/* Copyright (c) 1997-2024
   Ewgenij Gawrilow, Michael Joswig, and the polymake team
   Technische Universität Berlin, Germany
   https://polymake.org

   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 2, or (at your option) any
   later version: http://www.gnu.org/licenses/gpl.txt.

   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.
--------------------------------------------------------------------------------
*/

#include "polymake/client.h"
#include "polymake/Graph.h"
#include "polymake/Set.h"
#include "polymake/IncidenceMatrix.h"
#include "polymake/Matrix.h"
#include "polymake/linalg.h"
#include "polymake/Rational.h"

namespace polymake { namespace polytope {

template <typename IMatrix>
Graph<> points_graph_from_incidence(const Matrix<Rational>& points, const GenericIncidenceMatrix<IMatrix>& IM, const Matrix<Rational>& facets, Int dim)
{
  const Int n_vertices = IM.cols();
  Graph<> G(n_vertices);
  if (n_vertices < 3) {
    if (n_vertices == 2)
       G.edge(0, 1);
    return G;
  }

  EdgeMap<Undirected, Set<Int>> intersects(G);

  for (auto n1 = entire(nodes(G)); !n1.at_end(); ++n1) {
     const auto& min = facets.minor(IM.col(*n1), All);
     if (rank(min) >= dim-1) { //exclude vertices in the interior of some non-edge face
        auto n2 = n1;
        while (!(++n2).at_end()) {
           Set<Int> common = IM.col(*n1) * IM.col(*n2);
           if (common.empty()) continue;

           auto edge = n1.out_edges().begin();
           bool add = true;
           while (!edge.at_end()) {
              const Int inc = incl(intersects[*edge], common);
              if (inc == 2) {
                 ++edge;
              } else if (inc < 0) {
                 n1.out_edges().erase(edge++);
              } else if (inc > 0) {
                 add = false;
                 break;
              } else {
                 Int i = 1;
                 // compute lambda with lambda*points[*n1]+(1-lambda)*points[edge.to_node()]=points[*n2]
                 while (points(*n1, i) == points(*n2, i)) ++i;
                 if (points(edge.to_node(), i) == points(*n1, i)) {
                    add = false;
                    break;
                 }
                 const Rational lambda = (points(edge.to_node(), i) - points(*n2, i))/(points(edge.to_node(), i) - points(*n1, i));
                 if (lambda <= 0) {
                    add = false;
                 } else if (lambda <= 1) {
                    n1.out_edges().erase(edge++);
                 }
                 break;
              }
           }
           if (add)
              intersects[n1.edge(*n2)] = common;
        }
     }
  }
  return G;
}

FunctionTemplate4perl("points_graph_from_incidence(Matrix<Rational> IncidenceMatrix Matrix<Rational> Int)");

} }

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// End:
