// Copyright (c) 1999-2003,2006-2009   INRIA Sophia-Antipolis (France).
// All rights reserved.
//
// This file is part of CGAL (www.cgal.org); you may redistribute it under
// the terms of the Q Public License version 1.0.
// See the file LICENSE.QPL distributed with CGAL.
//
// 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: svn+ssh://scm.gforge.inria.fr/svn/cgal/trunk/Periodic_3_triangulation_3/include/CGAL/Periodic_3_triangulation_3.h $
// $Id: Periodic_3_triangulation_3.h 59685 2010-11-12 17:03:19Z mcaroli $
// 
//
// Author(s)     : Monique Teillaud <Monique.Teillaud@sophia.inria.fr>
//                 Sylvain Pion <Sylvain.Pion@sophia.inria.fr>
//                 Nico Kruithof <Nico.Kruithof@sophia.inria.fr>
//                 Manuel Caroli <Manuel.Caroli@sophia.inria.fr>

#ifndef CGAL_PERIODIC_3_TRIANGULATION_3_H
#define CGAL_PERIODIC_3_TRIANGULATION_3_H

#include <CGAL/basic.h>

#include <iostream>
#include <algorithm>
#include <cmath>
#include <functional>
#include <list>

#include <boost/tuple/tuple.hpp>
#include <boost/random/linear_congruential.hpp>
#include <boost/random/uniform_smallint.hpp>
#include <boost/random/variate_generator.hpp>

#include <CGAL/triangulation_assertions.h>

#include <CGAL/Triangulation_data_structure_3.h>
#include <CGAL/Periodic_3_triangulation_ds_cell_base_3.h>
#include <CGAL/Periodic_3_triangulation_ds_vertex_base_3.h>
#include <CGAL/Triangulation_cell_base_3.h>
#include <CGAL/Triangulation_vertex_base_3.h>

#include <CGAL/Periodic_3_triangulation_iterators_3.h>

#include <CGAL/Unique_hash_map.h>

#include <CGAL/internal/Exact_type_selector.h>
#include <CGAL/NT_converter.h>

namespace CGAL {

template < class GT, class TDS > class Periodic_3_triangulation_3;

template < class GT, class TDS > std::istream& operator>> 
    (std::istream& is, Periodic_3_triangulation_3<GT,TDS> &tr);
template < class GT, class TDS > std::ostream& operator<< 
    (std::ostream& os, const Periodic_3_triangulation_3<GT,TDS> &tr);

/**\class Periodic_3_triangulation_3
 * 
 * \brief Implements functionality for computing in periodic space.
 *
 * There are several things that are special to computing in $\mathbb{T}^3$
 * such as
 * - periodicity --> offsets
 * - no infinite vertex
 * - no degenerate dimensions
 * All functions that are affected can be found in this class. In case it is
 * necessary to provide different implementations for Delaunay and regular
 * triangulation, we work with visitors. 
 */

template < class GT,
            class TDS = Triangulation_data_structure_3 <
	      Triangulation_vertex_base_3<
		GT, Periodic_3_triangulation_ds_vertex_base_3<>
		>,
              Triangulation_cell_base_3<
                GT, Periodic_3_triangulation_ds_cell_base_3<>
              >
            >
          >
class Periodic_3_triangulation_3 
  : public Triangulation_utils_3
{
  friend std::istream& operator>> <>
  (std::istream& is, Periodic_3_triangulation_3<GT, TDS> &tr);

  typedef Periodic_3_triangulation_3<GT,TDS>   Self;

public:
  typedef GT                                   Geometric_traits;
  typedef TDS                                  Triangulation_data_structure;

  typedef typename GT::Periodic_3_offset_3     Offset;
  typedef typename GT::Iso_cuboid_3            Iso_cuboid;
  typedef array<int, 3>                        Covering_sheets;
  
  typedef typename GT::Point_3                 Point;
  typedef typename GT::Segment_3               Segment;
  typedef typename GT::Triangle_3              Triangle;
  typedef typename GT::Tetrahedron_3           Tetrahedron;

  typedef std::pair<Point,Offset>              Periodic_point;
  typedef array< std::pair<Point,Offset>, 2>   Periodic_segment;
  typedef array< std::pair<Point,Offset>, 3>   Periodic_triangle;
  typedef array< std::pair<Point,Offset>, 4>   Periodic_tetrahedron;

  typedef typename TDS::Vertex                 Vertex;
  typedef typename TDS::Cell                   Cell;
  typedef typename TDS::Facet                  Facet;
  typedef typename TDS::Edge                   Edge;

  typedef typename TDS::Vertex_handle          Vertex_handle;
  typedef typename TDS::Cell_handle            Cell_handle;
  
  typedef typename TDS::size_type              size_type;
  typedef typename TDS::difference_type        difference_type;

  typedef typename TDS::Cell_iterator          Cell_iterator;
  typedef typename TDS::Facet_iterator         Facet_iterator;
  typedef typename TDS::Edge_iterator          Edge_iterator;
  typedef typename TDS::Vertex_iterator        Vertex_iterator;

  typedef typename TDS::Cell_circulator        Cell_circulator;
  typedef typename TDS::Facet_circulator       Facet_circulator;

  typedef Cell_iterator                        All_cells_iterator;
  typedef Facet_iterator                       All_facets_iterator;
  typedef Edge_iterator                        All_edges_iterator;
  typedef Vertex_iterator                      All_vertices_iterator;
  typedef Periodic_3_triangulation_unique_vertex_iterator_3<Self>
                                               Unique_vertex_iterator;

private:
  typedef typename GT::FT                      FT;
  typedef std::pair< Vertex_handle, Offset >   Virtual_vertex;
  typedef std::map<Vertex_handle, Virtual_vertex>
                                               Virtual_vertex_map;
  typedef typename Virtual_vertex_map::const_iterator
                                               Virtual_vertex_map_it;
  typedef std::map<Vertex_handle, std::vector<Vertex_handle > >
                                               Virtual_vertex_reverse_map;
  typedef typename Virtual_vertex_reverse_map::const_iterator
                                               Virtual_vertex_reverse_map_it;
  typedef Triple< Vertex_handle, Vertex_handle, Vertex_handle >
                                               Vertex_triple;

public:
  typedef Periodic_3_triangulation_tetrahedron_iterator_3<Self>
                                               Periodic_tetrahedron_iterator;
  typedef Periodic_3_triangulation_triangle_iterator_3<Self>
                                               Periodic_triangle_iterator;
  typedef Periodic_3_triangulation_segment_iterator_3<Self>
                                               Periodic_segment_iterator;
  typedef Periodic_3_triangulation_point_iterator_3<Self>
                                               Periodic_point_iterator;

  typedef Point                                value_type;
  typedef const value_type&                    const_reference;

  typedef Tag_false Weighted_tag;

public:
  enum Iterator_type {
    STORED=0,
    UNIQUE, //1
    STORED_COVER_DOMAIN, //2
    UNIQUE_COVER_DOMAIN };//3

  enum Locate_type {
    VERTEX=0, 
    EDGE, //1
    FACET, //2
    CELL, //3
    EMPTY , //4
    OUTSIDE_CONVEX_HULL, // unused, for compatibility with Alpha_shape_3
    OUTSIDE_AFFINE_HULL }; // unused, for compatibility with Alpha_shape_3

private:
  Geometric_traits  _gt;
  Triangulation_data_structure _tds; 
  Iso_cuboid _domain;
  /// This threshold should be chosen such that if all edges are shorter,
  /// we can be sure that there are no self-edges anymore.
  FT edge_length_threshold;
  
  /// This adjacency list stores all edges that are longer than
  /// edge_length_threshold.
  std::map< Vertex_handle, std::list<Vertex_handle> > too_long_edges;
  unsigned int too_long_edge_counter;
  
  /// map of offsets for periodic copies of vertices
  Virtual_vertex_map virtual_vertices;
  Virtual_vertex_reverse_map  virtual_vertices_reverse;

protected:
  /// v_offsets temporarily stores all the vertices on the border of a
  /// conflict region.
  mutable std::vector<Vertex_handle> v_offsets;

private:
  /// Determines if we currently compute in 3-cover or 1-cover.
  Covering_sheets _cover;

public:
  /** @name Creation */ //@{
  Periodic_3_triangulation_3(
      const Iso_cuboid & domain = Iso_cuboid(0,0,0,1,1,1),
      const Geometric_traits & gt = Geometric_traits())
    : _gt(gt), _tds(), _domain(domain), too_long_edge_counter(0) {
    _gt.set_domain(_domain);
    typedef typename internal::Exact_type_selector<FT>::Type EFT;
    typedef NT_converter<FT,EFT> NTC;
    CGAL_triangulation_assertion_code( NTC ntc; )
    CGAL_triangulation_precondition(ntc(_domain.xmax())-ntc(_domain.xmin())
	== ntc(_domain.ymax())-ntc(_domain.ymin()));
    CGAL_triangulation_precondition(ntc(_domain.ymax())-ntc(_domain.ymin())
	== ntc(_domain.zmax())-ntc(_domain.zmin()));
    CGAL_triangulation_precondition(ntc(_domain.zmax())-ntc(_domain.zmin())
	== ntc(_domain.xmax())-ntc(_domain.xmin()));
    _cover = make_array(3,3,3);
    init_tds();
    edge_length_threshold = FT(0.166) * (_domain.xmax()-_domain.xmin())
                                      * (_domain.xmax()-_domain.xmin());
  }

private:
  // Copy constructor helpers
  class Finder;
  void copy_multiple_covering(const Periodic_3_triangulation_3 & tr);
public:
  // Copy constructor duplicates vertices and cells
  Periodic_3_triangulation_3(const Periodic_3_triangulation_3 & tr)
    : _gt(tr.geom_traits()),
      _domain(tr._domain),
      edge_length_threshold(tr.edge_length_threshold),
      _cover(tr._cover) {
    if (is_1_cover()) {
      _tds = tr.tds();
    } else {
      copy_multiple_covering(tr);
    }
    CGAL_triangulation_expensive_postcondition(*this == tr);
  }
  
  /** @name Assignment */ //@{
  Periodic_3_triangulation_3 & operator=(Periodic_3_triangulation_3 tr) {
    swap(tr);
    return *this;
  }
  
  void swap(Periodic_3_triangulation_3 &tr) {
    std::swap(tr._gt, _gt);
    _tds.swap(tr._tds);
    std::swap(_domain,tr._domain);
    std::swap(edge_length_threshold,tr.edge_length_threshold);
    std::swap(too_long_edges,tr.too_long_edges);
    std::swap(too_long_edge_counter,tr.too_long_edge_counter);
    std::swap(virtual_vertices,tr.virtual_vertices);
    std::swap(virtual_vertices_reverse,tr.virtual_vertices_reverse);
    std::swap(_cover, tr._cover);
  }

  /// Clears the triangulation and initializes it again.
  void clear() {
    _tds.clear();
    init_tds();
    too_long_edges.clear();
    too_long_edge_counter = 0;
    virtual_vertices.clear();
    virtual_vertices_reverse.clear();
    _cover = make_array(3,3,3);
    v_offsets.clear();
  }
  //@}

private:
  /// Initializes the triangulation data structure
  void init_tds() {
    _tds.set_dimension(-2);
    v_offsets.reserve(48);
  }

public:
  /** @name Access functions */ //@{
  const Geometric_traits& geom_traits() const { return _gt; }
  const TDS & tds() const { return _tds; }
  TDS & tds() { return _tds; }

  const Iso_cuboid & domain() const { return _domain; }
  // TODO: Documentation and tests
  void set_domain(const Iso_cuboid & domain) {
    clear();
    _domain = domain;
    _gt.set_domain(domain);
    edge_length_threshold = FT(0.166) * (_domain.xmax()-_domain.xmin())
                                      * (_domain.xmax()-_domain.xmin());
  }

  const Covering_sheets & number_of_sheets() const { return _cover; }
  const std::pair<Vertex_handle, Offset> original_vertex(
      const Vertex_handle v) const {
    return (virtual_vertices.find(v) == virtual_vertices.end()) ?
      std::make_pair(v,Offset()) : virtual_vertices.find(v)->second;
  }
  const std::vector<Vertex_handle>& periodic_copies(
      const Vertex_handle v) const {
    CGAL_triangulation_precondition(number_of_sheets() != make_array(1,1,1) );
    CGAL_triangulation_precondition(
	virtual_vertices.find(v) == virtual_vertices.end());
    CGAL_triangulation_assertion(
	virtual_vertices_reverse.find(v) != virtual_vertices_reverse.end());
    return virtual_vertices_reverse.find(v)->second;
  }

  bool is_extensible_triangulation_in_1_sheet_h1() const;
  bool is_extensible_triangulation_in_1_sheet_h2() const;
  bool is_triangulation_in_1_sheet() const;

  void convert_to_1_sheeted_covering();
  void convert_to_27_sheeted_covering();

  size_type number_of_cells() const {
    if (is_1_cover()) return _tds.number_of_cells();
    else return _tds.number_of_cells()/27;
  }
  size_type number_of_facets() const {
    if (is_1_cover()) return _tds.number_of_facets();
    else return _tds.number_of_facets()/27;
  }
  size_type number_of_edges() const {
    if (is_1_cover()) return _tds.number_of_edges();
    else return _tds.number_of_edges()/27;
  }
  size_type number_of_vertices() const {
    if (is_1_cover()) return _tds.number_of_vertices();
    else return _tds.number_of_vertices()/27;
  }

  size_type number_of_stored_cells() const {
    return _tds.number_of_cells();
  }
  size_type number_of_stored_facets() const {
    return _tds.number_of_facets();
  }
  size_type number_of_stored_edges() const {
    return _tds.number_of_edges();
  }
  size_type number_of_stored_vertices() const {
    return _tds.number_of_vertices();
  }

protected:
  bool is_1_cover() const {
    bool flag;
    flag = ((_cover[0] == 1) && (_cover[1] == 1) && (_cover[2] == 1));
    return flag;
  }

public:
  bool is_virtual(Vertex_handle v) {
    if (is_1_cover()) return false;
    return (virtual_vertices.find(v) != virtual_vertices.end());
  }

public:
  // Offset converters
  int off_to_int(const Offset & off) const {
    CGAL_triangulation_assertion( off.x()==0 || off.x() ==1 );
    CGAL_triangulation_assertion( off.y()==0 || off.y() ==1 );
    CGAL_triangulation_assertion( off.z()==0 || off.z() ==1 );
    int i = ((off.x()&1)<<2) + ((off.y()&1)<<1) + ((off.z()&1));
    return i;
  }
  Offset int_to_off(int i) const {
    return Offset((i>>2)&1,(i>>1)&1,i&1);
  }


  void set_offsets(Cell_handle c, int o0,int o1,int o2,int o3) {
    int off0[3] = {(o0>>2)&1,(o0>>1)&1,(o0&1)};
    int off1[3] = {(o1>>2)&1,(o1>>1)&1,(o1&1)};
    int off2[3] = {(o2>>2)&1,(o2>>1)&1,(o2&1)};
    int off3[3] = {(o3>>2)&1,(o3>>1)&1,(o3&1)};
    for (int i=0; i<3; i++) {
      int min_off = (std::min)((std::min)(off0[i],off1[i]),
			       (std::min)(off2[i],off3[i]));
      if (min_off != 0) {
	off0[i] -= min_off; off1[i] -= min_off;
	off2[i] -= min_off; off3[i] -= min_off;
      }
    }
    o0 = ((off0[0]&1)<<2)+((off0[1]&1)<<1)+(off0[2]&1);
    o1 = ((off1[0]&1)<<2)+((off1[1]&1)<<1)+(off1[2]&1);
    o2 = ((off2[0]&1)<<2)+((off2[1]&1)<<1)+(off2[2]&1);
    o3 = ((off3[0]&1)<<2)+((off3[1]&1)<<1)+(off3[2]&1);
    c->set_offsets(o0,o1,o2,o3);
  }
 
  template <class Offset> 
  void set_offsets(Cell_handle c, Offset o0,Offset o1,Offset o2,Offset o3) {
    int off0[3] = {o0.x(),o0.y(),o0.z()};
    int off1[3] = {o1.x(),o1.y(),o1.z()};
    int off2[3] = {o2.x(),o2.y(),o2.z()};
    int off3[3] = {o3.x(),o3.y(),o3.z()};
    for (int i=0; i<3; i++) {
      int min_off = (std::min)((std::min)(off0[i],off1[i]),
			       (std::min)(off2[i],off3[i]));
      if (min_off != 0) {
	off0[i] -= min_off; off1[i] -= min_off;
	off2[i] -= min_off; off3[i] -= min_off;
      }
    }

    CGAL_triangulation_assertion((std::min)((std::min)(off0[0],off1[0]),
			      (std::min)(off2[0],off3[0])) == 0);
    CGAL_triangulation_assertion((std::min)((std::min)(off0[1],off1[1]),
			      (std::min)(off2[1],off3[1])) == 0);
    CGAL_triangulation_assertion((std::min)((std::min)(off0[2],off1[2]),
			      (std::min)(off2[2],off3[2])) == 0);
    CGAL_triangulation_assertion((0 <= off0[0]) && (off0[0] < 2));
    CGAL_triangulation_assertion((0 <= off1[0]) && (off1[0] < 2));
    CGAL_triangulation_assertion((0 <= off2[0]) && (off2[0] < 2));
    CGAL_triangulation_assertion((0 <= off3[0]) && (off3[0] < 2));
    CGAL_triangulation_assertion((0 <= off0[1]) && (off0[1] < 2));
    CGAL_triangulation_assertion((0 <= off1[1]) && (off1[1] < 2));
    CGAL_triangulation_assertion((0 <= off2[1]) && (off2[1] < 2));
    CGAL_triangulation_assertion((0 <= off3[1]) && (off3[1] < 2));
    CGAL_triangulation_assertion((0 <= off0[2]) && (off0[2] < 2));
    CGAL_triangulation_assertion((0 <= off1[2]) && (off1[2] < 2));
    CGAL_triangulation_assertion((0 <= off2[2]) && (off2[2] < 2));
    CGAL_triangulation_assertion((0 <= off3[2]) && (off3[2] < 2));

    int o0i = ((off0[0]&1)<<2)+((off0[1]&1)<<1)+(off0[2]&1);
    int o1i = ((off1[0]&1)<<2)+((off1[1]&1)<<1)+(off1[2]&1);
    int o2i = ((off2[0]&1)<<2)+((off2[1]&1)<<1)+(off2[2]&1);
    int o3i = ((off3[0]&1)<<2)+((off3[1]&1)<<1)+(off3[2]&1);
    c->set_offsets(o0i,o1i,o2i,o3i);
  }

public:
  /** @name Wrapping the traits */ //@{
  Comparison_result compare_xyz(const Point &p1, const Point &p2) const {
    return geom_traits().compare_xyz_3_object()(p1,p2);
  }
  Comparison_result compare_xyz(const Point &p1, const Point&p2,
      const Offset &o1, const Offset &o2) const {
    return geom_traits().compare_xyz_3_object()(p1,p2,o1,o2);
  }

  Orientation orientation(
      const Point &p1, const Point &p2, const Point &p3, const Point &p4)
      const {
    return geom_traits().orientation_3_object()(p1,p2,p3,p4);
  }
  Orientation orientation(
      const Point &p1, const Point &p2, const Point &p3, const Point &p4,
      const Offset &o1, const Offset &o2, const Offset &o3, const Offset &o4)
      const {
    return geom_traits().orientation_3_object()(p1,p2,p3,p4,o1,o2,o3,o4);
  }

  bool equal(const Point &p1, const Point &p2) const {
    return compare_xyz(p1,p2) == EQUAL;
  }
  bool equal(const Point &p1, const Point &p2,
      const Offset &o1, const Offset &o2) const {
    return compare_xyz(p1,p2,o1,o2) == EQUAL;
  }

  bool coplanar(
      const Point &p1, const Point &p2, const Point &p3, const Point &p4)
      const {
    return orientation(p1,p2,p3,p4) == COPLANAR;
  }
  bool coplanar(
      const Point &p1, const Point &p2, const Point &p3, const Point &p4,
      const Offset &o1, const Offset &o2, const Offset &o3, const Offset &o4)
      const {
    return orientation(p1,p2,p3,p4,o1,o2,o3,o4) == COPLANAR;
  }

  Periodic_tetrahedron construct_periodic_3_tetrahedron(
      const Point &p1, const Point &p2, const Point &p3, const Point &p4)
      const {
    return make_array(std::make_pair(p1,Offset()), std::make_pair(p2,Offset()),
	std::make_pair(p3,Offset()), std::make_pair(p4,Offset()));
  }
  Periodic_tetrahedron construct_periodic_3_tetrahedron(
      const Point &p1, const Point &p2, const Point &p3, const Point &p4,
      const Offset &o1, const Offset &o2, const Offset &o3, const Offset &o4)
      const {
    return make_array(std::make_pair(p1,o1), std::make_pair(p2,o2),
	std::make_pair(p3,o3), std::make_pair(p4,o4));
  }

  Periodic_triangle construct_periodic_3_triangle(
      const Point &p1, const Point &p2, const Point &p3) const {
    return make_array(std::make_pair(p1,Offset()),
	std::make_pair(p2,Offset()), std::make_pair(p3,Offset()));
  }
  Periodic_triangle construct_periodic_3_triangle(
      const Point &p1, const Point &p2, const Point &p3,
      const Offset &o1, const Offset &o2, const Offset &o3) const {
    return make_array(std::make_pair(p1,o1), std::make_pair(p2,o2),
	std::make_pair(p3,o3));
  }

  Periodic_segment construct_periodic_3_segment(
      const Point &p1, const Point &p2) const {
    return make_array(std::make_pair(p1,Offset()), std::make_pair(p2,Offset()));
  }
  Periodic_segment construct_periodic_3_segment(
      const Point &p1, const Point &p2,
      const Offset &o1, const Offset &o2) const {
    return make_array(std::make_pair(p1,o1), std::make_pair(p2,o2));
  }

  Tetrahedron construct_tetrahedron(
      const Point &p1, const Point &p2, const Point &p3, const Point &p4)
      const {
    return geom_traits().construct_tetrahedron_3_object()(p1,p2,p3,p4);
  }
  Tetrahedron construct_tetrahedron(
      const Point &p1, const Point &p2, const Point &p3, const Point &p4,
      const Offset &o1, const Offset &o2, const Offset &o3, const Offset &o4)
      const {
    return geom_traits().construct_tetrahedron_3_object()(p1,p2,p3,p4,
	o1,o2,o3,o4);
  }
  Tetrahedron construct_tetrahedron(const Periodic_tetrahedron& tet) {
    return construct_tetrahedron(
	tet[0].first, tet[1].first, tet[2].first, tet[3].first,
	tet[0].second, tet[1].second, tet[2].second, tet[3].second);
  }

  Triangle construct_triangle(
      const Point &p1, const Point &p2, const Point &p3) const {
    return geom_traits().construct_triangle_3_object()(p1,p2,p3);
  }
  Triangle construct_triangle(
      const Point &p1, const Point &p2, const Point &p3,
      const Offset &o1, const Offset &o2, const Offset &o3) const {
    return geom_traits().construct_triangle_3_object()(p1,p2,p3,o1,o2,o3);
  }
  Triangle construct_triangle(const Periodic_triangle& tri) {
    return construct_triangle(tri[0].first, tri[1].first, tri[2].first,
       tri[0].second, tri[1].second, tri[2].second);
  }

  Segment construct_segment(const Point &p1, const Point &p2) const {
    return geom_traits().construct_segment_3_object()(p1, p2);
  }
  Segment construct_segment(const Point &p1, const Point &p2,
    const Offset &o1, const Offset &o2) const {
    return geom_traits().construct_segment_3_object()(p1,p2,o1,o2);
  }
  Segment construct_segment(const Periodic_segment& seg) const {
    return construct_segment(seg[0].first, seg[1].first,
	seg[0].second, seg[1].second);
  }

  Point construct_point(const Point& p, const Offset &o) const {
    return geom_traits().construct_point_3_object()(p,o);
  }
  Point construct_point(const Periodic_point& pp) const {
    return construct_point(pp.first, pp.second);
  }
  //@}

public:
  /** @name Geometric access functions */ //@{
  Periodic_point periodic_point( const Vertex_handle v ) const {
    if (is_1_cover()) return std::make_pair(v->point(), Offset(0,0,0));
    Virtual_vertex_map_it it = virtual_vertices.find(v);
    if (it == virtual_vertices.end()) {
      // if v is not contained in virtual_vertices, then it is in the
      // original domain.
      return std::make_pair(v->point(), Offset(0,0,0));
    } else {
      // otherwise it has to be looked up as well as its offset.
      return std::make_pair(it->second.first->point(), it->second.second);
    }
  }
  Periodic_point periodic_point( const Cell_handle c, int i) const {
    if (is_1_cover()) return std::make_pair(c->vertex(i)->point(),
					    int_to_off(c->offset(i)));
    Virtual_vertex_map_it it = virtual_vertices.find(c->vertex(i));
    if (it == virtual_vertices.end()) {
      // if c->vertex(i) is not contained in virtual_vertices, then it
      // is in the original domain.
      return std::make_pair(c->vertex(i)->point(), 
	  combine_offsets(Offset(),int_to_off(c->offset(i))) );
    } else {
      // otherwise it has to be looked up as well as its offset.
      return std::make_pair(it->second.first->point(),
	  combine_offsets(it->second.second, int_to_off(c->offset(i))) );
    }
  }

  Periodic_segment periodic_segment(const Cell_handle c, int i, int j) const {
    CGAL_triangulation_precondition( i != j );
    CGAL_triangulation_precondition( number_of_vertices() != 0 );
    CGAL_triangulation_precondition( i >= 0 && i <= 3 
        && j >= 0 && j <= 3 );
    return make_array( std::make_pair(c->vertex(i)->point(),
				      get_offset(c,i)),
		       std::make_pair(c->vertex(j)->point(),
				      get_offset(c,j)) );
  }
  Periodic_segment periodic_segment(const Edge & e) const {
    return periodic_segment(e.first,e.second,e.third);
  }

  Periodic_triangle periodic_triangle(const Cell_handle c, int i) const;
  Periodic_triangle periodic_triangle(const Facet & f) const {
    return periodic_triangle(f.first, f.second);
  }

  Periodic_tetrahedron periodic_tetrahedron(const Cell_handle c) const {
    CGAL_triangulation_precondition( number_of_vertices() != 0 );
    return make_array(
        std::make_pair(c->vertex(0)->point(), get_offset(c,0)),
	std::make_pair(c->vertex(1)->point(), get_offset(c,1)),
        std::make_pair(c->vertex(2)->point(), get_offset(c,2)),
	std::make_pair(c->vertex(3)->point(), get_offset(c,3)) );
  }

  Point point(const Periodic_point & pp) const {
    return construct_point(pp.first, pp.second);
  }
  Segment segment(const Periodic_segment & ps) const {
    return construct_segment(ps[0].first,ps[1].first,ps[0].second,ps[1].second);
  }
  Triangle triangle(const Periodic_triangle & pt) const {
    return construct_triangle(pt[0].first, pt[1].first, pt[2].first,
			      pt[0].second,pt[1].second,pt[2].second);
  }
  Tetrahedron tetrahedron(const Periodic_tetrahedron & pt) const {
    return construct_tetrahedron(pt[0].first, pt[1].first,
				 pt[2].first, pt[3].first,
				 pt[0].second,pt[1].second,
				 pt[2].second,pt[3].second);
  }
  // @}

  /** @name Queries */ //@{
  bool is_vertex(const Point & p, Vertex_handle & v) const;

  bool is_vertex(Vertex_handle v) const {
    return _tds.is_vertex(v);
  }
  bool is_edge(Vertex_handle u, Vertex_handle v,
      Cell_handle & c, int & i, int & j) const {
    return _tds.is_edge(u, v, c, i, j);
  }
  bool is_edge(Vertex_handle u, const Offset & off_u,
	       Vertex_handle v, const Offset & off_v,
      Cell_handle & c, int & i, int & j) const {
    if (!_tds.is_edge(u,v,c,i,j)) return false;
    if ((get_offset(c,i) == off_u) && (get_offset(c,j) == off_v))
      return true;
    // it might be that different cells containing (u,v) yield
    // different offsets, which forces us to test for all possibilities.
    else {
      Cell_circulator ccirc = incident_cells(c,i,j,c);
      while (++ccirc != c) {
	i = ccirc->index(u);
	j = ccirc->index(v);
	if ((get_offset(ccirc,i) == off_u) && (get_offset(ccirc,j) == off_v)) {
	  c = ccirc;
	  return true;
	}
      }
      return false;
    }
  }
  bool is_facet(Vertex_handle u, Vertex_handle v, Vertex_handle w,
      Cell_handle & c, int & i, int & j, int & k) const {
    return _tds.is_facet(u, v, w, c, i, j, k);
  }
  bool is_facet(Vertex_handle u, const Offset & off_u,
		Vertex_handle v, const Offset & off_v,
		Vertex_handle w, const Offset & off_w,
      Cell_handle & c, int & i, int & j, int & k) const {
    if (!_tds.is_facet(u,v,w,c,i,j,k)) return false;
    if ((get_offset(c,i) == off_u)
	&& (get_offset(c,j) == off_v)
	&& (get_offset(c,k) == off_w) )
      return true;
    // it might be that c and c->neighbor(l) yield different offsets
    // which forces us to test for both possibilities.
    else {
      int l = 6-i-j-k;
      c = c->neighbor(l);
      i = c->index(u);
      j = c->index(v);
      k = c->index(w);      
      return ((get_offset(c,i) == off_u)
	  && (get_offset(c,j) == off_v)
	  && (get_offset(c,k) == off_w) );
    }
  }
  bool is_cell(Cell_handle c) const {
    return _tds.is_cell(c);
  }
  bool is_cell(Vertex_handle u, Vertex_handle v,
      Vertex_handle w, Vertex_handle t,
      Cell_handle & c, int & i, int & j, int & k, int & l) const {
    return _tds.is_cell(u, v, w, t, c, i, j, k, l);
  }
  bool is_cell(Vertex_handle u, Vertex_handle v, Vertex_handle w,
      Vertex_handle t, Cell_handle & c) const {
    int i,j,k,l;
    return _tds.is_cell(u, v, w, t, c, i, j, k, l);
  }
  bool is_cell(Vertex_handle u, const Offset & off_u,
	       Vertex_handle v, const Offset & off_v,
	       Vertex_handle w, const Offset & off_w,
	       Vertex_handle t, const Offset & off_t,
      Cell_handle & c, int & i, int & j, int & k, int & l) const {
    if (!_tds.is_cell(u,v,w,t,c,i,j,k,l)) return false;
    return ((get_offset(c,i) == off_u)
	    && (get_offset(c,j) == off_v)
	    && (get_offset(c,k) == off_w)
	    && (get_offset(c,l) == off_t) );
    return false;
  }
  bool is_cell(Vertex_handle u, const Offset & off_u,
	       Vertex_handle v, const Offset & off_v,
	       Vertex_handle w, const Offset & off_w,
	       Vertex_handle t, const Offset & off_t,
	       Cell_handle & c) const {
    int i, j, k, l;
    return is_cell(u,off_u,v,off_v,w,off_w,t,off_t,c,i,j,k,l);
  }

  bool has_vertex(const Facet & f, Vertex_handle v, int & j) const {
    return _tds.has_vertex(f.first, f.second, v, j);
  }
  bool has_vertex(Cell_handle c, int i, Vertex_handle v, int & j) const {
    return _tds.has_vertex(c, i, v, j);
  }
  bool has_vertex(const Facet & f, Vertex_handle v) const {
    return _tds.has_vertex(f.first, f.second, v);
  }
  bool has_vertex(Cell_handle c, int i, Vertex_handle v) const {
    return _tds.has_vertex(c, i, v);
  }
  
  bool are_equal(Cell_handle c, int i, Cell_handle n, int j) const {
    return _tds.are_equal(c, i, n, j);
  }
  bool are_equal(const Facet & f, const Facet & g) const {
    return _tds.are_equal(f.first, f.second, g.first, g.second);
  }
  bool are_equal(const Facet & f, Cell_handle n, int j) const {
    return _tds.are_equal(f.first, f.second, n, j);
  }
  //@}
  
protected:
  /** @name Location helpers */ //@{
  Cell_handle periodic_locate(const Point & p, const Offset &o_p,
    Locate_type & lt, int & li, int & lj, Cell_handle start) const;

  Bounded_side side_of_cell(const Point & p, const Offset &off,
      Cell_handle c, Locate_type & lt, int & i, int & j) const;
  //@}
  
public:
  /** @name Point Location */ //@{
  /** Wrapper function for locate if only the request point is given.
    */
  Cell_handle locate(const Point & p, Cell_handle start=Cell_handle()) const {
    Locate_type lt;
    int li, lj;
    return locate( p, lt, li, lj, start);
  }
  
  /** Wrapper function calling locate with an empty offset if there was no
    * offset given.
    */
  Cell_handle locate(const Point & p, Locate_type & lt, int & li, int & lj,
      Cell_handle start = Cell_handle()) const {
    return periodic_locate(p, Offset(), lt, li, lj, start);
  }

  Bounded_side side_of_cell(const Point & p,
      Cell_handle c, Locate_type & lt, int & i, int & j) const {
    if (number_of_vertices() == 0) {
      lt = EMPTY;
      return ON_UNBOUNDED_SIDE;
    }
    return side_of_cell(p,Offset(),c,lt,i,j);
  }
  //@}

private:
  /** @name Insertion helpers */ //@{
  template <class CellIt>
  void insert_too_long_edges(Vertex_handle v,
      const CellIt begin, const CellIt end);

  template <class CellIt>
  void delete_too_long_edges(const CellIt begin, const CellIt end);

  template < class Conflict_tester, class Point_hider >
  Vertex_handle periodic_insert(const Point& p, const Offset& o, Locate_type lt,
      Cell_handle c, const Conflict_tester &tester,
      Point_hider &hider, Vertex_handle vh = Vertex_handle());

  template <class Point_iterator, class Offset_iterator>
  void periodic_sort(Point_iterator p_begin, Point_iterator p_end,
      Offset_iterator o_begin, Offset_iterator o_end) const {
    std::cout << "Periodic_sort not yet implemented" << std::endl;
  }

  Vertex_handle create_initial_triangulation(const Point &p);
public:
  std::vector<Vertex_handle> insert_dummy_points();
  
protected:
  // this is needed for compatibility reasons
  template <class Conflict_test, class OutputIteratorBoundaryFacets,
      class OutputIteratorCells, class OutputIteratorInternalFacets>
  Triple<OutputIteratorBoundaryFacets, OutputIteratorCells,
         OutputIteratorInternalFacets>
  find_conflicts(Cell_handle c,
      const Conflict_test &tester,
      Triple<OutputIteratorBoundaryFacets, OutputIteratorCells,
      OutputIteratorInternalFacets> it) const {
    Offset off = get_location_offset(tester, c);
    return find_conflicts(c,off,tester,it);
  }

  template <class Conflict_test, class OutputIteratorBoundaryFacets,
      class OutputIteratorCells, class OutputIteratorInternalFacets>
  Triple<OutputIteratorBoundaryFacets, OutputIteratorCells,
         OutputIteratorInternalFacets>
  find_conflicts(Cell_handle c, const Offset &current_off,
      const Conflict_test &tester,
      Triple<OutputIteratorBoundaryFacets, OutputIteratorCells,
      OutputIteratorInternalFacets> it) const;
  //@}
  
protected:
  // COMMON INSERTION for DELAUNAY and REGULAR TRIANGULATION
  template < class Conflict_tester, class Point_hider >
  Vertex_handle insert_in_conflict(const Point & p, Cell_handle start,
      const Conflict_tester &tester, Point_hider &hider) {
    Locate_type lt = Locate_type();
    int li=0, lj=0;
    Cell_handle c = periodic_locate(p, Offset(), lt, li, lj, start);
    return insert_in_conflict(p,lt,c,li,lj,tester,hider);
  }

  template < class Conflict_tester, class Point_hider >
  Vertex_handle insert_in_conflict(const Point & p, Locate_type lt,
    Cell_handle c, int li, int lj, const Conflict_tester &tester,
    Point_hider &hider);

  template < class InputIterator, class Conflict_tester,
      class Point_hider>
  std::vector<Vertex_handle> insert_in_conflict(
      InputIterator begin, InputIterator end, Cell_handle start,
      Conflict_tester &tester, Point_hider &hider) {
    Vertex_handle new_vertex;
    std::vector<Vertex_handle> double_vertices;
    Locate_type lt = Locate_type();
    int li=0, lj=0;
    CGAL_triangulation_assertion_code( Locate_type lta = Locate_type(); )
    CGAL_triangulation_assertion_code( int ia = 0; )
    CGAL_triangulation_assertion_code( int ja = 0; )
    Cell_handle hint;
    while (begin!=end) {
      tester.set_point(*begin);
      hint = periodic_locate(*begin, Offset(), lt, li, lj, start);
      CGAL_triangulation_assertion_code( if (number_of_vertices() != 0) { );
	CGAL_triangulation_assertion(side_of_cell(
		*begin,Offset(), hint, lta, ia, ja) != ON_UNBOUNDED_SIDE);
	CGAL_triangulation_assertion(lta == lt);
	CGAL_triangulation_assertion(ia == li);
	CGAL_triangulation_assertion(ja == lj);
      CGAL_triangulation_assertion_code( } );

      new_vertex = insert_in_conflict(*begin,lt,hint,li,lj,tester,hider);
      if (lt == VERTEX) double_vertices.push_back(new_vertex);
      start = new_vertex->cell();
      begin++;
    }
    return double_vertices;
  }
  //@}

private:
  /** @name Removal helpers */ //@{
  Vertex_triple make_vertex_triple(const Facet& f) const {
    Cell_handle ch = f.first;
    int i = f.second;
    return Vertex_triple(ch->vertex(vertex_triple_index(i,0)),
        ch->vertex(vertex_triple_index(i,1)),
        ch->vertex(vertex_triple_index(i,2))); 
  }

  void make_canonical(Vertex_triple& t) const;

  void make_hole(Vertex_handle v, std::map<Vertex_triple,Facet> &outer_map,
      std::vector<Cell_handle> &hole);

  template < class PointRemover >
  void periodic_remove(Vertex_handle v, PointRemover &remover); 
  //@}
  
protected:
  /** @name Removal */ //@{
  template < class PointRemover, class CT >
  void remove(Vertex_handle v, PointRemover &remover, CT &ct);
  //@}

public:
  /** @name Traversal */ //@{
  Cell_iterator cells_begin() const {
    return _tds.cells_begin();
  }
  Cell_iterator cells_end() const {
    return _tds.cells_end();
  }

  Vertex_iterator vertices_begin() const {
    return _tds.vertices_begin();
  }
  Vertex_iterator vertices_end() const {
    return _tds.vertices_end();
  }

  Edge_iterator edges_begin() const {
    return _tds.edges_begin();
  }
  Edge_iterator edges_end() const {
    return _tds.edges_end();
  }

  Facet_iterator facets_begin() const {
    return _tds.facets_begin();
  }
  Facet_iterator facets_end() const {
    return _tds.facets_end();
  }

  Cell_iterator finite_cells_begin() const {
    return _tds.cells_begin();
  }
  Cell_iterator finite_cells_end() const {
    return _tds.cells_end();
  }

  Vertex_iterator finite_vertices_begin() const {
    return _tds.vertices_begin();
  }
  Vertex_iterator finite_vertices_end() const {
    return _tds.vertices_end();
  }

  Edge_iterator finite_edges_begin() const {
    return _tds.edges_begin();
  }
  Edge_iterator finite_edges_end() const {
    return _tds.edges_end();
  }

  Facet_iterator finite_facets_begin() const {
    return _tds.facets_begin();
  }
  Facet_iterator finite_facets_end() const {
    return _tds.facets_end();
  }

  All_cells_iterator all_cells_begin() const {
    return _tds.cells_begin();
  }
  All_cells_iterator all_cells_end() const {
    return _tds.cells_end();
  }

  All_vertices_iterator all_vertices_begin() const {
    return _tds.vertices_begin();
  }
  All_vertices_iterator all_vertices_end() const {
    return _tds.vertices_end();
  }

  All_edges_iterator all_edges_begin() const {
    return _tds.edges_begin();
  }
  All_edges_iterator all_edges_end() const {
    return _tds.edges_end();
  }

  All_facets_iterator all_facets_begin() const {
    return _tds.facets_begin();
  }
  All_facets_iterator all_facets_end() const {
    return _tds.facets_end();
  }
  
  Unique_vertex_iterator unique_vertices_begin() const {
    return CGAL::filter_iterator(vertices_end(), Domain_tester<Self>(this),
	                         vertices_begin());
  }
  Unique_vertex_iterator unique_vertices_end() const {
    return CGAL::filter_iterator(vertices_end(), Domain_tester<Self>(this));
  }

  // Geometric iterators
  Periodic_tetrahedron_iterator periodic_tetrahedra_begin(
      Iterator_type it = STORED) const {
    return Periodic_tetrahedron_iterator(this, it);
  }
  Periodic_tetrahedron_iterator periodic_tetrahedra_end(
      Iterator_type it = STORED) const {
    return Periodic_tetrahedron_iterator(this, 1, it);
  }

  Periodic_triangle_iterator periodic_triangles_begin(
      Iterator_type it = STORED) const {
    return Periodic_triangle_iterator(this, it);
  }
  Periodic_triangle_iterator periodic_triangles_end(
      Iterator_type it = STORED) const {
    return Periodic_triangle_iterator(this, 1, it);
  }

  Periodic_segment_iterator periodic_segments_begin(
      Iterator_type it = STORED) const {
    return Periodic_segment_iterator(this, it);
  }
  Periodic_segment_iterator periodic_segments_end(
      Iterator_type it = STORED) const {
    return Periodic_segment_iterator(this, 1, it);
  }

  Periodic_point_iterator periodic_points_begin(
      Iterator_type it = STORED) const {
    return Periodic_point_iterator(this, it);
  }
  Periodic_point_iterator periodic_points_end(
      Iterator_type it = STORED) const  {
    return Periodic_point_iterator(this, 1, it);
  }

  // Circulators
  Cell_circulator incident_cells(const Edge & e) const {
    return _tds.incident_cells(e);
  }
  Cell_circulator incident_cells(Cell_handle c, int i, int j) const {
    return _tds.incident_cells(c, i, j);
  }
  Cell_circulator incident_cells(const Edge & e, Cell_handle start) const {
    return _tds.incident_cells(e, start);
  }
  Cell_circulator incident_cells(Cell_handle c, int i, int j, 
      Cell_handle start) const {
    return _tds.incident_cells(c, i, j, start);
  }

  Facet_circulator incident_facets(const Edge & e) const {
    return _tds.incident_facets(e);
  }
  Facet_circulator incident_facets(Cell_handle c, int i, int j) const {
    return _tds.incident_facets(c, i, j);
  }
  Facet_circulator incident_facets(const Edge & e, const Facet & start) const {
    return _tds.incident_facets(e, start);
  }
  Facet_circulator incident_facets(Cell_handle c, int i, int j, 
      const Facet & start) const {
    return _tds.incident_facets(c, i, j, start);
  }
  Facet_circulator incident_facets(const Edge & e,
      Cell_handle start, int f) const {
    return _tds.incident_facets(e, start, f);
  }
  Facet_circulator incident_facets(Cell_handle c, int i, int j, 
      Cell_handle start, int f) const {
    return _tds.incident_facets(c, i, j, start, f);
  }

  // around a vertex
  template <class OutputIterator>
  OutputIterator incident_cells(Vertex_handle v, OutputIterator cells) const {
    return _tds.incident_cells(v, cells);
  }

  template <class OutputIterator>
  OutputIterator incident_facets(Vertex_handle v, OutputIterator facets) const {
    return _tds.incident_facets(v, facets);
  }

  template <class OutputIterator>
  OutputIterator incident_edges(
      Vertex_handle v, OutputIterator edges) const {
    return _tds.incident_edges(v, edges);
  }

  template <class OutputIterator>
  OutputIterator adjacent_vertices(
      Vertex_handle v, OutputIterator vertices) const {
    return _tds.adjacent_vertices(v, vertices);
  }

  //deprecated, don't use anymore
  template <class OutputIterator>
  OutputIterator incident_vertices(
      Vertex_handle v, OutputIterator vertices) const {
    return _tds.adjacent_vertices(v, vertices);
  }

  size_type degree(Vertex_handle v) const {
    return _tds.degree(v);
  }

  // Functions forwarded from TDS.
  int mirror_index(Cell_handle c, int i) const {
    return _tds.mirror_index(c, i);
  }

  Vertex_handle mirror_vertex(Cell_handle c, int i) const {
    return _tds.mirror_vertex(c, i);
  }

  Facet mirror_facet(Facet f) const {
    return _tds.mirror_facet(f);
  }
  //@}
  
private:
  /** @name Checking helpers */ //@{
  /// calls has_self_edges for every cell of the triangulation
  bool has_self_edges() const {
    Cell_iterator it;
    for ( it = all_cells_begin(); it != all_cells_end(); ++it )
      if (has_self_edges(it)) return true;
    return false;
  }
  bool has_self_edges(Cell_handle c) const;
  //@}
  
public:
  /** @name Checking */ //@{
  bool is_valid(bool verbose = false, int level = 0) const;
  bool is_valid(Cell_handle c, bool verbose = false, int level = 0) const;
protected:
  template <class ConflictTester>
  bool is_valid_conflict(ConflictTester &tester, bool verbose = false,
      int level = 0) const;
  //@}

protected:
  /** @name Functors */ //@{
  template < class Cmp >
  class Perturbation_order;
  //@}

public:
  // undocumented access functions
  Offset get_offset(Cell_handle ch, int i) const {
    if (is_1_cover()) return int_to_off(ch->offset(i));
    Virtual_vertex_map_it it = virtual_vertices.find(ch->vertex(i));
    if (it != virtual_vertices.end())
      return combine_offsets(it->second.second, int_to_off(ch->offset(i)));
    else return combine_offsets(Offset(), int_to_off(ch->offset(i)));
  }
  Offset get_offset(Vertex_handle vh) const {
    if (is_1_cover()) return Offset();
    Virtual_vertex_map_it it = virtual_vertices.find(vh);
    if (it != virtual_vertices.end()) return it->second.second;
    else return Offset();
  }
  Vertex_handle get_original_vertex(Vertex_handle vh) const {
    if (is_1_cover()) return vh;
    Virtual_vertex_map_it it = virtual_vertices.find(vh);
    if (it != virtual_vertices.end()) return it->second.first;
    else return vh;
  }
  Offset combine_offsets(const Offset& o_c, const Offset& o_t) const {
    Offset o_ct(_cover[0]*o_t.x(),_cover[1]*o_t.y(),_cover[2]*o_t.z());
    return o_c + o_ct;
  }

  // These functions give the pair (vertex, offset) that corresponds to the
  // i-th vertex of cell ch or vertex vh, respectively.
  void get_vertex(Cell_handle ch, int i, Vertex_handle &vh, Offset &off) const;
  void get_vertex(Vertex_handle vh_i, Vertex_handle &vh, Offset &off) const;

protected:
  // Auxiliary functions
  int find_too_long_edges(std::map<Vertex_handle,
      std::list<Vertex_handle> >& edges) const;
  Cell_handle get_cell(const Vertex_handle* vh) const;
  template<class Conflict_tester>
  Offset get_location_offset(const Conflict_tester& tester,
	  Cell_handle c) const;

  Offset get_neighbor_offset(Cell_handle ch, int i, Cell_handle nb) const;
  
  /** @name Friends */ //@{
  friend class Perturbation_order<typename GT::Compare_xyz_3>;
  friend std::istream& operator>> <>
      (std::istream& is, Periodic_3_triangulation_3<GT,TDS> &tr);
  friend std::ostream& operator<< <>
      (std::ostream& os, const Periodic_3_triangulation_3<GT,TDS> &tr);
  //@}

  // unused and undocumented function required to be compatible to
  // Alpha_shape_3
public:
  Point point(Cell_handle c, int idx) const {
    //    if (is_1_cover())
      return point(periodic_point(c,idx));

    Offset vec_off[4];
    for (int i=0 ; i<4 ; i++) vec_off[i] = periodic_point(c,i).second;
    int ox = vec_off[0].x();
    int oy = vec_off[0].y();
    int oz = vec_off[0].z();
    for (int i=1 ; i<4 ; i++) {
      ox = std::min(ox,vec_off[i].x());
      oy = std::min(oy,vec_off[i].y());
      oz = std::min(oz,vec_off[i].z());
    }
    Offset diff_off(-ox,-oy,-oz);
    if (diff_off.is_null()) return point(periodic_point(c,idx));

    for (unsigned int i=0 ; i<4 ; i++) vec_off[i] += diff_off;
    Vertex_handle canonic_vh[4];
    for (int i=0 ; i<4 ; i++) {
      Virtual_vertex_map_it vvmit = virtual_vertices.find(c->vertex(i));
      Vertex_handle orig_vh;
      if (vvmit == virtual_vertices.end()) orig_vh = c->vertex(i);
      else orig_vh = vvmit->second.first;
      if (vec_off[i].is_null()) canonic_vh[i] = orig_vh;
      else {
	CGAL_assertion(virtual_vertices_reverse.find(orig_vh)
	    != virtual_vertices_reverse.end());
	canonic_vh[i] = virtual_vertices_reverse.find(orig_vh)
	  ->second[9*vec_off[i][0]+3*vec_off[i][1]+vec_off[i][2]-1];
      }
    }
    
    std::vector<Cell_handle> cells;
    incident_cells(canonic_vh[0], std::back_inserter(cells));
    for (unsigned int i=0 ; i<cells.size() ; i++) {
      CGAL_assertion(cells[i]->has_vertex(canonic_vh[0]));
      if (cells[i]->has_vertex(canonic_vh[1])
	  && cells[i]->has_vertex(canonic_vh[2])
	  && cells[i]->has_vertex(canonic_vh[3]) )
	return point(periodic_point(cells[i],cells[i]->index(canonic_vh[idx])));
    }
    CGAL_assertion(false);
  
  }
};

template < class GT, class TDS >
inline void
Periodic_3_triangulation_3<GT,TDS>::
copy_multiple_covering(const Periodic_3_triangulation_3<GT,TDS> & tr) {  
  // Write the respective offsets in the vertices to make them
  // automatically copy with the tds.
  for (Vertex_iterator vit = tr.vertices_begin() ;
       vit != tr.vertices_end() ; ++vit) {
    vit->set_offset(tr.get_offset(vit));
  }
  // copy the tds
  _tds = tr.tds();
  // make a list of all vertices that belong to the original
  // domain and initialize the basic structure of
  // virtual_vertices_reverse
  std::list<Vertex_handle> vlist;
  for (Vertex_iterator vit = vertices_begin() ;
       vit != vertices_end() ; ++vit) {
    if (vit->offset() == Offset()) {
      vlist.push_back(vit);
      virtual_vertices_reverse.insert(
	  std::make_pair(vit,std::vector<Vertex_handle>(26)));
      CGAL_triangulation_assertion(virtual_vertices_reverse.find(vit)
	  ->second.size() == 26);
    }
  }     
  // Iterate over all vertices that are not in the original domain
  // and construct the respective entries to virtual_vertices and
  // virtual_vertices_reverse
  for (Vertex_iterator vit2 = vertices_begin() ;
       vit2 != vertices_end() ; ++vit2) {
    if (vit2->offset() != Offset()) {
      //TODO: use some binding, maybe boost instead of the Finder.
      typename std::list<Vertex_handle>::iterator vlist_it
	= std::find_if(vlist.begin(), vlist.end(),
		       Finder(this,vit2->point()));
      Offset off = vit2->offset();
      virtual_vertices.insert(std::make_pair(vit2,
					     std::make_pair(*vlist_it,off)));
      virtual_vertices_reverse.find(*vlist_it)
	->second[9*off[0]+3*off[1]+off[2]-1]=vit2;
      CGAL_triangulation_assertion(get_offset(vit2) == off);
    }
  }
  // Cleanup vertex offsets
  for (Vertex_iterator vit = vertices_begin() ;
       vit != vertices_end() ; ++vit)
    vit->clear_offset();
  for (Vertex_iterator vit = tr.vertices_begin() ;
       vit != tr.vertices_end() ; ++vit)
    vit->clear_offset();
  // Build up the too_long_edges container
  too_long_edge_counter = 0;
  too_long_edges.clear();
  for (Vertex_iterator vit = vertices_begin() ;
       vit != vertices_end() ; ++vit) 
    too_long_edges[vit] = std::list<Vertex_handle>();
  std::pair<Vertex_handle, Vertex_handle> edge_to_add;
  Point p1,p2;
  int i,j;
  for (Edge_iterator eit = edges_begin() ;
       eit != edges_end() ; ++eit) {
    if (&*(eit->first->vertex(eit->second))
	< &*(eit->first->vertex(eit->third))) {
      i = eit->second; j = eit->third;
    } else {
      i = eit->third; j = eit->second;
    }
    edge_to_add = std::make_pair(eit->first->vertex(i),
				 eit->first->vertex(j));
    p1 = construct_point(eit->first->vertex(i)->point(),
	get_offset(eit->first, i));
    p2 = construct_point(eit->first->vertex(j)->point(),
	get_offset(eit->first, j));
    Vertex_handle v_no = eit->first->vertex(i);
    if (squared_distance(p1,p2) > edge_length_threshold) {
      CGAL_triangulation_assertion(
	  find(too_long_edges[v_no].begin(),
	       too_long_edges[v_no].end(),
	       edge_to_add.second) == too_long_edges[v_no].end());
      too_long_edges[v_no].push_back(edge_to_add.second);
      too_long_edge_counter++;
    }
  }
}

template < class GT, class TDS >
inline bool
Periodic_3_triangulation_3<GT,TDS>::
is_extensible_triangulation_in_1_sheet_h1() const {
  if (!is_1_cover()) {
    if (too_long_edge_counter == 0) return true;
    else return false;
  } else {
    typename Geometric_traits::FT longest_edge_squared_length(0);
    Segment s;
    for (Periodic_segment_iterator psit = periodic_segments_begin(UNIQUE);
 	 psit != periodic_segments_end(UNIQUE) ; ++psit) {
      s = construct_segment(*psit);
      longest_edge_squared_length = (std::max)(longest_edge_squared_length,
	  s.squared_length());
    }
    return (longest_edge_squared_length < edge_length_threshold);
  }
}

template < class GT, class TDS >
inline bool
Periodic_3_triangulation_3<GT,TDS>::
is_extensible_triangulation_in_1_sheet_h2() const {
  typedef typename Geometric_traits::Construct_circumcenter_3
    Construct_circumcenter;
  typedef typename Geometric_traits::FT FT;
  Construct_circumcenter construct_circumcenter
    = _gt.construct_circumcenter_3_object();
  for (Periodic_tetrahedron_iterator tit = periodic_tetrahedra_begin(UNIQUE) ;
       tit != periodic_tetrahedra_end(UNIQUE) ; ++tit) {
    Point cc = construct_circumcenter(
	tit->at(0).first, tit->at(1).first,
	tit->at(2).first, tit->at(3).first,
	tit->at(0).second, tit->at(1).second,
	tit->at(2).second, tit->at(3).second);

    if ( !(FT(16)*squared_distance(cc,point(tit->at(0)))
	    < (_domain.xmax()-_domain.xmin())*(_domain.xmax()-_domain.xmin())) )
      return false;
  }
  return true;
}

template < class GT, class TDS >
inline bool
Periodic_3_triangulation_3<GT,TDS>::
is_triangulation_in_1_sheet() const {
  if (is_1_cover()) return true;
  for (Vertex_iterator vit = vertices_begin(); vit != vertices_end(); ++vit) {
    if (virtual_vertices.find(vit) == virtual_vertices.end()) continue;
    std::vector<Vertex_handle> nb_v;
    std::set<Vertex_handle> nb_v_odom;
    Vertex_handle vh;
    Offset off;
    adjacent_vertices(vit, std::back_inserter(nb_v));
    for (unsigned int i=0; i<nb_v.size(); i++) {
      get_vertex(nb_v[i],vh,off);
      nb_v_odom.insert(vh);
    }
    if (nb_v.size() != nb_v_odom.size()) 
      return false;
  }
  return true;
}

template < class GT, class TDS >
inline bool
Periodic_3_triangulation_3<GT,TDS>::
is_vertex(const Point & p, Vertex_handle & v) const
{
  Locate_type lt;
  int li, lj;
  Cell_handle c = locate( p, lt, li, lj );
  if ( lt != VERTEX )
    return false;
  v = c->vertex(li);
  return true;
}

template < class GT, class TDS >
inline void
Periodic_3_triangulation_3<GT,TDS>::
make_canonical(Vertex_triple& t) const
{
  int i = (&*(t.first) < &*(t.second))? 0 : 1;
  if(i==0) {
    i = (&*(t.first) < &*(t.third))? 0 : 2;
  } else {
    i = (&*(t.second) < &*(t.third))? 1 : 2;
  }
  Vertex_handle tmp; 
  switch(i){
  case 0: return;
  case 1:
    tmp = t.first;
    t.first = t.second;
    t.second = t.third;
    t.third = tmp;
    return;
  default:
    tmp = t.first;
    t.first = t.third;
    t.third = t.second;
    t.second = tmp;
  }
}

template < class GT, class TDS >
inline typename Periodic_3_triangulation_3<GT,TDS>::Periodic_triangle
Periodic_3_triangulation_3<GT,TDS>::
periodic_triangle(const Cell_handle c, int i) const
{ 
  CGAL_triangulation_precondition( number_of_vertices() != 0 );
  CGAL_triangulation_precondition( i >= 0 && i <= 3 );
  if ( (i&1)==0 ) 
    return make_array(std::make_pair(c->vertex( (i+2)&3 )->point(),
				     get_offset(c,(i+2)&3)),
		      std::make_pair(c->vertex( (i+1)&3 )->point(),
				     get_offset(c,(i+1)&3)),
		      std::make_pair(c->vertex( (i+3)&3 )->point(),
				     get_offset(c,(i+3)&3)) );
  return make_array(std::make_pair(c->vertex( (i+1)&3 )->point(),
				   get_offset(c,(i+1)&3)),
		    std::make_pair(c->vertex( (i+2)&3 )->point(),
				   get_offset(c,(i+2)&3)),
		    std::make_pair(c->vertex( (i+3)&3 )->point(),
				   get_offset(c,(i+3)&3)) );
}

/** Assumes a point, an offset, and a cell to start from.
  * Gives the locate type and the simplex (cell and indices) containing p.
  *
  * Performs a remembering stochastic walk if the triangulation is not empty.
  * After the walk the type of the simplex containing p is determined.
  *
  * returns the cell p lies in
  * starts at cell "start"
  * returns a cell Cell_handel if lt == CELL
  * returns a facet (Cell_handle,li) if lt == FACET
  * returns an edge (Cell_handle,li,lj) if lt == EDGE
  * returns a vertex (Cell_handle,li) if lt == VERTEX
  */
template < class GT, class TDS >
typename Periodic_3_triangulation_3<GT,TDS>::Cell_handle
Periodic_3_triangulation_3<GT,TDS>::periodic_locate(
    const Point & p, const Offset &o_p,
    Locate_type & lt, int & li, int & lj, Cell_handle start) const {
  int cumm_off = 0;
  Offset off_query = o_p;
  if (number_of_vertices() == 0) {
    lt = EMPTY;
    return Cell_handle();
  }
  CGAL_triangulation_assertion(number_of_vertices() != 0);

  if (start == Cell_handle()) {
    start = cells_begin();
  }

  cumm_off = start->offset(0) | start->offset(1)
    | start->offset(2) | start->offset(3);
  if (is_1_cover() && cumm_off != 0) {
    if (((cumm_off & 4) == 4) && (FT(2)*p.x()<(_domain.xmax()+_domain.xmin())))
      off_query += Offset(1,0,0);
    if (((cumm_off & 2) == 2) && (FT(2)*p.y()<(_domain.ymax()+_domain.ymin())))
      off_query += Offset(0,1,0);
    if (((cumm_off & 1) == 1) && (FT(2)*p.z()<(_domain.zmax()+_domain.zmin())))
      off_query += Offset(0,0,1);
  }

  CGAL_triangulation_postcondition(start!=Cell_handle());
  CGAL_triangulation_assertion(start->neighbor(0)->neighbor(
      start->neighbor(0)->index(start))==start);
  CGAL_triangulation_assertion(start->neighbor(1)->neighbor(
      start->neighbor(1)->index(start))==start);
  CGAL_triangulation_assertion(start->neighbor(2)->neighbor(
      start->neighbor(2)->index(start))==start);
  CGAL_triangulation_assertion(start->neighbor(3)->neighbor(
      start->neighbor(3)->index(start))==start);

  // We implement the remembering visibility/stochastic walk.
  
  // Remembers the previous cell to avoid useless orientation tests.
  Cell_handle previous = Cell_handle();
  Cell_handle c = start;
  
  // Stores the results of the 4 orientation tests.  It will be used
  // at the end to decide if p lies on a face/edge/vertex/interior.
  Orientation o[4];
  
  boost::rand48 rng;      
  boost::uniform_smallint<> four(0, 3);
  boost::variate_generator<boost::rand48&, boost::uniform_smallint<> > die4(rng, four);

  
  // Now treat the cell c.
try_next_cell:
  // For the remembering stochastic walk,
  // we need to start trying with a random index :
  int i = die4();
  // For the remembering visibility walk (Delaunay only), we don't :
  // int i = 0;

  cumm_off =
    c->offset(0) | c->offset(1) | c->offset(2) | c->offset(3);

  bool simplicity_criterion = (cumm_off == 0) && (off_query.is_null());

  // We know that the 4 vertices of c are positively oriented.
  // So, in order to test if p is seen outside from one of c's facets,
  // we just replace the corresponding point by p in the orientation
  // test.  We do this using the arrays below.

  Offset off[4];
  const Point* pts[4] = { &(c->vertex(0)->point()),
      &(c->vertex(1)->point()),
      &(c->vertex(2)->point()),
      &(c->vertex(3)->point()) };

  if (!simplicity_criterion && is_1_cover() ) {
    for (int i=0; i<4; i++) {
      off[i] = int_to_off(c->offset(i));
    }
  }
  
  if (!is_1_cover()) {
    // Just fetch the vertices of c as points with offsets
    for (int i=0; i<4; i++) {
      pts[i] = &(c->vertex(i)->point());
      off[i] = get_offset(c,i);
    }
  }
  
  for (int j=0; j != 4; ++j, i = (i+1)&3) {
    Cell_handle next = c->neighbor(i);
    if (previous == next) {
      o[i] = POSITIVE;
      continue;
    }
    
    CGAL_triangulation_assertion(next->neighbor(next->index(c)) == c);
    
    // We temporarily put p at i's place in pts.
    const Point* backup = pts[i];
    pts[i] = &p;
    
    if (simplicity_criterion && is_1_cover() ) {
      o[i] = orientation(*pts[0], *pts[1], *pts[2], *pts[3]);
      
      if ( o[i] != NEGATIVE ) {
        pts[i] = backup;
        continue;
      }
    }
    else {
      Offset backup_off;
      
      backup_off = off[i];
      off[i] = off_query;
      o[i] = orientation(*pts[0], *pts[1], *pts[2], *pts[3],
          off[0], off[1], off[2], off[3]);

      if ( o[i] != NEGATIVE ) {
        pts[i] = backup;
        off[i] = backup_off;
        continue;
      }
    }

    // Test whether we need to adapt the offset of the query point.
    // This means, if we get out of the current cover.
    off_query = combine_offsets(off_query, get_neighbor_offset(c,i,next));
    previous = c;
    c = next;
    goto try_next_cell;
  }

  
  // Ok, now we have found the cell. It remains to find the dimension of the
  // intersected simplex.
  // now p is in c or on its boundary
  int sum = ( o[0] == COPLANAR )
    + ( o[1] == COPLANAR )
    + ( o[2] == COPLANAR )
    + ( o[3] == COPLANAR );
  switch (sum) {
  case 0:
      lt = CELL;
      break;
  case 1:
      lt = FACET;
      li = ( o[0] == COPLANAR ) ? 0 :
          ( o[1] == COPLANAR ) ? 1 :
          ( o[2] == COPLANAR ) ? 2 : 3;
      break;
  case 2:
    lt = EDGE;
    li = ( o[0] != COPLANAR ) ? 0 :
        ( o[1] != COPLANAR ) ? 1 : 2;
    lj = ( o[li+1] != COPLANAR ) ? li+1 :
        ( o[li+2] != COPLANAR ) ? li+2 : li+3;
    break;
  case 3:
    lt = VERTEX;
    li = ( o[0] != COPLANAR ) ? 0 :
        ( o[1] != COPLANAR ) ? 1 :
        ( o[2] != COPLANAR ) ? 2 : 3;
    break;
  default:
    // Vertex can not lie on four facets
    CGAL_triangulation_assertion(false);
  }
  return c;
}

/**
 * returns
 * ON_BOUNDED_SIDE if p inside the cell
 * (for an infinite cell this means that p lies strictly in the half space
 * limited by its finite facet)
 * ON_BOUNDARY if p on the boundary of the cell
 * (for an infinite cell this means that p lies on the *finite* facet)
 * ON_UNBOUNDED_SIDE if p lies outside the cell
 * (for an infinite cell this means that p is not in the preceding
 * two cases)
 * 
 * lt has a meaning only when ON_BOUNDED_SIDE or ON_BOUNDARY
 */
// TODO: currently off is not used. It could probably be optimized
// using off.
template < class GT, class TDS >
inline Bounded_side Periodic_3_triangulation_3<GT,TDS>::side_of_cell(
    const Point & q, const Offset &off, Cell_handle c,
    Locate_type & lt, int & i, int & j) const
{
  CGAL_triangulation_precondition( number_of_vertices() != 0 );

  Orientation o0,o1,o2,o3;
  o0 = o1 = o2 = o3 = ZERO;

  int cumm_off = c->offset(0) | c->offset(1) | c->offset(2) | c->offset(3);
  if ((cumm_off == 0) && (is_1_cover())) {
    CGAL_triangulation_assertion(off == Offset());
    const Point &p0  = c->vertex(0)->point();
    const Point &p1  = c->vertex(1)->point();
    const Point &p2  = c->vertex(2)->point();
    const Point &p3  = c->vertex(3)->point();
  
    if (((o0 = orientation(q ,p1,p2,p3)) == NEGATIVE) ||
        ((o1 = orientation(p0,q ,p2,p3)) == NEGATIVE) ||
        ((o2 = orientation(p0,p1,q ,p3)) == NEGATIVE) ||
        ((o3 = orientation(p0,p1,p2,q )) == NEGATIVE) ) {
      return ON_UNBOUNDED_SIDE;
    }
  } else { // Special case for the periodic space.
    Offset off_q;
    Offset offs[4];
    const Point *p[4];
    for (int i=0; i<4; i++) {
      p[i] = &(c->vertex(i)->point());
      offs[i] = get_offset(c,i);
    }
    CGAL_triangulation_assertion(orientation(*p[0], *p[1], *p[2], *p[3],
        offs[0], offs[1], offs[2], offs[3]) == POSITIVE);
    bool found=false;
    for (int i=0; (i<8)&&(!found); i++) {
      if ((cumm_off | ((~i)&7)) == 7) {
        o0 = o1 = o2 = o3 = NEGATIVE;
        off_q = combine_offsets(off, int_to_off(i));

        if (((o0 = orientation(      q,  *p[1],  *p[2],  *p[3], 
                off_q  ,offs[1],offs[2],offs[3])) != NEGATIVE)&&
            ((o1 = orientation(  *p[0],      q,  *p[2],  *p[3], 
                offs[0],  off_q,offs[2],offs[3])) != NEGATIVE)&&
            ((o2 = orientation(  *p[0],  *p[1],      q,  *p[3], 
                offs[0],offs[1],  off_q,offs[3])) != NEGATIVE)&&
            ((o3 = orientation(  *p[0],  *p[1],  *p[2],      q, 
                offs[0],offs[1],offs[2],  off_q)) != NEGATIVE)) {
          found = true;
        }
      }
    }
    if (!found) return ON_UNBOUNDED_SIDE;
  }

  // now all the oi's are >=0
  // sum gives the number of facets p lies on
  int sum = ( (o0 == ZERO) ? 1 : 0 ) 
    + ( (o1 == ZERO) ? 1 : 0 ) 
    + ( (o2 == ZERO) ? 1 : 0 ) 
    + ( (o3 == ZERO) ? 1 : 0 );

  switch (sum) {
  case 0:
    {
      lt = CELL;
      return ON_BOUNDED_SIDE;
    }
  case 1:
    {
      lt = FACET;
      // i = index such that p lies on facet(i)
      i = ( o0 == ZERO ) ? 0 :
      ( o1 == ZERO ) ? 1 :
      ( o2 == ZERO ) ? 2 :
      3;
      return ON_BOUNDARY;
    }
  case 2:
    {
      lt = EDGE;
      // i = smallest index such that p does not lie on facet(i)
      // i must be < 3 since p lies on 2 facets
      i = ( o0 == POSITIVE ) ? 0 :
      ( o1 == POSITIVE ) ? 1 :
      2;
      // j = larger index such that p not on facet(j)
      // j must be > 0 since p lies on 2 facets
      j = ( o3 == POSITIVE ) ? 3 :
      ( o2 == POSITIVE ) ? 2 :
      1;
      return ON_BOUNDARY;
    }
  case 3:
    {
      lt = VERTEX;
      // i = index such that p does not lie on facet(i)
      i = ( o0 == POSITIVE ) ? 0 :
      ( o1 == POSITIVE ) ? 1 :
      ( o2 == POSITIVE ) ? 2 :
      3;
      return ON_BOUNDARY;
    }
  default:
    {
      // impossible : cannot be on 4 facets for a real tetrahedron
      CGAL_triangulation_assertion(false);
      return ON_BOUNDARY;
    }
  }
} // side_of_cell

template< class GT, class TDS >
template< class CellIt >
inline void Periodic_3_triangulation_3<GT,TDS>::
    insert_too_long_edges(Vertex_handle v,
        const CellIt begin, const CellIt end) {
  CGAL_triangulation_precondition(number_of_vertices() != 0);
  // add newly added edges to too_long_edges, if necessary.
  Point p1,p2;
  Offset omin;
  std::pair< Vertex_handle, Vertex_handle > edge_to_add;
  std::pair< Offset, Offset > edge_to_add_off;
  std::list<Vertex_handle> empty_list;
  too_long_edges[v] = empty_list;
  // Iterate over all cells of the new star.
  for (CellIt it = begin ; it != end ; ++it) {
    // Consider all possible vertex pairs.
    for (int k=0; k<4 ; k++) {
    for (int j=0; j<4 ; j++) {
      if (j==k) continue;
      if (&*((*it)->vertex(j)) > &*((*it)->vertex(k))) continue;
      // make the offsets canonical (wrt. to some notion)
      // add to too_long_edges, if not yet added and if "too long"
      CGAL_triangulation_precondition(
	  &*((*it)->vertex(j))< &*((*it)->vertex(k)));

      edge_to_add = std::make_pair((*it)->vertex(j), (*it)->vertex(k));
      
      p1 = construct_point((*it)->vertex(j)->point(), get_offset(*it, j));
      p2 = construct_point((*it)->vertex(k)->point(), get_offset(*it, k));

      if ((squared_distance(p1,p2) > edge_length_threshold)
          && (find(too_long_edges[(*it)->vertex(j)].begin(),
		  too_long_edges[(*it)->vertex(j)].end(),
		  edge_to_add.second)
	      == too_long_edges[(*it)->vertex(j)].end())
      ){
        too_long_edges[(*it)->vertex(j)].push_back(edge_to_add.second);
        too_long_edge_counter++;
      }
    } }
  }
}

template < class GT, class TDS >
template < class CellIt >
inline void Periodic_3_triangulation_3<GT,TDS>::
    delete_too_long_edges(const CellIt begin, const CellIt end) {
  std::pair< Vertex_handle, Vertex_handle > edge_to_delete, edge_to_delete2;
  typename std::list< Vertex_handle >::iterator sit;
  // Iterate over all cells that are in the star. That means that those cells
  // are going to be deleted. Therefore, all of them have to be deleted from
  // too_long_edges, if they are contained in it.
  for (CellIt it = begin ; it != end ; ++it) {
    for (int j=0; j<4 ; j++) {
      for (int k=0; k<4; k++) {
        if (&*((*it)->vertex(j)) < &*((*it)->vertex(k))) {
          edge_to_delete = std::make_pair((*it)->vertex(j),(*it)->vertex(k));
        } else {
          edge_to_delete = std::make_pair((*it)->vertex(k),(*it)->vertex(j));
        }
        Vertex_handle v_no = edge_to_delete.first;
        sit = find(too_long_edges[v_no].begin(),
            too_long_edges[v_no].end(),
            edge_to_delete.second);
        if (sit != too_long_edges[v_no].end()) {
          too_long_edges[v_no].erase(sit);
          too_long_edge_counter--;
        }
      }
    }
  }
}

/*! \brief Insert point.
*
* Inserts the point p into the triangulation. It assumes that
* the cell c containing p is already known.
*
* Implementation:
* - some precondition checking
* - find and mark conflicting cells --> find_conflicts
* Conflicting cells are stored in the vector cells.
* - backup hidden points
* - Delete the edges of the marked cells from too_long_edges
* - Insert the new vertex in the hole obtained by removing the
*   conflicting cells (star-approach) --> _tds._insert_in_hole 
* - find out about offsets
* - Insert the newly added edges that are "too long"
*   to too_long_edges
* - reinsert hidden points
*/
template < class GT, class TDS >
template < class Conflict_tester, class Point_hider >
inline typename Periodic_3_triangulation_3<GT,TDS>::Vertex_handle
Periodic_3_triangulation_3<GT,TDS>::periodic_insert(
    const Point & p, const Offset& o,
    Locate_type lt, Cell_handle c, const Conflict_tester &tester,
    Point_hider &hider, Vertex_handle vh)
{
  Vertex_handle v;
  CGAL_triangulation_assertion(number_of_vertices() != 0);
  CGAL_triangulation_precondition_code(
      Locate_type lt_assert; int i_assert; int j_assert;);
  CGAL_triangulation_assertion(side_of_cell(tester.point(),o, c,
      lt_assert, i_assert, j_assert) != ON_UNBOUNDED_SIDE);

  tester.set_offset(o);

  // This only holds for Delaunay
  CGAL_triangulation_assertion(lt != VERTEX);

  // Choose the periodic copy of tester.point() that is inside c.
  Offset current_off = get_location_offset(tester, c);

  CGAL_triangulation_assertion(side_of_cell(tester.point(),
      combine_offsets(o,current_off),c,lt_assert,i_assert,j_assert)
      != ON_UNBOUNDED_SIDE);
  // If the new point is not in conflict with its cell, it is hidden.
  if (!tester.test_initial_cell(c, current_off)) {
    hider.hide_point(c,p);
    return Vertex_handle();
  }
  // Ok, we really insert the point now.
  // First, find the conflict region.
  std::vector<Cell_handle> cells;
  cells.reserve(32);

  Facet facet;

  find_conflicts(c, current_off, tester,
      make_triple(Oneset_iterator<Facet>(facet),
      std::back_inserter(cells),
      Emptyset_iterator()));

  // Remember the points that are hidden by the conflicting cells,
  // as they will be deleted during the insertion.
  hider.set_vertices(cells.begin(), cells.end());
  
  if (!is_1_cover())
    delete_too_long_edges(cells.begin(), cells.end());
  
  // Insertion. Attention: facets[0].first MUST be in conflict!
  // Compute the star and put it into the data structure.
  // Store the new cells from the star in nbs.
  v = _tds._insert_in_hole(cells.begin(), cells.end(),
      facet.first, facet.second);
  v->set_point(p);

  //TODO: this could be done within the _insert_in_hole without losing any
  //time because each cell is visited in any case.
  //- Do timings to argue to modify _insert_in_conflicts if need be
  //- Find the modified _insert_in_hole in the branch svn history of TDS
  std::vector<Cell_handle> nbs;
  incident_cells(v, std::back_inserter(nbs));
  // For all neighbors of the newly added vertex v: fetch their offsets from
  // the tester and reset them in the triangulation data structure.
  for (typename std::vector<Cell_handle>::iterator cit = nbs.begin();
      cit != nbs.end(); cit++) {
    Offset off[4];
    for (int i=0 ; i<4 ; i++) {
      off[i] = (*cit)->vertex(i)->offset();
    }
    set_offsets(*cit, off[0], off[1], off[2], off[3]);
  }
  
  for (typename std::vector<Vertex_handle>::iterator voit = v_offsets.begin();
      voit != v_offsets.end() ; ++voit) {
    (*voit)->clear_offset();
  }
  v_offsets.clear();

  if (vh != Vertex_handle()) {
    virtual_vertices[v] = Virtual_vertex(vh,o);
    virtual_vertices_reverse[vh].push_back(v);
  }

  if (!is_1_cover())
    insert_too_long_edges(v, nbs.begin(), nbs.end());

  // Store the hidden points in their new cells.
  hider.reinsert_vertices(v);
  return v;
}

/** Inserts the first point to a triangulation.
  *
  * With inserting the first point the 3-sheeted covering is constructed.
  * So first, the 27 vertices are inserted and are added to virtual_vertices
  * Then 6*27 cells are created.
  * Then all links are set.
 */
template < class GT, class TDS >
inline typename Periodic_3_triangulation_3<GT,TDS>::Vertex_handle
Periodic_3_triangulation_3<GT,TDS>::create_initial_triangulation(
    const Point &p) {
  /// Virtual vertices, one per periodic domain
  Vertex_handle vir_vertices[3][3][3];
  /// Virtual cells, 6 per periodic domain
  Cell_handle cells[3][3][3][6];

  // Initialise vertices:
  vir_vertices[0][0][0] = _tds.create_vertex();
  vir_vertices[0][0][0]->set_point(p);
  virtual_vertices_reverse[vir_vertices[0][0][0]] =
std::vector<Vertex_handle>();
  for (int i=0; i<_cover[0]; i++) {
    for (int j=0; j<_cover[1]; j++) {
      for (int k=0; k<_cover[2]; k++) {
        if ((i!=0)||(j!=0)||(k!=0)) {
          // Initialise virtual vertices out of the domain for debugging
          vir_vertices[i][j][k] =
            _tds.create_vertex();
          vir_vertices[i][j][k]->set_point(p); //+Offset(i,j,k));
          virtual_vertices[vir_vertices[i][j][k]] =
            Virtual_vertex(vir_vertices[0][0][0], Offset(i,j,k));
          virtual_vertices_reverse[vir_vertices[0][0][0]].push_back(
            vir_vertices[i][j][k]);
        }
        CGAL_triangulation_assertion(vir_vertices[i][j][k] != Vertex_handle());
        CGAL_triangulation_assertion(vir_vertices[0][0][0]->point() == p);
      }
    }
  }

  // Create cells:
  for (int i=0; i<_cover[0]; i++) {
    for (int j=0; j<_cover[1]; j++) {
      for (int k=0; k<_cover[2]; k++) {
        for (int l=0; l<6; l++) {
          // 6 cells per 'cube'
          cells[i][j][k][l] = _tds.create_cell();
          for (int n=0; n<4; n++)
            CGAL_triangulation_assertion(cells[i][j][k][l] != Cell_handle());
        }
      }
    }
  }
  // set vertex and neighbor information
  // index to the right vertex: [number of cells][vertex][offset]
  int vertex_ind[6][4][3] = {
    { {0, 0, 0},  {0, 1, 0},  {0, 0, 1},  {1, 0, 0} },
    { {1, 1, 0},  {0, 1, 1},  {1, 0, 1},  {1, 1, 1} },
    { {1, 0, 0},  {0, 1, 1},  {0, 1, 0},  {0, 0, 1} },
    { {1, 0, 0},  {0, 1, 1},  {0, 0, 1},  {1, 0, 1} },
    { {1, 0, 0},  {0, 1, 1},  {1, 0, 1},  {1, 1, 0} },
    { {1, 0, 0},  {0, 1, 1},  {1, 1, 0},  {0, 1, 0} }
  };
  int neighb_ind[6][4][4] = {
    { { 0, 0, 0, 2},  { 0,-1, 0, 5},  { 0, 0,-1, 3},  {-1, 0, 0, 4} },
    { { 0, 0, 1, 5},  { 1, 0, 0, 2},  { 0, 1, 0, 3},  { 0, 0, 0, 4} },
    { {-1, 0, 0, 1},  { 0, 0, 0, 0},  { 0, 0, 0, 3},  { 0, 0, 0, 5} },
    { { 0, 0, 1, 0},  { 0,-1, 0, 1},  { 0, 0, 0, 4},  { 0, 0, 0, 2} },
    { { 0, 0, 0, 1},  { 1, 0, 0, 0},  { 0, 0, 0, 5},  { 0, 0, 0, 3} },
    { { 0, 1, 0, 0},  { 0, 0,-1, 1},  { 0, 0, 0, 2},  { 0, 0, 0, 4} }
  };
  for (int i=0; i<_cover[0]; i++) {
    for (int j=0; j<_cover[1]; j++) {
      for (int k=0; k<_cover[2]; k++) {
        int offset = 
          (i==_cover[0]-1 ? 4 : 0) | 
	  (j==_cover[1]-1 ? 2 : 0) | 
	  (k==_cover[2]-1 ? 1 : 0);
        for (int l=0; l<6; l++) {
          // cell 0:
          cells[i][j][k][l]->set_vertices(
              vir_vertices
                [(i+vertex_ind[l][0][0])%_cover[0]]
                [(j+vertex_ind[l][0][1])%_cover[1]]
                [(k+vertex_ind[l][0][2])%_cover[2]],
              vir_vertices
                [(i+vertex_ind[l][1][0])%_cover[0]]
                [(j+vertex_ind[l][1][1])%_cover[1]]
                [(k+vertex_ind[l][1][2])%_cover[2]],
              vir_vertices
                [(i+vertex_ind[l][2][0])%_cover[0]]
                [(j+vertex_ind[l][2][1])%_cover[1]]
                [(k+vertex_ind[l][2][2])%_cover[2]],
              vir_vertices
                [(i+vertex_ind[l][3][0])%_cover[0]]
                [(j+vertex_ind[l][3][1])%_cover[1]]
                [(k+vertex_ind[l][3][2])%_cover[2]]);
          set_offsets(cells[i][j][k][l],
              offset & (vertex_ind[l][0][0]*4 +
                        vertex_ind[l][0][1]*2 +
                        vertex_ind[l][0][2]*1),
              offset & (vertex_ind[l][1][0]*4 +
                        vertex_ind[l][1][1]*2 +
                        vertex_ind[l][1][2]*1),
              offset & (vertex_ind[l][2][0]*4 +
                        vertex_ind[l][2][1]*2 +
                        vertex_ind[l][2][2]*1),
              offset & (vertex_ind[l][3][0]*4 +
                        vertex_ind[l][3][1]*2 +
                        vertex_ind[l][3][2]*1));
          cells[i][j][k][l]->set_neighbors(
              cells [(i+_cover[0]+neighb_ind[l][0][0])%_cover[0]]
                    [(j+_cover[1]+neighb_ind[l][0][1])%_cover[1]]
                    [(k+_cover[2]+neighb_ind[l][0][2])%_cover[2]]
                    [         neighb_ind[l][0][3]       ],
              cells [(i+_cover[0]+neighb_ind[l][1][0])%_cover[0]]
                    [(j+_cover[1]+neighb_ind[l][1][1])%_cover[1]]
                    [(k+_cover[2]+neighb_ind[l][1][2])%_cover[2]]
                    [         neighb_ind[l][1][3]       ],
              cells [(i+_cover[0]+neighb_ind[l][2][0])%_cover[0]]
                    [(j+_cover[1]+neighb_ind[l][2][1])%_cover[1]]
                    [(k+_cover[2]+neighb_ind[l][2][2])%_cover[2]]
                    [         neighb_ind[l][2][3]       ],
              cells [(i+_cover[0]+neighb_ind[l][3][0])%_cover[0]]
                    [(j+_cover[1]+neighb_ind[l][3][1])%_cover[1]]
                    [(k+_cover[2]+neighb_ind[l][3][2])%_cover[2]]
                    [         neighb_ind[l][3][3]       ]
          );
        }
      }
    }
  }
  // set pointers from the vertices to incident cells.
  for (int i=0; i<_cover[0]; i++) {
    for (int j=0; j<_cover[1]; j++) {
      for (int k=0; k<_cover[2]; k++) {
        vir_vertices[i][j][k]->set_cell(cells[i][j][k][0]);
      }
    }
  }
  
  _tds.set_dimension(3);

  // create the base for too_long_edges;
  CGAL_triangulation_assertion( too_long_edges.empty() );
  CGAL_triangulation_assertion(too_long_edge_counter == 0);

  for (Vertex_iterator vit = vertices_begin() ;
       vit !=vertices_end() ; ++vit )
    too_long_edges[vit] = std::list<Vertex_handle>();;

  std::vector<Cell_handle> temp_inc_cells;
  for (Vertex_iterator vit = vertices_begin() ;
       vit !=vertices_end() ; ++vit ) {
    temp_inc_cells.clear();
    incident_cells(vit, std::back_inserter(temp_inc_cells));
    for (unsigned int i=0 ; i<temp_inc_cells.size() ; i++) {
      int k = temp_inc_cells[i]->index(vit);
      for (int j=0; j<4 ; j++) {
        if (j==k) continue;
        if (&*vit > &*(temp_inc_cells[i]->vertex(j))) continue;
        if ((find(too_long_edges[vit].begin(),
                  too_long_edges[vit].end(),
            temp_inc_cells[i]->vertex(j)) ==
		too_long_edges[vit].end())
        ){
	  too_long_edges[vit].push_back(temp_inc_cells[i]->vertex(j));
          too_long_edge_counter++;
        }
      }
    }
  }
  return vir_vertices[0][0][0];
}

#include <CGAL/Periodic_3_triangulation_dummy_36.h>

/** finds all cells that are in conflict with the currently added point
  * (stored in tester).
  *
  * The result will be a hole of which the following data is returned:
  * - boundary facets
  * - cells
  * - internal facets.
  *
  * c is the current cell, which must be in conflict.
  * tester is the function object that tests if a cell is in conflict.
  */
template <class GT, class TDS>
template <class Conflict_test,
	  class OutputIteratorBoundaryFacets,
          class OutputIteratorCells,
	  class OutputIteratorInternalFacets>
Triple<OutputIteratorBoundaryFacets,
       OutputIteratorCells,
       OutputIteratorInternalFacets>
Periodic_3_triangulation_3<GT,TDS>::
find_conflicts(Cell_handle d, const Offset &current_off,
    const Conflict_test &tester,
    Triple<OutputIteratorBoundaryFacets,
    OutputIteratorCells,
    OutputIteratorInternalFacets> it) const {
  CGAL_triangulation_precondition( number_of_vertices() != 0 );
  CGAL_triangulation_precondition( tester(d, current_off) );

  std::stack<std::pair<Cell_handle, Offset> > cell_stack;
  cell_stack.push(std::make_pair(d,current_off));
  d->tds_data().mark_in_conflict();  
  *it.second++ = d;

  do {
    Cell_handle c = cell_stack.top().first;
    Offset current_off2 = cell_stack.top().second;
    cell_stack.pop();

    for (int i=0; i< 4; ++i) {
      Cell_handle test = c->neighbor(i);
      if (test->tds_data().is_in_conflict()) {
	if (c < test) {
	  *it.third++ = Facet(c, i); // Internal facet.
	}
	continue; // test was already in conflict.
      }
      if (test->tds_data().is_clear()) {
	Offset o_test = current_off2 + get_neighbor_offset(c, i, test);
	if (tester(test,o_test)) {
	  if (c < test)
	    *it.third++ = Facet(c, i); // Internal facet.
	  
	  cell_stack.push(std::make_pair(test,o_test));
	  test->tds_data().mark_in_conflict();
	  *it.second++ = test;
	  continue;
	}
	test->tds_data().mark_on_boundary(); // test is on the boundary.
      }
      *it.first++ = Facet(c, i);
      for (int j = 0 ; j<4 ; j++){
	if (j==i) continue;
	if (!c->vertex(j)->get_offset_flag()) {
	  c->vertex(j)->set_offset(int_to_off(c->offset(j))-current_off2);
	  v_offsets.push_back(c->vertex(j));
	}
      }
    }
  } while (!cell_stack.empty());
  return it;
}

/*! \brief Insert point into triangulation.
 *
 * Inserts the point p into the triangulation. It expects
 * - a cell to start the point location
 * - a testing function to determine cells in conflict
 * - a testing function to determine if a vertex is hidden.
 *
 * Implementation:
 * - If the triangulation is empty call a special function
 * (create_initial_triangulation) to construct the basic
 * triangulation.
 * - Run point location to get the cell c containing point p.
 * - Call periodic_insert to insert p into the 3-cover.
 * - Also insert the eight periodic copies of p.
 */
template < class GT, class TDS >
template < class Conflict_tester, class Point_hider >
inline typename Periodic_3_triangulation_3<GT,TDS>::Vertex_handle
Periodic_3_triangulation_3<GT,TDS>::insert_in_conflict(const Point & p,
    Locate_type lt, Cell_handle c, int li, int lj,
    const Conflict_tester &tester, Point_hider &hider) {

  CGAL_triangulation_assertion((_domain.xmin() <= p.x())
      && (p.x() < _domain.xmax()));
  CGAL_triangulation_assertion((_domain.ymin() <= p.y())
      && (p.y() < _domain.ymax()));
  CGAL_triangulation_assertion((_domain.zmin() <= p.z()) 
      && (p.z() < _domain.zmax()));

  if (number_of_vertices() == 0) {
    return create_initial_triangulation(p);
  }

  if ((lt == VERTEX) &&
      (tester.compare_weight(c->vertex(li)->point(),p)==0) ) {
    return c->vertex(li);
  }

  Vertex_handle vstart;
  if (!is_1_cover()) {
    Virtual_vertex_map_it vvmit = virtual_vertices.find(c->vertex(0));
    if (vvmit == virtual_vertices.end())
      vstart = c->vertex(0);
    else
      vstart = vvmit->second.first;
    CGAL_triangulation_assertion(virtual_vertices.find(vstart)
	==virtual_vertices.end());
    CGAL_triangulation_assertion(virtual_vertices_reverse.find(vstart)
        != virtual_vertices_reverse.end());
  }
  CGAL_triangulation_assertion( number_of_vertices() != 0 );
  CGAL_triangulation_expensive_assertion(is_valid());
  Vertex_handle vh = periodic_insert(p, Offset(), lt, c, tester, hider);
  if (is_1_cover()) {
    return vh;
  }
  
  for (Cell_iterator it = all_cells_begin() ;
      it != all_cells_end() ; it++){
    CGAL_triangulation_assertion(it->neighbor(0)->neighbor(
        it->neighbor(0)->index(it))==it);
    CGAL_triangulation_assertion(it->neighbor(1)->neighbor(
        it->neighbor(1)->index(it))==it);
    CGAL_triangulation_assertion(it->neighbor(2)->neighbor(
        it->neighbor(2)->index(it))==it);
    CGAL_triangulation_assertion(it->neighbor(3)->neighbor(
        it->neighbor(3)->index(it))==it);
  }

  std::vector<Vertex_handle> start_vertices
      = virtual_vertices_reverse.find(vstart)->second;
  Cell_handle start;
  virtual_vertices_reverse[vh] = std::vector<Vertex_handle>();
  // insert 26 periodic copies
  for (int i=0; i<_cover[0]; i++) {
    for (int j=0; j<_cover[1]; j++) {
      for (int k=0; k<_cover[2]; k++) {
        if ((i!=0)||(j!=0)||(k!=0)) {
          start = start_vertices[i*9+j*3+k-1]->cell();
          c = periodic_locate(p, Offset(i,j,k), lt, li, lj, start);
          periodic_insert(p, Offset(i,j,k), lt, c, tester, hider,vh);
        }
      }
    }
  }
  CGAL_triangulation_expensive_assertion(is_valid());

  // Fall back to 1-cover if the criterion that the longest edge is shorter
  // than sqrt(0.166) is fulfilled.
  if ( too_long_edge_counter == 0 ) {
    CGAL_triangulation_expensive_assertion(is_valid());
    convert_to_1_sheeted_covering();
    CGAL_triangulation_expensive_assertion( is_valid() );
  }
  return vh;
}

/// tests if two vertices of one cell are just periodic copies of each other
template < class GT, class TDS >
inline bool Periodic_3_triangulation_3<GT,TDS>::has_self_edges(Cell_handle c) const {
  CGAL_triangulation_assertion((c->vertex(0) != c->vertex(1)) || 
      (c->offset(0) != c->offset(1)));
  CGAL_triangulation_assertion((c->vertex(0) != c->vertex(2)) || 
      (c->offset(0) != c->offset(2)));
  CGAL_triangulation_assertion((c->vertex(0) != c->vertex(3)) || 
      (c->offset(0) != c->offset(3)));
  CGAL_triangulation_assertion((c->vertex(1) != c->vertex(2)) || 
      (c->offset(1) != c->offset(2)));
  CGAL_triangulation_assertion((c->vertex(1) != c->vertex(3)) || 
      (c->offset(1) != c->offset(3)));
  CGAL_triangulation_assertion((c->vertex(2) != c->vertex(3)) || 
      (c->offset(2) != c->offset(3)));
  return ((c->vertex(0) == c->vertex(1)) ||
      (c->vertex(0) == c->vertex(2)) ||
      (c->vertex(0) == c->vertex(3)) ||
      (c->vertex(1) == c->vertex(2)) ||
      (c->vertex(1) == c->vertex(3)) ||
      (c->vertex(2) == c->vertex(3)));
}

/*! \brief Tests if the triangulation is valid.
 *
 * A triangulation is valid if
 * - A cell is not its own neighbor.
 * - A cell has no two equal neighbors
 * - A cell has no two equal vertex-offset pairs
 * - A cell is positively oriented.
 * - The point of a neighbor of cell c that does not belong to c is not inside
 *   the circumcircle of c.
 */
template < class GT, class TDS >
bool
Periodic_3_triangulation_3<GT,TDS>::
is_valid(bool verbose, int level) const {
  bool error = false;
  for (Cell_iterator cit = cells_begin();
       cit != cells_end(); ++cit) {
    for (int i=0; i<4; i++) {
      CGAL_triangulation_assertion(cit != cit->neighbor(i));
      for (int j=i+1; j<4; j++) {
        CGAL_triangulation_assertion(cit->neighbor(i) != cit->neighbor(j));
        CGAL_triangulation_assertion(cit->vertex(i) != cit->vertex(j));
      }
    }
    // Check positive orientation:
    const Point *p[4]; Offset off[4];
    for (int i=0; i<4; i++) {
      p[i] = &cit->vertex(i)->point();
      off[i] = get_offset(cit,i);
    }
    if (orientation(*p[0], *p[1], *p[2], *p[3],
                   off[0], off[1], off[2], off[3]) != POSITIVE) {
      if (verbose) {
	std::cerr<<"Periodic_3_triangulation_3: wrong orientation:"<<std::endl;
	std::cerr<<off[0]<<'\t'<<*p[0]<<'\n'
		 <<off[1]<<'\t'<<*p[1]<<'\n'
		 <<off[2]<<'\t'<<*p[2]<<'\n'
		 <<off[3]<<'\t'<<*p[3]<<std::endl;
      }
      error = true;
    }
  }

  if (!has_self_edges()) {
    if (! _tds.is_valid(verbose, level) ) {
      return false;
    }
  }

  return !error;
}

template < class GT, class TDS >
bool Periodic_3_triangulation_3<GT,TDS>::is_valid(Cell_handle ch,
    bool verbose, int level) const {
  if ( ! _tds.is_valid(ch,verbose,level) )
    return false;
  bool error = false;
  const Point *p[4]; Offset off[4];
  for (int i=0; i<4; i++) {
    p[i] = &ch->vertex(i)->point();
    off[i] = get_offset(ch,i);
  }
  if (orientation(*p[0], *p[1], *p[2], *p[3], 
		  off[0], off[1], off[2], off[3]) != POSITIVE) {
    error = true;
  }
  
  return !error;
}

template < class GT, class TDS >
template < class ConflictTester >
bool Periodic_3_triangulation_3<GT,TDS>::
is_valid_conflict(ConflictTester &tester, bool verbose, int level) const {
  Cell_iterator it;
  for ( it = cells_begin(); it != cells_end(); ++it ) {
    is_valid(it, verbose, level);
    for (int i=0; i<4; i++ ) {
      Offset o_nb = get_neighbor_offset(it,i,it->neighbor(i));
      Offset o_vt = get_offset(it->neighbor(i),
				      it->neighbor(i)->index(it));
      if (tester(it,
		 it->neighbor(i)->vertex(it->neighbor(i)->index(it))->point(),
		 o_vt-o_nb)) {
        if (verbose)
          std::cerr << "non-empty sphere: "
              <<it->vertex(0)->point()<<'\t'
              <<it->vertex(1)->point()<<'\t'
              <<it->vertex(2)->point()<<'\t'
              <<it->vertex(3)->point()<<'\n'
	      <<it->neighbor(i)->vertex(it->neighbor(i)->index(it))->point()
              <<'\t'<<o_vt-o_nb
              << std::endl;
        return false;
      }
    }
  }
  return true;
}

template < class GT, class TDS >
inline void Periodic_3_triangulation_3<GT,TDS>::make_hole(Vertex_handle v, 
    std::map<Vertex_triple,Facet> &outer_map, std::vector<Cell_handle> &hole) {
  
  //CGAL_triangulation_precondition( all_vertices_begin()++ 
  //    != all_vertices_end() );
  
  incident_cells(v, std::back_inserter(hole));

  for (typename std::vector<Cell_handle>::iterator cit = hole.begin();
       cit != hole.end(); ++cit) {
    int indv = (*cit)->index(v);
    Cell_handle opp_cit = (*cit)->neighbor( indv );
    Facet f(opp_cit, opp_cit->index(*cit)); 
    Vertex_triple vt = make_vertex_triple(f);
    make_canonical(vt);
    outer_map[vt] = f;
    for (int i=0; i<4; i++)
      if ( i != indv )
        (*cit)->vertex(i)->set_cell(opp_cit);
  }
}

/*! \brief Remove a vertex from the triangulation.
 *
 * Removes vertex v from the triangulation.
 */
template < class GT, class TDS >
template < class PointRemover, class Conflict_tester>
inline void Periodic_3_triangulation_3<GT,TDS>::remove(Vertex_handle v,
    PointRemover &r, Conflict_tester &t) {
  CGAL_expensive_precondition(is_vertex(v));
  std::vector<Vertex_handle> vhrem;
  if (!is_1_cover()) {
    if (number_of_vertices() == 1) {
      clear();
      return;
    }
    Virtual_vertex_map_it vvmit = virtual_vertices.find(v);
    if (vvmit != virtual_vertices.end()) v = vvmit->second.first;
    CGAL_triangulation_assertion(virtual_vertices_reverse.find(v)
        != virtual_vertices_reverse.end());
    vhrem = virtual_vertices_reverse.find(v)->second;
    virtual_vertices_reverse.erase(v);
    CGAL_triangulation_assertion(vhrem.size()==26);
    for (int i=0 ; i<26 ; i++) {
      periodic_remove(vhrem[i],r);
      virtual_vertices.erase(vhrem[i]);
      CGAL_triangulation_expensive_assertion(is_valid());
    }
    periodic_remove(v,r);
  } else {
    periodic_remove(v,r);
    if (!is_1_cover()) remove(v,r,t);
  }
  
}

/*! \brief Remove a vertex from the triangulation.
 *
 * Removes vertex v from the triangulation.
 * It expects a reference to an instance of a PointRemover.
 * 
 * Implementation:
 * - Compute the hole, that is, all cells incident to v. Cells outside of
 *   this hole are not affected by the deletion of v.
 * - Triangulate the hole. This is done computing the triangulation
 *   in Euclidean space for the points on the border of the hole.
 * - Sew this triangulation into the hole.
 * - Test for all newly added edges, whether they are shorter than the
 *   edge_length_threshold. If not, convert to 3-cover.
 */
template < class GT, class TDS >
template < class PointRemover >
inline void Periodic_3_triangulation_3<GT,TDS>::periodic_remove(Vertex_handle v,
    PointRemover &remover) {

  // Construct the set of vertex triples on the boundary
  // with the facet just behind
  typedef std::map<Vertex_triple,Facet> Vertex_triple_Facet_map;
  typedef PointRemover Point_remover;
  typedef typename Point_remover::TDSE            TDSE;
  typedef typename Point_remover::CellE_handle    CellE_handle;
  typedef typename Point_remover::VertexE_handle  VertexE_handle;
  typedef typename Point_remover::FacetE          FacetE;
  typedef typename Point_remover::VertexE_triple  VertexE_triple;
  typedef typename Point_remover::Finite_cellsE_iterator
      Finite_cellsE_iterator;
  typedef typename Point_remover::Vertex_triple_FacetE_map
      Vertex_triple_FacetE_map;
  typedef typename Point_remover::Vertex_triple_FacetE_map_it
      Vertex_triple_FacetE_map_it;

  // First compute the hole and its boundary vertices.
  std::vector<Cell_handle> hole;
  hole.reserve(64);
  Vertex_triple_Facet_map outer_map;
  Vertex_triple_FacetE_map inner_map;

  make_hole(v, outer_map, hole);

  CGAL_triangulation_assertion(outer_map.size()==hole.size());
  CGAL_triangulation_assertion(remover.hidden_points_begin() == 
      remover.hidden_points_end());

  if (!is_1_cover()) {
    delete_too_long_edges(hole.begin(), hole.end());
  }
  
  // Output the hidden points.
  for (typename std::vector<Cell_handle>::iterator
      hi = hole.begin(), hend = hole.end(); hi != hend; ++hi)
  {
    remover.add_hidden_points(*hi);
  }

  // Build up the map between Vertices on the boundary and offsets
  // collect all vertices on the boundary
  std::vector<Vertex_handle> vertices;
  vertices.reserve(64);

  // The set is needed to ensure that each vertex is inserted only once.
  std::set<Vertex_handle> tmp_vertices;
  // The map connects vertices to offsets in the hole
  std::map<Vertex_handle, Offset> vh_off_map;

  for(typename std::vector<Cell_handle>::iterator cit = hole.begin();
      cit != hole.end(); ++cit)
  {
    // Put all incident vertices in tmp_vertices.
    for (int j=0; j<4; ++j){
      if ((*cit)->vertex(j) != v){
        tmp_vertices.insert((*cit)->vertex(j));
        vh_off_map[(*cit)->vertex(j)] = int_to_off((*cit)->offset(j))
            - int_to_off((*cit)->offset((*cit)->index(v)));
      }
    }
  }

  // Now output the vertices.
  std::copy(tmp_vertices.begin(), tmp_vertices.end(),
      std::back_inserter(vertices));

  // create a Delaunay/Regular triangulation of the points on the boundary
  // in Euclidean space and make a map from the vertices in remover.tmp
  // towards the vertices in *this
  
  Unique_hash_map<VertexE_handle,Vertex_handle> vmap;
  CellE_handle ch;
  remover.tmp.clear();
  
  for(unsigned int i=0; i < vertices.size(); i++){
    typedef typename Point_remover::Triangulation_R3::Point TRPoint;
    CGAL_triangulation_assertion(get_offset(vertices[i])
	+ combine_offsets(Offset(), vh_off_map[vertices[i]])
	== combine_offsets(get_offset(vertices[i]),vh_off_map[vertices[i]]));
    TRPoint trp = std::make_pair(vertices[i]->point(),
	combine_offsets( get_offset(vertices[i]), vh_off_map[vertices[i]]) );
    VertexE_handle vh = remover.tmp.insert(trp, ch);
    vmap[vh] = vertices[i];
    CGAL_triangulation_assertion(vmap.is_defined(vh));
  }
  CGAL_triangulation_assertion(remover.tmp.number_of_vertices() != 0);

  // Construct the set of vertex triples of tmp
  // We reorient the vertex triple so that it matches those from outer_map
  // Also note that we use the vertices of *this, not of tmp
  for(Finite_cellsE_iterator it = remover.tmp.finite_cells_begin();
      it != remover.tmp.finite_cells_end();
      ++it){
    VertexE_triple vt_aux;
    for(int i=0; i < 4; i++){
      FacetE f = std::pair<CellE_handle,int>(it,i);
      vt_aux = VertexE_triple(
          f.first->vertex(vertex_triple_index(f.second,0)),
          f.first->vertex(vertex_triple_index(f.second,1)),
          f.first->vertex(vertex_triple_index(f.second,2)));
      if (vmap.is_defined(vt_aux.first)
          && vmap.is_defined(vt_aux.second)
          && vmap.is_defined(vt_aux.third) ) {
        Vertex_triple vt(vmap[vt_aux.first],vmap[vt_aux.third],
            vmap[vt_aux.second]);
        make_canonical(vt);
        inner_map[vt]= f;
      }
    }
  }

  // A structure for storing the new neighboring relations
  typedef boost::tuple<Cell_handle, int, Cell_handle> Neighbor_relation;
  std::vector<Neighbor_relation> nr_vec;
  std::vector<Cell_handle> new_cells;

  // Grow inside the hole, by extending the surface
  while(! outer_map.empty()){
    typename Vertex_triple_Facet_map::iterator oit = outer_map.begin();
    
    typename Vertex_triple_Facet_map::value_type o_vt_f_pair = *oit;
    Cell_handle o_ch = o_vt_f_pair.second.first;
    unsigned int o_i = o_vt_f_pair.second.second;
    
    typename Vertex_triple_FacetE_map::iterator iit =
        inner_map.find(o_vt_f_pair.first);
    
    CGAL_triangulation_assertion(iit != inner_map.end());
    typename Vertex_triple_FacetE_map::value_type i_vt_f_pair = *iit;
    CellE_handle i_ch = i_vt_f_pair.second.first;
    unsigned int i_i = i_vt_f_pair.second.second;
    
    // create a new cell to glue to the outer surface
    Cell_handle new_ch = _tds.create_cell();
    new_cells.push_back(new_ch);
    new_ch->set_vertices(vmap[i_ch->vertex(0)], vmap[i_ch->vertex(1)],
                        vmap[i_ch->vertex(2)], vmap[i_ch->vertex(3)]);
    set_offsets(new_ch, vh_off_map[vmap[i_ch->vertex(0)]],
                        vh_off_map[vmap[i_ch->vertex(1)]],
                        vh_off_map[vmap[i_ch->vertex(2)]],
                        vh_off_map[vmap[i_ch->vertex(3)]]);
    
    // Update the edge length management
    for( int i=0 ; i < 4 ; i++ ) {
      for (int j=0 ; j < 4 ; j++) {
        if (j==i) continue;
        if (&*(new_ch->vertex(i)) > &*(new_ch->vertex(j))) continue;

	Point p1 = construct_point(new_ch->vertex(i)->point(),
	    get_offset(new_ch, i));
	Point p2 = construct_point(new_ch->vertex(j)->point(),
	    get_offset(new_ch, j));
        Vertex_handle v_no = new_ch->vertex(i);

        if (squared_distance(p1,p2) > edge_length_threshold) {
	  // If the cell does not fulfill the edge-length criterion
	  // revert all changes to the triangulation and transform it
	  // to a triangulation in the needed covering space.
          if (is_1_cover()) {
	    _tds.delete_cells(new_cells.begin(), new_cells.end());
	    convert_to_27_sheeted_covering();
            return;
          }
          else if (find(too_long_edges[v_no].begin(),
			too_long_edges[v_no].end(),
			new_ch->vertex(j))
		   == too_long_edges[v_no].end()) {
            too_long_edges[v_no].push_back(new_ch->vertex(j));
            too_long_edge_counter++;
          }
        }
      }
    }

    // The neighboring relation needs to be stored temporarily in
    // nr_vec. It cannot be applied directly because then we could not
    // easily cancel the removing process if a cell is encountered
    // that does not obey the edge-length criterion.
    nr_vec.push_back(boost::make_tuple(o_ch,o_i,new_ch));
    nr_vec.push_back(boost::make_tuple(new_ch,i_i,o_ch));

    // for the other faces check, if they can also be glued
    for(unsigned int i = 0; i < 4; i++){
      if(i != i_i){
	Facet f = std::pair<Cell_handle,int>(new_ch,i);
        Vertex_triple vt = make_vertex_triple(f);
        make_canonical(vt);
        std::swap(vt.second,vt.third);
        typename Vertex_triple_Facet_map::iterator oit2 = outer_map.find(vt);
        if(oit2 == outer_map.end()){
          std::swap(vt.second,vt.third);
          outer_map[vt]= f;
        } else {
        // glue the faces
          typename Vertex_triple_Facet_map::value_type o_vt_f_pair2 = *oit2;
          Cell_handle o_ch2 = o_vt_f_pair2.second.first;
          int o_i2 = o_vt_f_pair2.second.second;
	  nr_vec.push_back(boost::make_tuple(o_ch2,o_i2,new_ch));
	  nr_vec.push_back(boost::make_tuple(new_ch,i,o_ch2));
          outer_map.erase(oit2);
        }
      }
    }
    outer_map.erase(oit);
  }

  // finally set the neighboring relations
  for (unsigned int i=0 ; i<nr_vec.size() ; i++) {
    nr_vec[i].template get<0>()->set_neighbor(nr_vec[i].template get<1>(),nr_vec[i].template get<2>());
  }
  
  _tds.delete_vertex(v);
  _tds.delete_cells(hole.begin(), hole.end());
  CGAL_triangulation_expensive_assertion(is_valid());
}

// ############################################################################
// ############################################################################
// ############################################################################
template < class GT, class TDS >
template < class Cmp >
class Periodic_3_triangulation_3<GT, TDS>::Perturbation_order {

  typedef GT Geometric_traits;
  typedef typename Geometric_traits::Point_3 Point;
  typedef typename Geometric_traits::Compare_xyz_3 Compare_xyz_3;
  typedef typename Periodic_3_triangulation_3<GT, TDS>::Periodic_point
      Periodic_point;
  
  Cmp _cmp;

public:
  Perturbation_order(const Cmp & cmp) : _cmp(cmp) {}

  bool operator()(const Periodic_point *p, const Periodic_point *q) const {
    return (_cmp(p->first, q->first, p->second, q->second) == SMALLER);
  }
};

// ############################################################################
// ############################################################################
// ############################################################################

/** \brief Delete each redundant cell and the not anymore needed data
 *  structures.
 * 
 *  This function consists of four iterations over all cells and one
 *  iteration over all vertices:
 *  1. cell iteration: mark all cells that are to delete
 *  2. cell iteration: redirect neighbors of remaining cells
 *  3. cell iteration: redirect vertices of remaining cells
 *  4. cell iteration: delete all cells marked in the 1. iteration
 *  Vertex iteration: delete all vertices outside the original domain.
 */
template < class GT, class TDS >
inline void 
Periodic_3_triangulation_3<GT,TDS>::convert_to_1_sheeted_covering() {
  // ###################################################################
  // ### First cell iteration ##########################################
  // ###################################################################
  {
    if (is_1_cover()) return;
    bool to_delete, has_simplifiable_offset;
    Virtual_vertex_map_it vvmit;
    // First iteration over all cells: Mark the cells that are to delete.
    // Cells are to delete if they cannot be translated anymore in the
    // direction of one of the axes without yielding negative offsets.
    for( Cell_iterator it = all_cells_begin() ;
    it != all_cells_end() ; ++it ) {
      to_delete = false;
      // for all directions in 3D Space
      for( int j=0 ; j<3 ; j++ ) {
        has_simplifiable_offset = true;
        // for all vertices of cell it
        for( int i=0 ; i<4 ; i++ ) {
          vvmit = virtual_vertices.find(it->vertex(i));
          // if it->vertex(i) lies inside the original domain:
          if (vvmit == virtual_vertices.end()) {
            // the cell cannot be moved any more because if we did, then
            // it->vertex(i) will get at least one negative offset.
            has_simplifiable_offset = false;
            // if it->vertex(i) lies outside the original domain:
          } else {
            // The cell can certainly be deleted if the offset contains a 2
            to_delete = to_delete
                || (vvmit->second.second[j] == 2) ;
            // The cell can be moved into one direction only if the offset of
            // all for vertices is >=1 for this direction. Since we already
            // tested for 2 it is sufficient to test here for 1.
            has_simplifiable_offset = has_simplifiable_offset
                && (vvmit->second.second[j] == 1) ;
          }
        } 
        // if the offset can be simplified, i.e. the cell can be moved, then
        // it can be deleted.
        if (has_simplifiable_offset)
          to_delete = true;
      }
      // Mark all cells that are to delete. They cannot be deleted yet,
      // because neighboring information still needs to be extracted.
      if (to_delete) {
        it->set_additional_flag(1);
      }
    }
  }

  // ###################################################################
  // ### Second cell iteration #########################################
  // ###################################################################
  {
    Vertex_handle vert[4], nbv[4];
    Offset off[4];
    Cell_handle nb, new_neighbor;
    std::vector<Triple<Cell_handle, int, Cell_handle> > new_neighbor_relations;

    // Second iteration over all cells: redirect neighbors where necessary
    for (Cell_iterator it = all_cells_begin() ;
        it != all_cells_end() ; ++it) {
      // Skip all cells that are to delete.
      if (it->get_additional_flag() == 1) continue;
      
      // Redirect neighbors: Only neighbors that are marked by the
      // additional_flag have to be substituted by one of their periodic
      // copies. The unmarked neighbors stay the same.
      for ( int i = 0 ; i < 4 ; i++ ) {
        if ( it->neighbor(i)->get_additional_flag() != 1 ) continue;
        
        nb = it->neighbor(i);
        
        for ( int j = 0 ; j < 4 ; j++ ) {
          off[j] = Offset();
          get_vertex( nb, j, vert[j], off[j]);
        }
        int x,y,z;
        x = (std::min) ( (std::min) ( off[0][0], off[1][0] ),
            (std::min) ( off[2][0], off[3][0] ) );
        y = (std::min) ( (std::min) ( off[0][1], off[1][1] ),
            (std::min) ( off[2][1], off[3][1] ) );
        z = (std::min) ( (std::min) ( off[0][2], off[1][2] ),
            (std::min) ( off[2][2], off[3][2] ) );
        
        // The vector from nb to the "original" periodic copy of nb, that is
        // the copy that will not be deleted.
        Offset difference_offset(x,y,z);
        CGAL_triangulation_assertion( !difference_offset.is_null() );
        
        // We now have to find the "original" periodic copy of nb from
        // its vertices. Therefore, we first have to find the vertices.
        for ( int j = 0 ; j < 4 ; j++ ) {
          CGAL_triangulation_assertion( (off[j]-difference_offset)[0] >= 0);
          CGAL_triangulation_assertion( (off[j]-difference_offset)[1] >= 0);
          CGAL_triangulation_assertion( (off[j]-difference_offset)[2] >= 0);
          CGAL_triangulation_assertion( (off[j]-difference_offset)[0] < 3);
          CGAL_triangulation_assertion( (off[j]-difference_offset)[1] < 3);
          CGAL_triangulation_assertion( (off[j]-difference_offset)[2] < 3);
          
          // find the Vertex_handles of the vertices of the "original"
          // periodic copy of nb. If the vertex is inside the original
          // domain, there is nothing to do
          if ( (off[j]-difference_offset).is_null() ) {
            nbv[j] = vert[j];
            // If the vertex is outside the original domain, we have to search
            // in virtual_vertices in the "wrong" direction. That means, we
            // cannot use virtual_vertices.find but have to use
            // virtual_vertices_reverse.
          } else {
            Offset nbo = off[j]-difference_offset;
            nbv[j] = virtual_vertices_reverse.find(vert[j])
              ->second[nbo[0]*9+nbo[1]*3+nbo[2]-1];
          }
        }
        // Find the new neighbor by its 4 vertices
        new_neighbor = get_cell( nbv );
        
        // Store the new neighbor relation. This cannot be applied yet because
        // it would disturb the functioning of get_cell( ... )
        new_neighbor_relations.push_back(make_triple(it, i, new_neighbor));
      }
    }
    // Apply the new neighbor relations now.
    for (unsigned int i=0 ; i<new_neighbor_relations.size() ; i++){
      new_neighbor_relations[i].first->set_neighbor(
          new_neighbor_relations[i].second,
          new_neighbor_relations[i].third);
    }
  }
  
  // ###################################################################
  // ### Third cell iteration ##########################################
  // ###################################################################
  {
    Vertex_handle vert[4];
    Offset off[4];
    // Third iteration over all cells: redirect vertices where necessary
    for (Cell_iterator it = all_cells_begin() ;
        it != all_cells_end() ; ++it) {
      // Skip all cells that are marked to delete
      if (it->get_additional_flag() == 1) continue;
      // Find the corresponding vertices of it in the original domain
      // and set them as new vertices of it.
      for ( int i = 0 ; i < 4 ; i++ ) {
        off[i] = Offset();
        get_vertex( it, i, vert[i], off[i]);
        it->set_vertex( i, vert[i]);
        CGAL_triangulation_assertion(vert[i]->point()[0] < _domain.xmax());
        CGAL_triangulation_assertion(vert[i]->point()[1] < _domain.ymax());
        CGAL_triangulation_assertion(vert[i]->point()[2] < _domain.zmax());
        CGAL_triangulation_assertion(vert[i]->point()[0] >= _domain.xmin());
        CGAL_triangulation_assertion(vert[i]->point()[1] >= _domain.ymin());
        CGAL_triangulation_assertion(vert[i]->point()[2] >= _domain.zmin());
        
        // redirect also the cell pointer of the vertex.
        it->vertex(i)->set_cell(it);
      }
      // Set the offsets.
      set_offsets(it, off[0], off[1], off[2], off[3] );
      CGAL_triangulation_assertion( int_to_off(it->offset(0)) == off[0] );
      CGAL_triangulation_assertion( int_to_off(it->offset(1)) == off[1] );
      CGAL_triangulation_assertion( int_to_off(it->offset(2)) == off[2] );
      CGAL_triangulation_assertion( int_to_off(it->offset(3)) == off[3] );
    }
  }
  
  // ###################################################################
  // ### Fourth cell iteration #########################################
  // ###################################################################
  {
    // Delete the marked cells.
    std::vector<Cell_handle> cells_to_delete;
    for ( Cell_iterator cit = all_cells_begin() ;
    cit != all_cells_end() ; ++cit ) {
      if ( cit->get_additional_flag() == 1 )
        cells_to_delete.push_back( cit );
    }
    _tds.delete_cells(cells_to_delete.begin(), cells_to_delete.end());
  }
  
  // ###################################################################
  // ### Vertex iteration ##############################################
  // ###################################################################
  {
    // Delete all the vertices in virtual_vertices, that is all vertices
    // outside the original domain.
    std::vector<Vertex_handle> vertices_to_delete;
    for ( Vertex_iterator vit = all_vertices_begin() ;
	  vit != all_vertices_end() ; ++vit ) {
      if ( virtual_vertices.count( vit ) != 0 ) {
        CGAL_triangulation_assertion( virtual_vertices.count( vit ) == 1 );
        vertices_to_delete.push_back( vit ) ;
      }
    }
    _tds.delete_vertices(vertices_to_delete.begin(), vertices_to_delete.end());
  }
  _cover = make_array(1,1,1);
  virtual_vertices.clear();
  virtual_vertices_reverse.clear();
}

template < class GT, class TDS >
inline void
Periodic_3_triangulation_3<GT,TDS>::convert_to_27_sheeted_covering() {
  if (_cover == make_array(3,3,3)) return;
  CGAL_triangulation_precondition(is_1_cover());

  // Create 27 copies of each vertex and write virtual_vertices and
  // virtual_vertices_reverse
  std::list<Vertex_handle> original_vertices;
  // try to use std::copy instead of the following loop.
  for (Vertex_iterator vit = vertices_begin() ; vit != vertices_end() ; ++vit)
    original_vertices.push_back(vit);
  for (typename std::list<Vertex_handle>::iterator vit
	 = original_vertices.begin() ; vit != original_vertices.end() ; ++vit) {
    Vertex_handle v_cp;
    std::vector<Vertex_handle> copies;
    for (int i=0; i<3; i++)
      for (int j=0; j<3; j++)
	for (int k=0; k<3; k++) {
	  if (i==0 && j==0 && k==0) continue;
	  v_cp = _tds.create_vertex(*vit);
	  copies.push_back(v_cp);
	  virtual_vertices.insert(std::make_pair(v_cp,
	      std::make_pair(*vit,Offset(i,j,k))));
	}
    virtual_vertices_reverse.insert(std::make_pair(*vit,copies));
  }

  // Create 27 copies of each cell from the respective copies of the
  // vertices and write virtual_cells and virtual_cells_reverse.
  typedef std::map<Cell_handle, std::pair<Cell_handle, Offset> >
    Virtual_cell_map;
  typedef std::map<Cell_handle, std::vector<Cell_handle > >
    Virtual_cell_reverse_map;
  typedef typename Virtual_cell_map::const_iterator VCMIT;
  typedef typename Virtual_cell_reverse_map::const_iterator VCRMIT;

  Virtual_cell_map virtual_cells;
  Virtual_cell_reverse_map virtual_cells_reverse;
  
  std::list<Cell_handle> original_cells;
  for (Cell_iterator cit = cells_begin() ; cit != cells_end() ; ++cit)
    original_cells.push_back(cit);

  // Store vertex offsets in a separate data structure
  std::list< Offset > off_v;
  for (typename std::list<Vertex_handle>::iterator vit
	 = original_vertices.begin() ; vit != original_vertices.end() ; ++vit) {
    Cell_handle ccc = (*vit)->cell();
    int v_index = ccc->index(*vit);
    off_v.push_back(int_to_off(ccc->offset(v_index)));
  }

  // Store neighboring offsets in a separate data structure
  std::list< array<Offset,4> > off_nb;
  for (typename std::list<Cell_handle>::iterator cit = original_cells.begin() ;
       cit != original_cells.end() ; ++cit) {
    array<Offset,4> off_nb_c;
    for (int i=0; i<4; i++){
      Cell_handle ccc = *cit;
      Cell_handle nnn = ccc->neighbor(i);
      off_nb_c[i] = get_neighbor_offset(ccc,i,nnn);
    }
    off_nb.push_back(off_nb_c);
  }

  // Create copies of cells
  for (typename std::list<Cell_handle>::iterator cit = original_cells.begin() ;
       cit != original_cells.end() ; ++cit) {
    Cell_handle c_cp;
    Vertex_handle v0,v1,v2,v3;
    std::vector<Cell_handle> copies;
    Virtual_vertex_reverse_map_it vvrmit[4];
    Offset vvoff[4];
    for (int i=0; i<4; i++) {
	vvrmit[i] = virtual_vertices_reverse.find((*cit)->vertex(i));
	CGAL_triangulation_assertion(
	    vvrmit[i] != virtual_vertices_reverse.end());
	vvoff[i] = int_to_off((*cit)->offset(i));
    }
    Vertex_handle vvh[4];
    for (int n=0; n<26; n++) {
      for (int i=0; i<4; i++) {
	// Decomposition of n into an offset (nx,ny,nz):
	// nx = (n+1)/9, ny = ((n+1)/3)%3, nz = (n+1)%3
	int o_i = ((n+1)/9+vvoff[i].x()+3)%3;
	int o_j = ((n+1)/3+vvoff[i].y()+3)%3;
	int o_k = ((n+1)+vvoff[i].z()+3)%3;
	int n_c = 9*o_i+3*o_j+o_k-1;
	CGAL_triangulation_assertion(n_c >= -1);
	if (n_c == -1) vvh[i] = (*cit)->vertex(i);
	else           vvh[i] = vvrmit[i]->second[n_c];
      }
      c_cp = _tds.create_cell(vvh[0], vvh[1], vvh[2], vvh[3]);
      copies.push_back(c_cp);
    }
    virtual_cells_reverse.insert(std::make_pair(*cit,copies));
  }

  // Set new vertices of boundary cells of the original domain.
  for (typename std::list<Cell_handle>::iterator cit = original_cells.begin() ;
       cit != original_cells.end() ; ++cit) {
    for (int i=0; i<4; i++) {
      Virtual_vertex_reverse_map_it vvrmit
	= virtual_vertices_reverse.find((*cit)->vertex(i));
      CGAL_triangulation_assertion(vvrmit != virtual_vertices_reverse.end());
      Offset vvoff = int_to_off((*cit)->offset(i));
      if (!vvoff.is_null()) {
	int n_c = 9*vvoff.x()+3*vvoff.y()+vvoff.z()-1;
	CGAL_triangulation_assertion(n_c >= 0);
	CGAL_triangulation_assertion(static_cast<unsigned int>(n_c) 
	    < vvrmit->second.size());
	(*cit)->set_vertex(i,vvrmit->second[n_c]);
      }
    }
  }

  // Set neighboring relations of cell copies
  typename std::list< array<Offset,4> >::iterator oit = off_nb.begin() ; 
  for (typename std::list<Cell_handle>::iterator cit = original_cells.begin();
       cit != original_cells.end() ; ++cit, ++oit) {
    CGAL_triangulation_assertion( oit != off_nb.end() );
    VCRMIT c_cp = virtual_cells_reverse.find(*cit);
    CGAL_triangulation_assertion(c_cp != virtual_cells_reverse.end());
    for (int i=0; i<4; i++) {
      Cell_handle cit_nb = (*cit)->neighbor(i);
      VCRMIT c_cp_nb = virtual_cells_reverse.find(cit_nb);
      CGAL_triangulation_assertion(c_cp_nb != virtual_cells_reverse.end());
      Offset nboff = (*oit)[i];
      for (int n=0; n<26; n++) {
	int n_nb;
 	if (nboff.is_null()) n_nb = n;
 	else {
 	  int o_i = ((n+1)/9-nboff.x()+3)%3;
 	  int o_j = ((n+1)/3-nboff.y()+3)%3;
 	  int o_k = (n+1-nboff.z()+3)%3;
 	  n_nb = 9*o_i+3*o_j+o_k-1;
 	}
	if (n_nb == -1) {
	  CGAL_triangulation_assertion(cit_nb->has_vertex(
		  c_cp->second[n]->vertex((i+1)%4)) );
	  CGAL_triangulation_assertion(cit_nb->has_vertex(
		  c_cp->second[n]->vertex((i+2)%4)) );
	  CGAL_triangulation_assertion(cit_nb->has_vertex(
		  c_cp->second[n]->vertex((i+3)%4)) );
	  c_cp->second[n]->set_neighbor(i,cit_nb);
	}
	else {
	  CGAL_triangulation_assertion(n_nb >= 0);
	  CGAL_triangulation_assertion(static_cast<unsigned int>(n_nb)
	      <= c_cp_nb->second.size());
	  CGAL_triangulation_assertion(c_cp_nb->second[n_nb]
			 ->has_vertex(c_cp->second[n]->vertex((i+1)%4)) );
	  CGAL_triangulation_assertion(c_cp_nb->second[n_nb]
			 ->has_vertex(c_cp->second[n]->vertex((i+2)%4)) );
	  CGAL_triangulation_assertion(c_cp_nb->second[n_nb]
			 ->has_vertex(c_cp->second[n]->vertex((i+3)%4)) );
	  c_cp->second[n]->set_neighbor(i,c_cp_nb->second[n_nb]);
	}
      }
    }
  }

  // Set neighboring relations of original cells
  oit = off_nb.begin();
  for (typename std::list<Cell_handle>::iterator cit = original_cells.begin();
       cit != original_cells.end() ; ++cit, ++oit) {
    CGAL_triangulation_assertion( oit != off_nb.end() );
    for (int i=0; i<4; i++) {
      Offset nboff = (*oit)[i];
      if (!nboff.is_null()) {
	Cell_handle cit_nb = (*cit)->neighbor(i);
	VCRMIT c_cp_nb = virtual_cells_reverse.find(cit_nb);
	CGAL_triangulation_assertion(c_cp_nb != virtual_cells_reverse.end());
	int o_i = (3-nboff.x())%3;
	int o_j = (3-nboff.y())%3;
	int o_k = (3-nboff.z())%3;
	int n_nb = 9*o_i+3*o_j+o_k-1;
	CGAL_triangulation_assertion(n_nb >= 0);
	CGAL_triangulation_assertion(static_cast<unsigned int>(n_nb)
	    <= c_cp_nb->second.size());
	CGAL_triangulation_assertion(c_cp_nb->second[n_nb]
		       ->has_vertex((*cit)->vertex((i+1)%4)) );
	CGAL_triangulation_assertion(c_cp_nb->second[n_nb]
		       ->has_vertex((*cit)->vertex((i+2)%4)) );
	CGAL_triangulation_assertion(c_cp_nb->second[n_nb]
		       ->has_vertex((*cit)->vertex((i+3)%4)) );
	(*cit)->set_neighbor(i,c_cp_nb->second[n_nb]);
      }
    }
  }

  // Set incident cells 
  for (Cell_iterator cit = cells_begin() ; cit != cells_end() ; ++cit) {
    for (int i=0 ; i<4 ; i++) {
      cit->vertex(i)->set_cell(cit);
    }
  }

  // Set offsets where necessary
  for (typename std::list<Cell_handle>::iterator cit = original_cells.begin() ;
       cit != original_cells.end() ; ++cit) {
    VCRMIT c_cp = virtual_cells_reverse.find(*cit);
    CGAL_triangulation_assertion( c_cp != virtual_cells_reverse.end());
    Offset off[4];
    for (int i=0; i<4; i++)
      off[i] = int_to_off((*cit)->offset(i));
    if (off[0].is_null() && off[1].is_null()
	&& off[2].is_null() && off[3].is_null()) continue;
    for (int n=0; n<26; n++) {
      Offset off_cp[4];
      int o_i = (n+1)/9;
      int o_j = ((n+1)/3)%3;
      int o_k = (n+1)%3;
      if (o_i!=2 && o_j!=2 && o_k !=2) continue;
      for (int i=0; i<4; i++) {
	off_cp[i] = Offset((o_i==2)?off[i].x():0,
			   (o_j==2)?off[i].y():0,
			   (o_k==2)?off[i].z():0);
	CGAL_triangulation_assertion(off_cp[i].x() == 0 || off_cp[i].x() == 1);
	CGAL_triangulation_assertion(off_cp[i].y() == 0 || off_cp[i].y() == 1);
	CGAL_triangulation_assertion(off_cp[i].z() == 0 || off_cp[i].z() == 1);
      }
      set_offsets(c_cp->second[n],off_cp[0],off_cp[1],off_cp[2],off_cp[3]);
    }
  }

  // Iterate over all original cells and reset offsets.
  for (typename std::list<Cell_handle>::iterator cit = original_cells.begin() ;
       cit != original_cells.end() ; ++cit) {
    //This statement does not seem to have any effect
    set_offsets(*cit, 0,0,0,0);
    CGAL_triangulation_assertion((*cit)->offset(0) == 0);
    CGAL_triangulation_assertion((*cit)->offset(1) == 0);
    CGAL_triangulation_assertion((*cit)->offset(2) == 0);
    CGAL_triangulation_assertion((*cit)->offset(3) == 0);
  }

  _cover = make_array(3,3,3);
  CGAL_triangulation_expensive_assertion(is_valid());

  // Set up too long edges data structure
  int i=0;
  for (Vertex_iterator vit = vertices_begin(); vit != vertices_end(); ++vit) {
    too_long_edges[vit] = std::list<Vertex_handle>();
    ++i;
  }
  too_long_edge_counter = find_too_long_edges(too_long_edges);
}

// iterate over all edges and store the ones that are longer than
// edge_length_threshold in edges. Return the number of too long edges.
template < class GT, class TDS >
inline int
Periodic_3_triangulation_3<GT,TDS>::find_too_long_edges(
    std::map<Vertex_handle, std::list<Vertex_handle> >& edges)
const {
  Point p1, p2;
  int counter = 0;
  Vertex_handle v_no,vh;
  for (Edge_iterator eit = edges_begin();
       eit != edges_end() ; eit++) {
    p1 = construct_point(eit->first->vertex(eit->second)->point(),
	get_offset(eit->first, eit->second));
    p2 = construct_point(eit->first->vertex(eit->third)->point(),
	get_offset(eit->first, eit->third));
    if (squared_distance(p1,p2) > edge_length_threshold) {
      if (&*(eit->first->vertex(eit->second)) <
	  &*(eit->first->vertex(eit->third))) {
	v_no = eit->first->vertex(eit->second);
	vh = eit->first->vertex(eit->third);
      } else {
	v_no = eit->first->vertex(eit->third);
	vh = eit->first->vertex(eit->second);
      }
      edges[v_no].push_back(vh);
      counter++;
    }
  }
  return counter;
}

template < class GT, class TDS >
class Periodic_3_triangulation_3<GT,TDS>::Finder {
  const Self* _t;
  const Point & _p;
public:
  Finder(const Self* t, const Point &p) : _t(t), _p(p) {}
  bool operator()(const Vertex_handle v) {
    return _t->equal(v->point(), _p);
  }
};

/** Find the cell that consists of the four given vertices
 *
 *  Iterates over all cells and compare the four vertices of each cell
 *  with the four vertices in vh.
 */
template < class GT, class TDS >
inline typename Periodic_3_triangulation_3<GT,TDS>::Cell_handle
Periodic_3_triangulation_3<GT,TDS>::get_cell(const Vertex_handle* vh) const {
  bool contains_v[4];
  std::vector<Cell_handle> cells;
  incident_cells(vh[3],std::back_inserter(cells));
  for ( typename std::vector<Cell_handle>::iterator it = cells.begin();
       it != cells.end(); it++ ) {
    CGAL_triangulation_assertion(
	(*it)->vertex(0) == vh[3] || (*it)->vertex(1) == vh[3]
      ||(*it)->vertex(2) == vh[3] || (*it)->vertex(3) == vh[3]) ;
    for ( int j=0 ; j<3 ; j++ ) {
      contains_v[j] = false;
      contains_v[j] = ( (*it)->vertex(0) == vh[j] )
          || ( (*it)->vertex(1) == vh[j] )
          || ( (*it)->vertex(2) == vh[j] )
          || ( (*it)->vertex(3) == vh[j] );
    }
    if (contains_v[0] && contains_v[1] && contains_v[2]) {
      return (*it);
    }
  }
  CGAL_triangulation_assertion(false);
  return Cell_handle();
}

/*! \brief Get the offset of tester.point() such that 
 * this point is in conflict with c w.r.t tester.get_offset().
 *
 * Implementation: Just try all eight possibilities.
 */
template < class GT, class TDS >
template < class Conflict_tester >
inline typename Periodic_3_triangulation_3<GT,TDS>::Offset
Periodic_3_triangulation_3<GT,TDS>::get_location_offset(
    const Conflict_tester& tester, Cell_handle c) const {
  CGAL_triangulation_precondition( number_of_vertices() != 0 );

  //  CGAL_triangulation_precondition_code(Locate_type lt; int i; int j;);
  //  CGAL_triangulation_precondition(side_of_cell(q,o,c,lt,i,j)
  //      != ON_UNBOUNDED_SIDE);

  int cumm_off = c->offset(0) | c->offset(1) | c->offset(2) | c->offset(3);
  if (cumm_off == 0) {
    // default case:
    return Offset();
  } else {
    // Main idea seems to just test all possibilities.
    for (int i=0; i<8; i++) {
      if (((cumm_off | (~i))&7) == 7) {
		  if (tester(c,int_to_off(i))) {
			return int_to_off(i);
        }
      }
    }
  }
  CGAL_triangulation_assertion(false);
  return Offset();
}

/** Get the offset between the origins of the internal offset coordinate
  * systems of two neighboring cells with respect from ch to nb.
  *
  * - Find two corresponding vertices from each cell
  * - Return the difference of their offsets.
  */
template < class GT, class TDS >
inline typename Periodic_3_triangulation_3<GT,TDS>::Offset
Periodic_3_triangulation_3<GT,TDS>::get_neighbor_offset(
    Cell_handle ch, int i, Cell_handle nb) const {
  // Redundance in the signature!
  CGAL_triangulation_precondition(ch->neighbor(i) == nb);
  CGAL_triangulation_precondition(nb->neighbor(nb->index(ch)) == ch);
  
  Vertex_handle vertex_ch;
  int index_ch, index_nb;
  // ensure that vertex_ch \in nb and vertex_nb \in ch
  index_ch = (i==0? 1 : 0);
  vertex_ch = ch->vertex(index_ch);
  index_nb = nb->index(vertex_ch);

  return int_to_off(nb->offset(index_nb)) - int_to_off(ch->offset(index_ch));
}

/**
 * - ch->offset(i) is an bit triple encapsulated in an integer. Each bit
 *   represents the offset in one direction --> 2-cover!
 * - it_to_off(int) decodes this again.
 * - Finally the offset vector is multiplied by cover.
 *   So if we are working in 3-cover we translate it to the neighboring
 *   3-cover and not only to the neighboring domain.
 */
template < class GT, class TDS >
inline void Periodic_3_triangulation_3<GT, TDS>::get_vertex(
    Cell_handle ch, int i, Vertex_handle &vh, Offset &off) const {

  off = combine_offsets(Offset(),int_to_off(ch->offset(i)));
  vh = ch->vertex(i);
  
  if (is_1_cover()) return;
  Vertex_handle vh_i = vh;
  get_vertex(vh_i, vh, off);
  return;
}

template < class GT, class TDS >
inline void Periodic_3_triangulation_3<GT, TDS>::get_vertex(
    Vertex_handle vh_i, Vertex_handle &vh, Offset &off) const {
  
  Virtual_vertex_map_it it = virtual_vertices.find(vh_i);

  if (it == virtual_vertices.end()) {
    // if ch->vertex(i) is not contained in virtual_vertices, then it is in
    // the original domain.
    vh = vh_i;
    CGAL_triangulation_assertion(vh != Vertex_handle());
  } else {
    // otherwise it has to be looked up as well as its offset.
    vh = it->second.first;
    off += it->second.second;
    CGAL_triangulation_assertion(vh->point().x() < _domain.xmax());
    CGAL_triangulation_assertion(vh->point().y() < _domain.ymax());
    CGAL_triangulation_assertion(vh->point().z() < _domain.zmax());
    CGAL_triangulation_assertion(vh->point().x() >= _domain.xmin());
    CGAL_triangulation_assertion(vh->point().y() >= _domain.ymin());
    CGAL_triangulation_assertion(vh->point().z() >= _domain.zmin());
  }
}

template < class GT, class TDS >
std::istream & 
operator>> (std::istream& is, Periodic_3_triangulation_3<GT,TDS> &tr)
  // reads
  // the current covering that guarantees the triangulation to be a
  //     simplicial complex
  // the number of vertices
  // the non combinatorial information on vertices (points in case of 1-sheeted
  //     covering, point-offset pairs otherwise)
  //     ALL PERIODIC COPIES OF ONE VERTEX MUST BE STORED CONSECUTIVELY
  // the number of cells
  // the cells by the indices of their vertices in the preceding list
  // of vertices, plus the non combinatorial information on each cell
  // the neighbors of each cell by their index in the preceding list of cells
{
  CGAL_triangulation_precondition(is.good());

  typedef Periodic_3_triangulation_3<GT,TDS>       Triangulation;
  typedef typename GT::FT FT;
  typedef typename Triangulation::size_type             size_type;
  typedef typename Triangulation::Vertex_handle         Vertex_handle;
  typedef typename Triangulation::Cell_handle           Cell_handle;
  typedef typename Triangulation::Offset                Offset;
  typedef typename Triangulation::Iso_cuboid            Iso_cuboid;

  tr.clear();

  Iso_cuboid domain(0,0,0,1,1,1);
  int cx=0, cy=0, cz=0;
  size_type n=0;

  if (is_ascii(is)) {
    is >> domain;
    is >> cx >> cy >> cz;
    is >> n;
  }
  else {
    is >> domain;
    read(is,cx);
    read(is,cy);
    read(is,cz);
    read(is,n);
  }
 
  CGAL_triangulation_assertion((n/(cx*cy*cz))*cx*cy*cz == n);

  tr.tds().set_dimension((n==0?-2:3));
  tr._domain = domain;
  tr._gt.set_domain(domain);
  tr._cover = make_array(cx,cy,cz);

  if ( n==0 ) return is;

  std::map< std::size_t, Vertex_handle > V;

  if (cx==1 && cy==1 && cz==1) {
    for (std::size_t i=0; i < n; i++) {
      V[i] = tr.tds().create_vertex();
      is >> *V[i];
    }
  } else {
    Vertex_handle v,w;
    std::vector<Vertex_handle> vv;
    Offset off;
    for (std::size_t i=0; i < n; i++) {
      v = tr.tds().create_vertex();
      V[i] = v;
      is >> *V[i] >> off;
      vv.clear();
      for (int j=1; j<cx*cy*cz; j++) {
        i++;
        w = tr.tds().create_vertex();
        V[i] = w;
        is >> *V[i] >> off;
        vv.push_back(w);
        tr.virtual_vertices[w]=std::make_pair(v,off);
      }
      tr.virtual_vertices_reverse[v]=vv;
    }
  }
  
  std::map< std::size_t, Cell_handle > C;
  std::size_t m;
  tr._tds.read_cells(is, V, m, C);

  // read offsets
  int off[4] = {0,0,0,0};
  for (std::size_t j=0 ; j < m; j++) {
    if (is_ascii(is))
      is >> off[0] >> off[1] >> off[2] >> off[3];
    else {
      read(is,off[0]);
      read(is,off[1]);
      read(is,off[2]);
      read(is,off[3]);
    }
    tr.set_offsets(C[j],off[0],off[1],off[2],off[3]);
  }
  
  // read potential other information
  for (std::size_t j=0 ; j < m; j++)
    is >> *(C[j]);

  typedef typename Triangulation::Vertex_iterator VI;

  int i=0;
  for (VI vi = tr.vertices_begin();
      vi != tr.vertices_end(); ++vi) {
    tr.too_long_edges[vi]=std::list<Vertex_handle>();
    ++i;
  }

  tr.edge_length_threshold = FT(0.166) * (tr._domain.xmax()-tr._domain.xmin())
                                       * (tr._domain.xmax()-tr._domain.xmin());
  tr.too_long_edge_counter = tr.find_too_long_edges(tr.too_long_edges);

  CGAL_triangulation_expensive_assertion( tr.is_valid() );
  return is;
}
    
template < class GT, class TDS >
std::ostream & 
operator<< (std::ostream& os,const Periodic_3_triangulation_3<GT,TDS> &tr)
// writes :
// the number of vertices
// the domain as six coordinates: xmin ymin zmin xmax ymax zmax
// the current covering that guarantees the triangulation to be a
//     simplicial complex
// the non combinatorial information on vertices (points in case of 1-sheeted
//     covering, point-offset pairs otherwise)
//     ALL PERIODIC COPIES OF ONE VERTEX MUST BE STORED CONSECUTIVELY
// the number of cells
// the cells by the indices of their vertices in the preceding list
// of vertices, plus the non combinatorial information on each cell
// the neighbors of each cell by their index in the preceding list of cells
{
  typedef Periodic_3_triangulation_3<GT,TDS>       Triangulation;
  typedef typename Triangulation::size_type        size_type;
  typedef typename Triangulation::Vertex_handle    Vertex_handle;
  typedef typename Triangulation::Vertex_iterator  Vertex_iterator;
  typedef typename Triangulation::Cell_handle      Cell_handle;
  typedef typename Triangulation::Cell_iterator    Cell_iterator;
  typedef typename Triangulation::Edge_iterator    Edge_iterator;
  typedef typename Triangulation::Facet_iterator   Facet_iterator;
  typedef typename Triangulation::Covering_sheets  Covering_sheets;
  typedef typename Triangulation::Offset           Offset;
  typedef typename Triangulation::Virtual_vertex_map_it Virtual_vertex_map_it;
  typedef typename Triangulation::Iso_cuboid       Iso_cuboid;

  // outputs dimension, domain and number of vertices
  Iso_cuboid domain = tr.domain();
  Covering_sheets cover = tr.number_of_sheets();
  size_type n = tr.number_of_vertices();

  if (is_ascii(os))
    os << domain << std::endl
       << cover[0] << " " << cover[1] << " " << cover[2] << std::endl
       << n*cover[0]*cover[1]*cover[2] << std::endl;       
  else {
    os << domain;
    write(os,cover[0]);
    write(os,cover[1]);
    write(os,cover[2]);
    write(os,n*cover[0]*cover[1]*cover[2]);
  }

  if (n == 0)
    return os;
 
  // write the vertices
  std::map<Vertex_handle, std::size_t > V;
  std::size_t i=0;
  if (tr.is_1_cover()) {
    for (Vertex_iterator it=tr.vertices_begin(); it!=tr.vertices_end(); ++it) {
      V[it] = i++;
      os << it->point();
      if (is_ascii(os))
        os << std::endl;
    }
  } else {
    Virtual_vertex_map_it vit, vvit;
    std::vector<Vertex_handle> vv;
    for (Vertex_iterator it=tr.vertices_begin(); it!=tr.vertices_end(); ++it) {
      vit = tr.virtual_vertices.find(it);
      if (vit != tr.virtual_vertices.end()) continue;
      V[it]=i++;
      if (is_ascii(os))
        os << it->point() << std::endl
           << Offset(0,0,0) << std::endl;
      else os << it->point() << Offset(0,0,0);
      CGAL_triangulation_assertion(tr.virtual_vertices_reverse.find(it)
          != tr.virtual_vertices_reverse.end());
      vv = tr.virtual_vertices_reverse.find(it)->second;
      CGAL_triangulation_assertion(vv.size() == 26);
      for (std::size_t j=0; j<vv.size(); j++) {
        vvit = tr.virtual_vertices.find(vv[j]);
        CGAL_triangulation_assertion(vvit != tr.virtual_vertices.end());
        V[vv[j]] = i++;
        if (is_ascii(os))
          os << vv[j]->point() << std::endl
             << vvit->second.second << std::endl;
        else os << vv[j]->point() << vvit->second.second;
      }
    }
  }
  CGAL_triangulation_postcondition(i==tr._cover[0]*tr._cover[1]*tr._cover[2]*n);
  
  // asks the tds for the combinatorial information
  tr.tds().print_cells(os, V);
  
  // write offsets
  //for (unsigned int i=0 ; i<tr.number_of_cells() ; i++) {
  for (Cell_iterator it=tr.cells_begin(); it!=tr.cells_end(); ++it) {
    //Cell_handle ch = std::find(tr.cells_begin(), tr.cells_end(), i);
    Cell_handle ch(it);
    for (int j=0; j<4; j++) {
      if(is_ascii(os)) {
	os << ch->offset(j);
        if ( j==3 )
          os << std::endl;
        else
          os << ' ';
      }
      else write(os,ch->offset(j));
    }
  }
  
  // write the non combinatorial information on the cells
  // using the << operator of Cell
  // works because the iterator of the tds traverses the cells in the
  // same order as the iterator of the triangulation
  if(tr.number_of_vertices() != 0) {
      for(Cell_iterator it=tr.cells_begin(); it != tr.cells_end(); ++it) {
    os << *it; // other information
    if(is_ascii(os))
      os << std::endl;
    }
  }
  return os ;
}

namespace internal {

  /// Internal function used by operator==.
  //TODO: introduce offsets
  template <class GT, class TDS1, class TDS2>
  bool
  test_next(const Periodic_3_triangulation_3<GT, TDS1> &t1,
      const Periodic_3_triangulation_3<GT, TDS2> &t2,
      typename Periodic_3_triangulation_3<GT, TDS1>::Cell_handle c1,
      typename Periodic_3_triangulation_3<GT, TDS2>::Cell_handle c2,
      std::map<typename Periodic_3_triangulation_3<GT, TDS1>::Cell_handle,
          typename Periodic_3_triangulation_3<GT, TDS2>::Cell_handle> &Cmap,
      std::map<typename Periodic_3_triangulation_3<GT, TDS1>::Vertex_handle,
          typename Periodic_3_triangulation_3<GT, TDS2>::Vertex_handle> &Vmap)
  {
    // This function tests and registers the 4 neighbors of c1/c2,
    // and recursively calls itself over them.
    // Returns false if an inequality has been found.

    // Precondition: c1, c2 have been registered as well as their 4 vertices.
    CGAL_triangulation_precondition(t1.number_of_vertices() != 0);
    CGAL_triangulation_precondition(Cmap[c1] == c2);
    CGAL_triangulation_precondition(Vmap.find(c1->vertex(0)) != Vmap.end());
    CGAL_triangulation_precondition(Vmap.find(c1->vertex(1)) != Vmap.end());
    CGAL_triangulation_precondition(Vmap.find(c1->vertex(2)) != Vmap.end());
    CGAL_triangulation_precondition(Vmap.find(c1->vertex(3)) != Vmap.end());

    typedef Periodic_3_triangulation_3<GT, TDS1> Tr1;
    typedef Periodic_3_triangulation_3<GT, TDS2> Tr2;
    typedef typename Tr1::Vertex_handle  Vertex_handle1;
    typedef typename Tr1::Cell_handle    Cell_handle1;
    typedef typename Tr2::Vertex_handle  Vertex_handle2;
    typedef typename Tr2::Cell_handle    Cell_handle2;
    typedef typename std::map<Cell_handle1, Cell_handle2>::const_iterator  Cit;
    typedef typename std::map<Vertex_handle1,
      Vertex_handle2>::const_iterator Vit;

    for (int i=0; i <= 3; ++i) {
      Cell_handle1 n1 = c1->neighbor(i);
      Cit cit = Cmap.find(n1);
      Vertex_handle1 v1 = c1->vertex(i);
      Vertex_handle2 v2 = Vmap[v1];
      Cell_handle2 n2 = c2->neighbor(c2->index(v2));
      if (cit != Cmap.end()) {
        // n1 was already registered.
        if (cit->second != n2)
          return false;
        continue;
      }
      // n1 has not yet been registered.
      // We check that the new vertices match geometrically.
      // And we register them.
      Vertex_handle1 vn1 = n1->vertex(n1->index(c1));
      Vertex_handle2 vn2 = n2->vertex(n2->index(c2));
      Vit vit = Vmap.find(vn1);
      if (vit != Vmap.end()) {
        // vn1 already registered
        if (vit->second != vn2)
          return false;
      }
      else {
        if (t1.geom_traits().compare_xyz_3_object()(vn1->point(),
		vn2->point()) != 0)
          return false;

        // We register vn1/vn2.
        Vmap.insert(std::make_pair(vn1, vn2));
      }

      // We register n1/n2.
      Cmap.insert(std::make_pair(n1, n2));
      // We recurse on n1/n2.
      if (!test_next(t1, t2, n1, n2, Cmap, Vmap))
    return false;
    }

    return true;
  }

} // namespace internal


template < class GT, class TDS1, class TDS2  >
bool
operator==(const Periodic_3_triangulation_3<GT,TDS1> &t1,
     const Periodic_3_triangulation_3<GT,TDS2> &t2)
{
  typedef typename Periodic_3_triangulation_3<GT,TDS1>::Vertex_handle
      Vertex_handle1;
  typedef typename Periodic_3_triangulation_3<GT,TDS1>::Cell_handle  
      Cell_handle1;
  typedef typename Periodic_3_triangulation_3<GT,TDS2>::Vertex_handle
      Vertex_handle2;
  typedef typename Periodic_3_triangulation_3<GT,TDS2>::Vertex_handle
      Vertex_iterator2;
  typedef typename Periodic_3_triangulation_3<GT,TDS2>::Cell_handle
      Cell_handle2;
  
  typedef typename Periodic_3_triangulation_3<GT,TDS1>::Point      Point;
  typedef typename Periodic_3_triangulation_3<GT,TDS1>::Offset     Offset;
  typedef typename Periodic_3_triangulation_3<GT,TDS1>
      ::Geometric_traits::Compare_xyz_3                       Compare_xyz_3;
  
  Compare_xyz_3 cmp1 = t1.geom_traits().compare_xyz_3_object();
  Compare_xyz_3 cmp2 = t2.geom_traits().compare_xyz_3_object();
  
  // Some quick checks.
  if (   t1.domain()           != t2.domain()
      || t1.number_of_sheets() != t2.number_of_sheets())
    return false;

  if (   t1.number_of_vertices() != t2.number_of_vertices()
      || t1.number_of_cells() != t2.number_of_cells())
    return false;

  // Special case for empty triangulations
  if (t1.number_of_vertices() == 0)
    return true;

  // We will store the mapping between the 2 triangulations vertices and
  // cells in 2 maps.
  std::map<Vertex_handle1, Vertex_handle2> Vmap;
  std::map<Cell_handle1, Cell_handle2> Cmap;

  // find a common point
  Vertex_handle1 v1 = static_cast<Vertex_handle1>(t1.vertices_begin());
  Vertex_handle2 iv2;
  for (Vertex_iterator2 vit2 = t2.vertices_begin() ;
      vit2 != t2.vertices_end(); ++vit2) {
    if (!t1.equal(vit2->point(), v1->point(),
		  t2.get_offset(vit2), t1.get_offset(v1)))
      continue;
    iv2 = static_cast<Vertex_handle2>(vit2);
    break;
  }
  if (iv2 == Vertex_handle2())
    return false;
  Vmap.insert(std::make_pair(v1, iv2));

  // We pick one cell of t1, and try to match it against the
  // cells of t2.
  Cell_handle1 c = v1->cell();
  Vertex_handle1 v2 = c->vertex((c->index(v1)+1)%4);
  Vertex_handle1 v3 = c->vertex((c->index(v1)+2)%4);
  Vertex_handle1 v4 = c->vertex((c->index(v1)+3)%4);
  Point p2 = v2->point();
  Point p3 = v3->point();
  Point p4 = v4->point();
  Offset o2 = t1.get_offset(v2);
  Offset o3 = t1.get_offset(v3);
  Offset o4 = t1.get_offset(v4);

  std::vector<Cell_handle2> ics;
  t2.incident_cells(iv2, std::back_inserter(ics));
  for (typename std::vector<Cell_handle2>::const_iterator cit = ics.begin();
       cit != ics.end(); ++cit) {
    int inf = (*cit)->index(iv2);

    if (t1.equal(p2, (*cit)->vertex((inf+1)%4)->point(),
	      o2, t2.get_offset((*cit)->vertex((inf+1)%4))))
      Vmap.insert(std::make_pair(v2, (*cit)->vertex((inf+1)%4)));
    else if (t1.equal(p2, (*cit)->vertex((inf+2)%4)->point(),
	      o2, t2.get_offset((*cit)->vertex((inf+2)%4))))
      Vmap.insert(std::make_pair(v2, (*cit)->vertex((inf+2)%4)));
    else if (t1.equal(p2, (*cit)->vertex((inf+3)%4)->point(),
	      o2, t2.get_offset((*cit)->vertex((inf+3)%4))))
      Vmap.insert(std::make_pair(v2, (*cit)->vertex((inf+3)%4)));
    else
      continue; // None matched v2.

    if (t1.equal(p3, (*cit)->vertex((inf+1)%4)->point(),
	      o3, t2.get_offset((*cit)->vertex((inf+1)%4))))
      Vmap.insert(std::make_pair(v3, (*cit)->vertex((inf+1)%4)));
    else if (t1.equal(p3, (*cit)->vertex((inf+2)%4)->point(),
	      o3, t2.get_offset((*cit)->vertex((inf+2)%4))))
      Vmap.insert(std::make_pair(v3, (*cit)->vertex((inf+2)%4)));
    else if (t1.equal(p3, (*cit)->vertex((inf+3)%4)->point(),
	      o3, t2.get_offset((*cit)->vertex((inf+3)%4))))
      Vmap.insert(std::make_pair(v3, (*cit)->vertex((inf+3)%4)));
    else
      continue; // None matched v3.

    if (t1.equal(p4, (*cit)->vertex((inf+1)%4)->point(),
	      o4, t2.get_offset((*cit)->vertex((inf+1)%4))))
      Vmap.insert(std::make_pair(v4,(*cit)->vertex((inf+1)%4)));
    else if (t1.equal(p4, (*cit)->vertex((inf+2)%4)->point(),
	      o4, t2.get_offset((*cit)->vertex((inf+2)%4))))
      Vmap.insert(std::make_pair(v4,(*cit)->vertex((inf+2)%4)));
    else if (t1.equal(p4, (*cit)->vertex((inf+3)%4)->point(),
	      o4, t2.get_offset((*cit)->vertex((inf+3)%4))))
      Vmap.insert(std::make_pair(v4,(*cit)->vertex((inf+3)%4)));
    else
      continue; // None matched v4.

    // Found it !
    Cmap.insert(std::make_pair(c, *cit));
    break;
  }

  if (Cmap.size() == 0)
    return false;

  // We now have one cell, we need to propagate recursively.
  return internal::test_next(t1, t2,
      Cmap.begin()->first, Cmap.begin()->second, Cmap, Vmap);
}

template < class GT, class TDS1, class TDS2 >
inline
bool
operator!=(const Periodic_3_triangulation_3<GT,TDS1> &t1,
    const Periodic_3_triangulation_3<GT,TDS2> &t2)
{
  return ! (t1 == t2);
}

} //namespace CGAL

#endif // CGAL_PERIODIC_3_TRIANGULATION_3_H
