    }
  }

  static void write_region_to_OFF(std::ostream& out, const CDT_2& cdt_2) {
    out.precision(17);
    auto color_fn = [](CDT_2_face_handle fh_2d) -> CGAL::IO::Color {
      if(fh_2d->info().is_outside_the_face) return CGAL::IO::gray();
      if(fh_2d->info().is_in_region) {
        if(fh_2d->info().is_in_region == 1) return CGAL::IO::violet();
        else return CGAL::IO::red();
      }
      return CGAL::IO::green();
    };
    auto color_pmap = boost::make_function_property_map<CDT_2_face_handle>(color_fn);
    CGAL::IO::write_OFF(out, cdt_2, CGAL::parameters::face_color_map(color_pmap));
  }

  template <typename Region>
  void write_region(std::ostream& out, const Region& region)
  {
    for(const auto fh_2d : region) {
      write_2d_triangle(out, fh_2d);
    }
  }

  void write_3d_triangulation_to_OFF(std::ostream& out, const Constrained_Delaunay_triangulation_3& tr) {
    write_facets(out, tr, tr.finite_facets());
  }

  void dump_3d_triangulation(CDT_3_face_index face_index,
                             int region_index,
                             std::string type,
                             const Constrained_Delaunay_triangulation_3& tr)
  {
    std::ofstream dump(std::string("dump_") + type + "_cavity_" + std::to_string(face_index) + "_" +
                       std::to_string(region_index) + ".off");
    dump.precision(17);
    write_3d_triangulation_to_OFF(dump, tr);
  }

  void dump_triangulation() const {
    std::ofstream dump("dump.binary.cgal", std::ios::binary);
    CGAL::IO::save_binary_file(dump, *this);
  }

  void dump_region(CDT_3_face_index face_index, int region_index, const CDT_2& cdt_2) {
    std::ofstream dump_region(std::string("dump_region_") + std::to_string(face_index) + "_" +
                              std::to_string(region_index) + ".off");
    dump_region.precision(17);
    write_region_to_OFF(dump_region, cdt_2);
  }

  void dump_region(CDT_3_face_index face_index, int region_index) {
    const auto& cdt_2 = face_cdt_2[face_index];
    dump_region(face_index, region_index, cdt_2);
  }

  void write_triangle(std::ostream &out,
                      Vertex_handle v0, Vertex_handle v1, Vertex_handle v2)
  {
    out.precision(17);
    out << "4"
        << " " << tr.point(v0) << " " << tr.point(v1) << " " << tr.point(v2)
        << " " << tr.point(v0) << '\n';
  }

  static void write_segment(std::ostream &out, Point_3 p0, Point_3 p1)
  {
    out.precision(17);
    out << "2" << " " << p0 << " " << p1 << '\n';
  }

  static void write_segment(std::ostream &out, Segment_3 seg) {
    write_segment(out, seg.source(), seg.target());
  }

  void write_segment(std::ostream& out, Vertex_handle v0, Vertex_handle v1)
  {
    write_segment(out, tr.point(v0), tr.point(v1));
  }

  void write_segment(std::ostream& out, Edge edge) {
    const auto [c, i, j] = edge;
    write_segment(out, c->vertex(i), c->vertex(j));
  }

  void dump_edge_link(std::string filename, Edge edge) {
    std::ofstream out(filename);
    out.precision(17);
    const auto [c, i, j] = edge;
    const Vertex_handle va = c->vertex(i);
    const Vertex_handle vb = c->vertex(j);
    auto cell_circ = this->incident_cells(edge), end = cell_circ;
    CGAL_assertion(cell_circ != nullptr);
    do {
      if(this->is_infinite(cell_circ)) {
        continue;
      }
      const auto index_va = cell_circ->index(va);
      const auto index_vb = cell_circ->index(vb);
      const auto index_vc = this->next_around_edge(index_va, index_vb);
      const auto index_vd = this->next_around_edge(index_vb, index_va);
      write_segment(out, cell_circ->vertex(index_vc), cell_circ->vertex(index_vd));
    } while(++cell_circ != end);
  }

  template <typename ...Args>
  void dump_segment(std::string filename, Args&& ...args)
  {
    std::ofstream out(filename);
    out.precision(17);
    write_segment(out, std::forward<Args>(args)...);
  }

  template <typename Tr, typename Facets>
  static auto export_facets_to_surface_mesh(const Tr& tr, Facets&& facets_range) {
    return CGAL::export_facets_to_surface_mesh(tr, std::forward<Facets>(facets_range));
  }

  template <typename Tr, typename Facets>
  static void write_facets(std::ostream& out, const Tr& tr, Facets&& facets_range) {
    return CGAL::write_facets(out, tr, std::forward<Facets>(facets_range));
  }

  template <typename Facets_range>
  void dump_facets_of_cavity(CDT_3_face_index face_index, int region_index, std::string type,
                             const Facets_range& facets_range)
  {
    std::ofstream out(std::string("dump_facets_of_region_") + std::to_string(face_index) + "_" +
                      std::to_string(region_index) + "_" + type + ".off");
    out.precision(17);
    write_facets(out, *this, facets_range);
  }

  void write_2d_triangle(std::ostream &out, const CDT_2_face_handle fh)
  {
    const auto v0 = fh->vertex(0)->info().vertex_handle_3d;
    const auto v1 = fh->vertex(1)->info().vertex_handle_3d;
    const auto v2 = fh->vertex(2)->info().vertex_handle_3d;
    write_triangle(out, v0, v1, v2);
  }

  bool write_missing_subfaces_file(std::ostream& out) {
    const auto npos = face_constraint_misses_subfaces.npos;
    auto i = face_constraint_misses_subfaces.find_first();
    bool has_missing_subfaces = i != npos;
    while(i != npos) {
      const CDT_2& cdt = face_cdt_2[i];
      for(const auto fh: cdt.finite_face_handles()) {
        if (false == fh->info().is_outside_the_face &&
            true == fh->info().missing_subface)
        {
          write_2d_triangle(out, fh);
        }
      }
      i = face_constraint_misses_subfaces.find_next(i);
    }
    return has_missing_subfaces;
  }

  /// @{
  /// remove functions cannot be called
  void remove(Vertex_handle) = delete;
  void remove_cluster() = delete;
  /// @}

protected:
  T_3 &tr = *this;
  Conforming_Dt &conforming_dt = *this;
  Insert_in_conflict_visitor insert_in_conflict_visitor = {*this};
  std::vector<CDT_2> face_cdt_2;
  bool cdt_2_are_initialized = false;
  struct Face_edge {
    Constraint_id constraint_id;
    bool is_reverse = false;
  };
  std::vector<std::vector<std::vector<Face_edge>>> face_borders;
  std::multimap<Constraint_id, CDT_3_face_index> constraint_to_faces;
  boost::dynamic_bitset<> face_constraint_misses_subfaces;
  std::vector<std::size_t> faces_region_numbers;
};

} // end CGAL

#endif // not DOXYGEN_RUNNING

#endif // CGAL_CONSTRAINED_DELAUNAY_TRIANGULATION_3_H