// Copyright (c) 2014  INRIA Sophia-Antipolis (France).
// All rights reserved.
//
// This file is part of CGAL (www.cgal.org).
//
// $URL: https://github.com/CGAL/cgal/blob/v6.1/Convex_hull_3/include/CGAL/Convex_hull_3/dual/halfspace_intersection_interior_point_3.h $
// $Id: include/CGAL/Convex_hull_3/dual/halfspace_intersection_interior_point_3.h b26b07a1242 $
// SPDX-License-Identifier: GPL-3.0-or-later OR LicenseRef-Commercial
//
//
// Author(s)     : Jocelyn Meyron
//                 Pierre Alliez
//

#ifndef CGAL_CH3_DUAL_HALFSPACE_INTERIOR_POINT_3_H
#define CGAL_CH3_DUAL_HALFSPACE_INTERIOR_POINT_3_H

#include <CGAL/license/Convex_hull_3.h>

#include <CGAL/disable_warnings.h>

// LP solver to compute an interior point of a polyhedron
#include <CGAL/QP_functions.h>
#include <CGAL/QP_models.h>
#include <limits>
#include <CGAL/number_utils.h>
#include <CGAL/assertions.h>
#include <CGAL/Number_types/internal/Exact_type_selector.h>

// Description taken from http://www.qhull.org/html/qhalf.htm

// If you do not know an interior point for the halfspaces, use linear programming
// to find one. Assume, n halfspaces defined by: aj*x1+bj*x2+cj*x3+dj>=0, j=1..n.
// Perform the following linear program:
//                max(x5) aj*x1+bj*x2+cj*x3+dj*x4-x5>=0, j=1..n

// Then, if [x1,x2,x3,x4,x5] is an optimal m_solution with x4,x5>0 we get:
//                aj*(x1/x4)+bj*(x2/x4)+cj*(x3/x4)+dj>=(x5/x4)>0, j=1..n
// and conclude that the point [x1/x4,x2/x4,x3/x4] is in the interior of all
// the halfspaces. Note that x5 is optimal, so this point is "way in" the
// interior (good for precision errors).

// After finding an interior point, the rest of the intersection algorithm is
// from Preparata & Shamos ['85, p. 316, "A simple case ..."]. Translate the
// halfspaces so that the interior point is the origin. Calculate the dual
// polytope. The dual polytope is the convex hull of the vertices dual to the
// original faces in regard to the unit sphere (i.e., halfspaces at distance
// d from the origin are dual to vertices at distance 1/d). Then calculate
// the resulting polytope, which is the dual of the dual polytope, and
// translate the origin back to the interior point [S. Spitz and S. Teller].

// NOTE here we change this to max(x4) under constraints aj*x1 + bj*x2 + cj*x3 + dj - x4 >= 0, j=1..n
//  i.e. aj*x1 + bj*x2 + cj*x3 - x4 >= -dj, j=1..n
// Then, if [x1,x2,x3,x4] is an optimal m_solution with x3 > 0 we pick
// the point [x1,x2,x3] as inside point.

namespace CGAL {

namespace internal {

template <class Kernel, class ET>
class Interior_polyhedron_3 {
  // 3D
  typedef typename Kernel::FT FT;
  typedef typename Kernel::Plane_3 Plane;
  typedef typename Kernel::Point_3 Point;

  // program and solution types
  typedef CGAL::Quadratic_program<double> LP;
  typedef typename CGAL::Quadratic_program_solution<ET> Solution;
  typedef typename Solution::Variable_value_iterator Variable_value_iterator;
  typedef CGAL::Real_embeddable_traits<typename Variable_value_iterator::value_type> RE_traits;
  typename RE_traits::To_double to_double;
  Solution m_solution;
  Point m_inside_point;
  Point m_optimal_point;

public:
  Point& inside_point() { return m_inside_point; }
  const Point& inside_point() const { return m_inside_point; }
  Point& optimal_point() { return m_optimal_point; }
  const Point& optimal_point() const { return m_optimal_point; }

  // Determines if a value is infinite or not
  template<typename T>
  inline bool isinf(T value) {
      return value == std::numeric_limits<T>::infinity();
  }

  // Find a point inside the polyhedron defined by a list of planes
  // InputIterator::value_type = Plane
  template < class InputIterator >
  bool find(InputIterator begin, InputIterator end) {
    // solve linear program
    LP lp(CGAL::LARGER,false); // with constraints Ax >= b

    // column indices
    const int index_x1 = 0;
    const int index_x2 = 1;
    const int index_x3 = 2;
    const int index_x4 = 3;

    // assemble linear program
    int j = 0; // row index
    // iterate over segments
    InputIterator it;
    for(it = begin; it != end; ++it, j++) {
      const Plane& plane = *it;
      const double aj = CGAL::to_double(plane.a());
      const double bj = CGAL::to_double(plane.b());
      const double cj = CGAL::to_double(plane.c());
      const double dj = CGAL::to_double(plane.d());

      CGAL_assertion(!isinf(aj));
      CGAL_assertion(!isinf(bj));
      CGAL_assertion(!isinf(cj));
      CGAL_assertion(!isinf(dj));

      // plane defined the halfspace: aj * x1 + bj * x2 + cj * x3 + dj <= 0
      // <=> - (aj * x1 + bj * x2 + cj * x3 + dj) >= 0
      // j^th constraint: -(aj * x1 + bj * x2 + cj * x3 + x4) >= dj
      lp.set_a(index_x1, j,   -aj);
      lp.set_a(index_x2, j,   -bj);
      lp.set_a(index_x3, j,   -cj);
      lp.set_a(index_x4, j, -1.0);

      // right hand side
      lp.set_b(j, dj);
    }

    // objective function -> max x4 (negative sign set because
    // the lp solver always minimizes an objective function)
    lp.set_c(index_x4,-1.0);

    // solve the linear program
    m_solution = CGAL::solve_linear_program(lp, ET());

    if(m_solution.is_infeasible())
        return false;

    if(!m_solution.is_optimal())
        return false;

    // get variables
    Variable_value_iterator X = m_solution.variable_values_begin();

    // solution if x4 > 0
    double x4 = to_double(X[index_x4]);
    if(x4 <= 0.0)
        return false;

    // define inside point as (x1;x2;x3)
    double x1 = to_double(X[index_x1]);
    double x2 = to_double(X[index_x2]);
    double x3 = to_double(X[index_x3]);
    m_inside_point = Point(x1,x2,x3);

    return true;
  }
};

} // end of internal namespace

/*!
\ingroup PkgConvexHull3Functions

computes a point belonging to the intersection of the halfspaces defined by the planes contained in the range `[begin, end)`.
If the intersection is empty, `std::nullopt` is returned.

\attention Halfspaces are considered as lower halfspaces that is to say if the plane's equation
is \f$ a\, x +b\, y +c\, z + d = 0 \f$ then the corresponding halfspace is defined by \f$ a\, x +b\, y +c\, z + d \le 0 \f$ .

\tparam PlaneIterator must be an input iterator with the value type being a `Plane_3` object from \cgal Kernel
*/
template <class PlaneIterator>
std::optional<typename Kernel_traits<typename std::iterator_traits<PlaneIterator>::value_type>::Kernel::Point_3>
halfspace_intersection_interior_point_3(PlaneIterator begin, PlaneIterator end)
{
  // Types
  typedef typename Kernel_traits<typename std::iterator_traits<PlaneIterator>::value_type>::Kernel K;

  // choose exact integral type
  typedef typename internal::Exact_field_selector<typename K::FT>::Type ET;

  // find a point inside the intersection
  internal::Interior_polyhedron_3<K, ET> interior;
  if (!interior.find(begin, end))
    return std::nullopt;

  return std::make_optional(interior.inside_point());
}

} // namespace CGAL

#include <CGAL/enable_warnings.h>

#endif // CGAL_CH3_DUAL_HALFSPACE_INTERIOR_POINT_3_H


