Simplex_tree.h

/*    This file is part of the Gudhi Library. The Gudhi library
 *    (Geometric Understanding in Higher Dimensions) is a generic C++
 *    library for computational topology.
 *
 *    Author(s):       Clément Maria
 *
 *    Copyright (C) 2014  INRIA Sophia Antipolis-Méditerranée (France)
 *
 *    This program is free software: you can redistribute it and/or modify
 *    it under the terms of the GNU General Public License as published by
 *    the Free Software Foundation, either version 3 of the License, or
 *    (at your option) any later version.
 *
 *    This program is distributed in the hope that it will be useful,
 *    but WITHOUT ANY WARRANTY; without even the implied warranty of
 *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *    GNU General Public License for more details.
 *
 *    You should have received a copy of the GNU General Public License
 *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#ifndef SIMPLEX_TREE_H_
#define SIMPLEX_TREE_H_

#include <gudhi/Simplex_tree/Simplex_tree_node_explicit_storage.h>
#include <gudhi/Simplex_tree/Simplex_tree_siblings.h>
#include <gudhi/Simplex_tree/Simplex_tree_iterators.h>
#include <gudhi/Simplex_tree/indexing_tag.h>

#include <gudhi/reader_utils.h>
#include <gudhi/graph_simplicial_complex.h>

#include <boost/container/flat_map.hpp>
#include <boost/iterator/transform_iterator.hpp>
#include <boost/graph/adjacency_list.hpp>

#include <algorithm>
#include <utility>
#include <vector>
#include <functional>  // for greater<>

namespace Gudhi {

struct Simplex_tree_options_full_featured {
  typedef linear_indexing_tag Indexing_tag;
  typedef int Vertex_handle;
  typedef double Filtration_value;
  typedef int Simplex_key;
  static const bool store_key = true;
  static const bool store_filtration = true;
};

template<typename SimplexTreeOptions = Simplex_tree_options_full_featured>
class Simplex_tree {
 public:
  typedef SimplexTreeOptions Options;
  typedef typename Options::Indexing_tag Indexing_tag;
  typedef typename Options::Filtration_value Filtration_value;
  typedef typename Options::Simplex_key Simplex_key;
  typedef typename Options::Vertex_handle Vertex_handle;

  /* Type of node in the simplex tree. */
  typedef Simplex_tree_node_explicit_storage<Simplex_tree> Node;
  /* Type of dictionary Vertex_handle -> Node for traversing the simplex tree. */
  // Note: this wastes space when Vertex_handle is 32 bits and Node is aligned on 64 bits. It would be better to use a
  // flat_set (with our own comparator) where we can control the layout of the struct (put Vertex_handle and
  // Simplex_key next to each other).
  typedef typename boost::container::flat_map<Vertex_handle, Node> Dictionary;

  /* \brief Set of nodes sharing a same parent in the simplex tree. */
  /* \brief Set of nodes sharing a same parent in the simplex tree. */
  typedef Simplex_tree_siblings<Simplex_tree, Dictionary> Siblings;

  struct Key_simplex_base_real {
    Key_simplex_base_real() : key_(-1) {}
    void assign_key(Simplex_key k) { key_ = k; }
    Simplex_key key() const { return key_; }
   private:
    Simplex_key key_;
  };
  struct Key_simplex_base_dummy {
    Key_simplex_base_dummy() {}
    void assign_key(Simplex_key) { }
    Simplex_key key() const { assert(false); return -1; }
  };
  typedef typename std::conditional<Options::store_key, Key_simplex_base_real, Key_simplex_base_dummy>::type Key_simplex_base;

  struct Filtration_simplex_base_real {
    Filtration_simplex_base_real() : filt_(0) {}
    void assign_filtration(Filtration_value f) { filt_ = f; }
    Filtration_value filtration() const { return filt_; }
   private:
    Filtration_value filt_;
  };
  struct Filtration_simplex_base_dummy {
    Filtration_simplex_base_dummy() {}
    void assign_filtration(Filtration_value f) { assert(f == 0); }
    Filtration_value filtration() const { return 0; }
  };
  typedef typename std::conditional<Options::store_filtration, Filtration_simplex_base_real,
    Filtration_simplex_base_dummy>::type Filtration_simplex_base;

 public:
  typedef typename Dictionary::iterator Simplex_handle;

 private:
  typedef typename Dictionary::iterator Dictionary_it;
  typedef typename Dictionary_it::value_type Dit_value_t;

  struct return_first {
    Vertex_handle operator()(const Dit_value_t& p_sh) const {
      return p_sh.first;
    }
  };

 public:
  typedef boost::transform_iterator<return_first, Dictionary_it> Complex_vertex_iterator;
  typedef boost::iterator_range<Complex_vertex_iterator> Complex_vertex_range;
  typedef Simplex_tree_simplex_vertex_iterator<Simplex_tree> Simplex_vertex_iterator;
  typedef boost::iterator_range<Simplex_vertex_iterator> Simplex_vertex_range;
  typedef std::vector<Simplex_handle> Cofaces_simplex_range;
  typedef Simplex_tree_boundary_simplex_iterator<Simplex_tree> Boundary_simplex_iterator;
  typedef boost::iterator_range<Boundary_simplex_iterator> Boundary_simplex_range;
  typedef Simplex_tree_complex_simplex_iterator<Simplex_tree> Complex_simplex_iterator;
  typedef boost::iterator_range<Complex_simplex_iterator> Complex_simplex_range;
  typedef Simplex_tree_skeleton_simplex_iterator<Simplex_tree> Skeleton_simplex_iterator;
  typedef boost::iterator_range<Skeleton_simplex_iterator> Skeleton_simplex_range;
  typedef std::vector<Simplex_handle> Filtration_simplex_range;
  typedef typename Filtration_simplex_range::const_iterator Filtration_simplex_iterator;

  /* @} */  // end name range and iterator types
  Complex_vertex_range complex_vertex_range() {
    return Complex_vertex_range(
                                boost::make_transform_iterator(root_.members_.begin(), return_first()),
                                boost::make_transform_iterator(root_.members_.end(), return_first()));
  }

  Complex_simplex_range complex_simplex_range() {
    return Complex_simplex_range(Complex_simplex_iterator(this),
                                 Complex_simplex_iterator());
  }

  Skeleton_simplex_range skeleton_simplex_range(int dim) {
    return Skeleton_simplex_range(Skeleton_simplex_iterator(this, dim),
                                  Skeleton_simplex_iterator());
  }

  Filtration_simplex_range const& filtration_simplex_range(Indexing_tag = Indexing_tag()) {
    if (filtration_vect_.empty()) {
      initialize_filtration();
    }
    return filtration_vect_;
  }

  Simplex_vertex_range simplex_vertex_range(Simplex_handle sh) {
    assert(sh != null_simplex());  // Empty simplex
    return Simplex_vertex_range(Simplex_vertex_iterator(this, sh),
                                Simplex_vertex_iterator(this));
  }

  Boundary_simplex_range boundary_simplex_range(Simplex_handle sh) {
    return Boundary_simplex_range(Boundary_simplex_iterator(this, sh),
                                  Boundary_simplex_iterator(this));
  }
  // end range and iterator methods
  Simplex_tree()
      : null_vertex_(-1),
      threshold_(0),
      root_(nullptr, null_vertex_),
      filtration_vect_(),
      dimension_(-1) { }

  Simplex_tree(const Simplex_tree& simplex_source)
      : null_vertex_(simplex_source.null_vertex_),
      threshold_(simplex_source.threshold_),
      filtration_vect_(),
      dimension_(simplex_source.dimension_) {
    auto root_source = simplex_source.root_;
    auto memb_source = root_source.members();
    root_ = Siblings(nullptr, null_vertex_, memb_source);
    rec_copy(&root_, &root_source);
  }

  void rec_copy(Siblings *sib, Siblings *sib_source) {
    for (auto sh = sib->members().begin(), sh_source = sib_source->members().begin();
         sh != sib->members().end(); ++sh, ++sh_source) {
      if (has_children(sh_source)) {
        Siblings * newsib = new Siblings(sib, sh_source->first);
        newsib->members_.reserve(sh_source->second.children()->members().size());
        for (auto & child : sh_source->second.children()->members())
          newsib->members_.emplace_hint(newsib->members_.end(), child.first, Node(sib, child.second.filtration()));
        rec_copy(newsib, sh_source->second.children());
        sh->second.assign_children(newsib);
      }
    }
  }

  Simplex_tree(Simplex_tree && old)
      : null_vertex_(std::move(old.null_vertex_)),
      threshold_(std::move(old.threshold_)),
      root_(std::move(old.root_)),
      filtration_vect_(std::move(old.filtration_vect_)),
      dimension_(std::move(old.dimension_)) {
    old.dimension_ = -1;
    old.threshold_ = 0;
    old.root_ = Siblings(nullptr, null_vertex_);
  }

  ~Simplex_tree() {
    for (auto sh = root_.members().begin(); sh != root_.members().end(); ++sh) {
      if (has_children(sh)) {
        rec_delete(sh->second.children());
      }
    }
  }  // end constructor/destructor
 private:
  // Recursive deletion
  void rec_delete(Siblings * sib) {
    for (auto sh = sib->members().begin(); sh != sib->members().end(); ++sh) {
      if (has_children(sh)) {
        rec_delete(sh->second.children());
      }
    }
    delete sib;
  }

 public:
  bool operator==(Simplex_tree& st2) {
    if ((null_vertex_ != st2.null_vertex_) ||
        (threshold_ != st2.threshold_) ||
        (dimension_ != st2.dimension_))
      return false;
    return rec_equal(&root_, &st2.root_);
  }

  bool operator!=(Simplex_tree& st2) {
    return (!(*this == st2));
  }

 private:
  bool rec_equal(Siblings* s1, Siblings* s2) {
    if (s1->members().size() != s2->members().size())
      return false;
    for (auto sh1 = s1->members().begin(), sh2 = s2->members().begin();
         (sh1 != s1->members().end() && sh2 != s2->members().end()); ++sh1, ++sh2) {
      if (sh1->first != sh2->first || sh1->second.filtration() != sh2->second.filtration())
        return false;
      if (has_children(sh1) != has_children(sh2))
        return false;
      // Recursivity on children only if both have children
      else if (has_children(sh1))
        if (!rec_equal(sh1->second.children(), sh2->second.children()))
          return false;
    }
    return true;
  }

 public:
  static Simplex_key key(Simplex_handle sh) {
    return sh->second.key();
  }

  Simplex_handle simplex(Simplex_key key) const {
    return filtration_vect_[key];
  }

  static Filtration_value filtration(Simplex_handle sh) {
    if (sh != null_simplex()) {
      return sh->second.filtration();
    } else {
      return INFINITY;
    }
  }

  Filtration_value filtration() const {
    return threshold_;
  }

  static Simplex_handle null_simplex() {
    return Dictionary_it(nullptr);
  }

  static Simplex_key null_key() {
    return -1;
  }

  Vertex_handle null_vertex() const {
    return null_vertex_;
  }

  size_t num_vertices() const {
    return root_.members_.size();
  }

 public:
  size_t num_simplices() {
    return num_simplices(&root_);
  }

 private:
  size_t num_simplices(Siblings * sib) {
    auto sib_begin = sib->members().begin();
    auto sib_end = sib->members().end();
    size_t simplices_number = sib_end - sib_begin;
    for (auto sh = sib_begin; sh != sib_end; ++sh) {
      if (has_children(sh)) {
        simplices_number += num_simplices(sh->second.children());
      }
    }
    return simplices_number;
  }

 public:
  int dimension(Simplex_handle sh) {
    Siblings * curr_sib = self_siblings(sh);
    int dim = 0;
    while (curr_sib != nullptr) {
      ++dim;
      curr_sib = curr_sib->oncles();
    }
    return dim - 1;
  }

  int dimension() const {
    return dimension_;
  }

  bool has_children(Simplex_handle sh) const {
    return (sh->second.children()->parent() == sh->first);
  }

  template<class InputVertexRange>
  Simplex_handle find(const InputVertexRange & s) {
    auto first = std::begin(s);
    auto last = std::end(s);

    if (first == last)
      return null_simplex();  // ----->>

    // Copy before sorting
    std::vector<Vertex_handle> copy(first, last);
    std::sort(std::begin(copy), std::end(copy));
    return find_simplex(copy);
  }

 private:
  Simplex_handle find_simplex(const std::vector<Vertex_handle> & simplex) {
    Siblings * tmp_sib = &root_;
    Dictionary_it tmp_dit;
    Vertex_handle last = simplex.back();
    for (auto v : simplex) {
      tmp_dit = tmp_sib->members_.find(v);
      if (tmp_dit == tmp_sib->members_.end()) {
        return null_simplex();
      }
      if (!has_children(tmp_dit) && v != last) {
        return null_simplex();
      }
      tmp_sib = tmp_dit->second.children();
    }
    return tmp_dit;
  }

  Simplex_handle find_vertex(Vertex_handle v) {
    return root_.members_.begin() + v;
  }
  //{ return root_.members_.find(v); }

 private:
  std::pair<Simplex_handle, bool> insert_vertex_vector(const std::vector<Vertex_handle>& simplex,
                                                     Filtration_value filtration) {
    Siblings * curr_sib = &root_;
    std::pair<Simplex_handle, bool> res_insert;
    auto vi = simplex.begin();
    for (; vi != simplex.end() - 1; ++vi) {
      res_insert = curr_sib->members_.emplace(*vi, Node(curr_sib, filtration));
      if (!(has_children(res_insert.first))) {
        res_insert.first->second.assign_children(new Siblings(curr_sib, *vi));
      }
      curr_sib = res_insert.first->second.children();
    }
    res_insert = curr_sib->members_.emplace(*vi, Node(curr_sib, filtration));
    if (!res_insert.second) {
      // if already in the complex
      if (res_insert.first->second.filtration() > filtration) {
        // if filtration value modified
        res_insert.first->second.assign_filtration(filtration);
        return res_insert;
      }
      // if filtration value unchanged
      return std::pair<Simplex_handle, bool>(null_simplex(), false);
    }
    // otherwise the insertion has succeeded
    return res_insert;
  }

 public:
  template<class InputVertexRange>
  std::pair<Simplex_handle, bool> insert_simplex(const InputVertexRange & simplex,
                                                 Filtration_value filtration = 0) {
    auto first = std::begin(simplex);
    auto last = std::end(simplex);

    if (first == last)
      return std::pair<Simplex_handle, bool>(null_simplex(), true);  // ----->>

    // Copy before sorting
    std::vector<Vertex_handle> copy(first, last);
    std::sort(std::begin(copy), std::end(copy));
    return insert_vertex_vector(copy, filtration);
  }

  template<class InputVertexRange>
  std::pair<Simplex_handle, bool> insert_simplex_and_subfaces(const InputVertexRange& Nsimplex,
                                   Filtration_value filtration = 0) {
    auto first = std::begin(Nsimplex);
    auto last = std::end(Nsimplex);

    if (first == last)
      return std::pair<Simplex_handle, bool>(null_simplex(), true);  // ----->>

    // Copy before sorting
    std::vector<Vertex_handle> copy(first, last);
    std::sort(std::begin(copy), std::end(copy));

    std::vector<std::vector<Vertex_handle>> to_be_inserted;
    std::vector<std::vector<Vertex_handle>> to_be_propagated;
    return rec_insert_simplex_and_subfaces(copy, to_be_inserted, to_be_propagated, filtration);
  }

 private:
  std::pair<Simplex_handle, bool> rec_insert_simplex_and_subfaces(std::vector<Vertex_handle>& the_simplex,
                                                             std::vector<std::vector<Vertex_handle>>& to_be_inserted,
                                                             std::vector<std::vector<Vertex_handle>>& to_be_propagated,
                                                             Filtration_value filtration = 0.0) {
    std::pair<Simplex_handle, bool> insert_result;
    if (the_simplex.size() > 1) {
      // Get and remove last vertex
      Vertex_handle last_vertex = the_simplex.back();
      the_simplex.pop_back();
      // Recursive call after last vertex removal
      insert_result = rec_insert_simplex_and_subfaces(the_simplex, to_be_inserted, to_be_propagated, filtration);

      // Concatenation of to_be_inserted and to_be_propagated
      to_be_inserted.insert(to_be_inserted.begin(), to_be_propagated.begin(), to_be_propagated.end());
      to_be_propagated = to_be_inserted;

      // to_be_inserted treatment
      for (auto& simplex_tbi : to_be_inserted) {
        simplex_tbi.push_back(last_vertex);
      }
      std::vector<Vertex_handle> last_simplex(1, last_vertex);
      to_be_inserted.insert(to_be_inserted.begin(), last_simplex);
      // i.e. (0,1,2) =>
      // [to_be_inserted | to_be_propagated] = [(1) (0,1) | (0)]
      // [to_be_inserted | to_be_propagated] = [(2) (0,2) (1,2) (0,1,2) | (0) (1) (0,1)]
      // N.B. : it is important the last inserted to be the highest in dimension
      // in order to return the "last" insert_simplex result

      // insert all to_be_inserted
      for (auto& simplex_tbi : to_be_inserted) {
        insert_result = insert_vertex_vector(simplex_tbi, filtration);
      }
    } else if (the_simplex.size() == 1) {
      // When reaching the end of recursivity, vector of simplices shall be empty and filled on back recursive
      if ((to_be_inserted.size() != 0) || (to_be_propagated.size() != 0)) {
        std::cerr << "Simplex_tree::rec_insert_simplex_and_subfaces - Error vector not empty" << std::endl;
        exit(-1);
      }
      std::vector<Vertex_handle> first_simplex(1, the_simplex.back());
      // i.e. (0,1,2) => [to_be_inserted | to_be_propagated] = [(0) | ]
      to_be_inserted.push_back(first_simplex);

      insert_result = insert_vertex_vector(first_simplex, filtration);
    } else {
        std::cerr << "Simplex_tree::rec_insert_simplex_and_subfaces - Recursivity error" << std::endl;
        exit(-1);
    }
    return insert_result;
  }

 public:
  void assign_key(Simplex_handle sh, Simplex_key key) {
    sh->second.assign_key(key);
  }

 public:
  std::pair<Simplex_handle, Simplex_handle> endpoints(Simplex_handle sh) {
    assert(dimension(sh) == 1);
    return { find_vertex(sh->first), find_vertex(self_siblings(sh)->parent()) };
  }

  Siblings * self_siblings(Simplex_handle sh) {
    if (sh->second.children()->parent() == sh->first)
      return sh->second.children()->oncles();
    else
      return sh->second.children();
  }

 public:
  Siblings * root() {
    return &root_;
  }

  void set_filtration(Filtration_value fil) {
    threshold_ = fil;
  }

  void set_dimension(int dimension) {
    dimension_ = dimension;
  }

 public:
  void initialize_filtration() {
    filtration_vect_.clear();
    filtration_vect_.reserve(num_simplices());
    for (Simplex_handle sh : complex_simplex_range())
      filtration_vect_.push_back(sh);

    /* We use stable_sort here because with libstdc++ it is faster than sort.
     * is_before_in_filtration is now a total order, but we used to call
     * stable_sort for the following heuristic:
     * The use of a depth-first traversal of the simplex tree, provided by
     * complex_simplex_range(), combined with a stable sort is meant to
     * optimize the order of simplices with same filtration value. The
     * heuristic consists in inserting the cofaces of a simplex as soon as
     * possible.
     */
    std::stable_sort(filtration_vect_.begin(), filtration_vect_.end(),
                     is_before_in_filtration(this));
  }

 private:
  void rec_coface(std::vector<Vertex_handle> &vertices, Siblings *curr_sib, int curr_nbVertices,
                  std::vector<Simplex_handle>& cofaces, bool star, int nbVertices) {
    if (!(star || curr_nbVertices <= nbVertices))  // dimension of actual simplex <= nbVertices
      return;
    for (Simplex_handle simplex = curr_sib->members().begin(); simplex != curr_sib->members().end(); ++simplex) {
      if (vertices.empty()) {
        // If we reached the end of the vertices, and the simplex has more vertices than the given simplex
        // => we found a coface

        // Add a coface if we wan't the star or if the number of vertices of the current simplex matches with nbVertices
        bool addCoface = (star || curr_nbVertices == nbVertices);
        if (addCoface)
          cofaces.push_back(simplex);
        if ((!addCoface || star) && has_children(simplex))  // Rec call
          rec_coface(vertices, simplex->second.children(), curr_nbVertices + 1, cofaces, star, nbVertices);
      } else {
        if (simplex->first == vertices.back()) {
          // If curr_sib matches with the top vertex
          bool equalDim = (star || curr_nbVertices == nbVertices);  // dimension of actual simplex == nbVertices
          bool addCoface = vertices.size() == 1 && equalDim;
          if (addCoface)
            cofaces.push_back(simplex);
          if ((!addCoface || star) && has_children(simplex)) {
            // Rec call
            Vertex_handle tmp = vertices.back();
            vertices.pop_back();
            rec_coface(vertices, simplex->second.children(), curr_nbVertices + 1, cofaces, star, nbVertices);
            vertices.push_back(tmp);
          }
        } else if (simplex->first > vertices.back()) {
          return;
        } else {
          // (simplex->first < vertices.back()
          if (has_children(simplex))
            rec_coface(vertices, simplex->second.children(), curr_nbVertices + 1, cofaces, star, nbVertices);
        }
      }
    }
  }

 public:
  Cofaces_simplex_range star_simplex_range(const Simplex_handle simplex) {
    return cofaces_simplex_range(simplex, 0);
  }

  Cofaces_simplex_range cofaces_simplex_range(const Simplex_handle simplex, int codimension) {
    Cofaces_simplex_range cofaces;
    // codimension must be positive or null integer
    assert(codimension >= 0);
    Simplex_vertex_range rg = simplex_vertex_range(simplex);
    std::vector<Vertex_handle> copy(rg.begin(), rg.end());
    if (codimension + static_cast<int>(copy.size()) > dimension_ + 1 ||
        (codimension == 0 && static_cast<int>(copy.size()) > dimension_))  // n+codimension greater than dimension_
      return cofaces;
    // must be sorted in decreasing order
    assert(std::is_sorted(copy.begin(), copy.end(), std::greater<Vertex_handle>()));
    bool star = codimension == 0;
    rec_coface(copy, &root_, 1, cofaces, star, codimension + static_cast<int>(copy.size()));
    return cofaces;
  }

 private:
  bool reverse_lexicographic_order(Simplex_handle sh1, Simplex_handle sh2) {
    Simplex_vertex_range rg1 = simplex_vertex_range(sh1);
    Simplex_vertex_range rg2 = simplex_vertex_range(sh2);
    Simplex_vertex_iterator it1 = rg1.begin();
    Simplex_vertex_iterator it2 = rg2.begin();
    while (it1 != rg1.end() && it2 != rg2.end()) {
      if (*it1 == *it2) {
        ++it1;
        ++it2;
      } else {
        return *it1 < *it2;
      }
    }
    return ((it1 == rg1.end()) && (it2 != rg2.end()));
  }

  struct is_before_in_filtration {
    explicit is_before_in_filtration(Simplex_tree * st)
        : st_(st) { }

    bool operator()(const Simplex_handle sh1, const Simplex_handle sh2) const {
      // Not using st_->filtration(sh1) because it uselessly tests for null_simplex.
      if (sh1->second.filtration() != sh2->second.filtration()) {
    return sh1->second.filtration() < sh2->second.filtration();
      }
      // is sh1 a proper subface of sh2
      return st_->reverse_lexicographic_order(sh1, sh2);
    }

    Simplex_tree * st_;
  };

 public:
  template<class OneSkeletonGraph>
  void insert_graph(const OneSkeletonGraph& skel_graph) {
    // the simplex tree must be empty
    assert(num_simplices() == 0);

    if (boost::num_vertices(skel_graph) == 0) {
      return;
    }
    if (num_edges(skel_graph) == 0) {
      dimension_ = 0;
    } else {
      dimension_ = 1;
    }

    root_.members_.reserve(boost::num_vertices(skel_graph));

    typename boost::graph_traits<OneSkeletonGraph>::vertex_iterator v_it,
        v_it_end;
    for (std::tie(v_it, v_it_end) = boost::vertices(skel_graph); v_it != v_it_end;
         ++v_it) {
      root_.members_.emplace_hint(
                                  root_.members_.end(), *v_it,
                                  Node(&root_, boost::get(vertex_filtration_t(), skel_graph, *v_it)));
    }
    typename boost::graph_traits<OneSkeletonGraph>::edge_iterator e_it,
        e_it_end;
    for (std::tie(e_it, e_it_end) = boost::edges(skel_graph); e_it != e_it_end;
         ++e_it) {
      auto u = source(*e_it, skel_graph);
      auto v = target(*e_it, skel_graph);
      if (u < v) {
        // count edges only once { std::swap(u,v); } // u < v
        auto sh = find_vertex(u);
        if (!has_children(sh)) {
          sh->second.assign_children(new Siblings(&root_, sh->first));
        }

        sh->second.children()->members().emplace(
                                                 v,
                                                 Node(sh->second.children(),
                                                      boost::get(edge_filtration_t(), skel_graph, *e_it)));
      }
    }
  }

  void expansion(int max_dim) {
    dimension_ = max_dim;
    for (Dictionary_it root_it = root_.members_.begin();
         root_it != root_.members_.end(); ++root_it) {
      if (has_children(root_it)) {
        siblings_expansion(root_it->second.children(), max_dim - 1);
      }
    }
    dimension_ = max_dim - dimension_;
  }

 private:
  void siblings_expansion(Siblings * siblings,  // must contain elements
                          int k) {
    if (dimension_ > k) {
      dimension_ = k;
    }
    if (k == 0)
      return;
    Dictionary_it next = siblings->members().begin();
    ++next;

    static std::vector<std::pair<Vertex_handle, Node> > inter;  // static, not thread-safe.
    for (Dictionary_it s_h = siblings->members().begin();
         s_h != siblings->members().end(); ++s_h, ++next) {
      Simplex_handle root_sh = find_vertex(s_h->first);
      if (has_children(root_sh)) {
        intersection(
                     inter,  // output intersection
                     next,  // begin
                     siblings->members().end(),  // end
                     root_sh->second.children()->members().begin(),
                     root_sh->second.children()->members().end(),
                     s_h->second.filtration());
        if (inter.size() != 0) {
          Siblings * new_sib = new Siblings(siblings,  // oncles
                                            s_h->first,  // parent
                                            inter);  // boost::container::ordered_unique_range_t
          inter.clear();
          s_h->second.assign_children(new_sib);
          siblings_expansion(new_sib, k - 1);
        } else {
          // ensure the children property
          s_h->second.assign_children(siblings);
          inter.clear();
        }
      }
    }
  }

  static void intersection(std::vector<std::pair<Vertex_handle, Node> >& intersection,
                           Dictionary_it begin1, Dictionary_it end1,
                           Dictionary_it begin2, Dictionary_it end2,
                           Filtration_value filtration_) {
    if (begin1 == end1 || begin2 == end2)
      return;  // ----->>
    while (true) {
      if (begin1->first == begin2->first) {
        Filtration_value filt = (std::max)({begin1->second.filtration(), begin2->second.filtration(), filtration_});
        intersection.emplace_back(begin1->first, Node(nullptr, filt));
        if (++begin1 == end1 || ++begin2 == end2)
          return;  // ----->>
      } else if (begin1->first < begin2->first) {
        if (++begin1 == end1)
          return;
      } else /* begin1->first > begin2->first */ {
        if (++begin2 == end2)
          return;  // ----->>
      }
    }
  }

 public:
  void print_hasse(std::ostream& os) {
    os << num_simplices() << " " << std::endl;
    for (auto sh : filtration_simplex_range()) {
      os << dimension(sh) << " ";
      for (auto b_sh : boundary_simplex_range(sh)) {
        os << key(b_sh) << " ";
      }
      os << filtration(sh) << " \n";
    }
  }

 private:
  Vertex_handle null_vertex_;
  Filtration_value threshold_;
  Siblings root_;
  std::vector<Simplex_handle> filtration_vect_;
  int dimension_;
};

// Print a Simplex_tree in os.

template<typename...T>
std::ostream& operator<<(std::ostream & os, Simplex_tree<T...> & st) {
  for (auto sh : st.filtration_simplex_range()) {
    os << st.dimension(sh) << " ";
    for (auto v : st.simplex_vertex_range(sh)) {
      os << v << " ";
    }
    os << st.filtration(sh) << "\n";  // TODO(VR): why adding the key ?? not read ?? << "     " << st.key(sh) << " \n";
  }
  return os;
}

template<typename...T>
std::istream& operator>>(std::istream & is, Simplex_tree<T...> & st) {
  typedef Simplex_tree<T...> ST;
  std::vector<typename ST::Vertex_handle> simplex;
  typename ST::Filtration_value fil;
  typename ST::Filtration_value max_fil = 0;
  int max_dim = -1;
  while (read_simplex(is, simplex, fil)) {
    // read all simplices in the file as a list of vertices
    // Warning : simplex_size needs to be casted in int - Can be 0
    int dim = static_cast<int> (simplex.size() - 1);
    if (max_dim < dim) {
      max_dim = dim;
    }
    if (max_fil < fil) {
      max_fil = fil;
    }
    // insert every simplex in the simplex tree
    st.insert_simplex(simplex, fil);
    simplex.clear();
  }
  st.set_dimension(max_dim);
  st.set_filtration(max_fil);

  return is;
}  // end defgroup simplex_tree

}  // namespace Gudhi

#endif  // SIMPLEX_TREE_H_