// Copyright (c) 1997-2001  ETH Zurich (Switzerland).
// All rights reserved.
//
// This file is part of CGAL (www.cgal.org).
//
// $URL: https://github.com/CGAL/cgal/blob/v5.6.1/Bounding_volumes/include/CGAL/Approximate_min_ellipsoid_d.h $
// $Id: Approximate_min_ellipsoid_d.h 62a31ba 2022-11-14T17:44:33+01:00 albert-github
// SPDX-License-Identifier: GPL-3.0-or-later OR LicenseRef-Commercial
//
//
// Author(s)     : Kaspar Fischer <fischerk@inf.ethz.ch>

#ifndef CGAL_CGAL_APPROX_MIN_ELL_D_H
#define CGAL_CGAL_APPROX_MIN_ELL_D_H

#include <CGAL/license/Bounding_volumes.h>


#include <cstdlib>
#include <cmath>
#include <vector>
#include <iostream>

#include <CGAL/Simple_cartesian.h>
#include <CGAL/Approximate_min_ellipsoid_d/Khachiyan_approximation.h>

namespace CGAL {

  template<class Traits>
  class Approximate_min_ellipsoid_d
  // An instance of class Approximate_min_ellipsoid_d represents an
  // eps-approximation of the smallest enclosing ellipsoid of the
  // point set P in R^d, that is, an ellipsoid E satisfying
  //
  //  (i) E contains P,
  //
  //  (ii) the volume of E is at most (1+eps) times larger than the
  //    volume of the smallest enclosing ellipsoid of P.
  {
  public: // types:
    typedef typename Traits::FT                       FT;
    typedef typename Traits::Point                    Point;
    typedef typename Traits::Cartesian_const_iterator Cartesian_const_iterator;

  private:
    typedef std::vector<Point>                        Point_list;
    typedef typename Traits::Cartesian_const_iterator C_it;

  protected: // member variables:
    Traits tco;                           // traits class object
    const Point_list P;                   // the input points in R^d
    int d;                                // dimension of the input points
    const double eps;                     // the desired epsilon
    double e_eps;                         // (see below)

    // We obtain our eps-approximating ellipsoid by embedding the input
    // points P into R^{d+1} by mapping p to (p,1).  Then we compute in
    // R^{d+1} an (1+e_eps)-approximating centrally symmetric ellipsoid E for
    // the embedded points from which the desired (1+eps)-approximating
    // ellipsoid of the original points can be obtained by projection.  The
    // following variable E represents the e_eps-approximating centrally
    // symmetric ellipsoid in R^{d+1}:
    Khachiyan_approximation<true,Traits> *E;

    // When the input points do not affinely span the whole space
    // (i.e., if dim(aff(P)) < d), then the smallest enclosing
    // ellipsoid of P has no volume in R^d and so the points are
    // called "degenerate" (see is_degenerate()) below.

    // As discussed below (before (*)), the centrally symmetric ellipsoid
    // E':= sqrt{(1+a_eps)(d+1)} E contains (under exact arithmetic) the
    // embedded points. (Here, a_eps is the value obtained by
    // achieved_epsilon().) Denote by M the defining matrix of E; we
    // then have
    //
    //      p^T M p <= alpha,
    //
    // for all p in E and alpha = (1+a_eps)(d+1).  Since this is equivalent
    // to
    //
    //      p^T M/alpha p <= 1,                                 (***)
    //
    // we see that E' = sqrt{alpha} E has M/alpha as its defining
    // matrix. Consequently, when we return M in the routines
    // defining_matrix(), defining_vector(), and defining_scalar() below,
    // these numbers need to be scaled (by the user) with the factor
    // 1/alpha. (We do not perform the scaling ourselves because we cannot do
    // it exactly in double arithmetic.)

  public: // construction & destruction:

    template<typename InputIterator>
    Approximate_min_ellipsoid_d(double eps,
                            InputIterator first,InputIterator last,
                            const Traits& traits = Traits())
    // Given a range [first,last) of n points P, constructs an
    // (1+eps)-approximation of the smallest enclosing ellipsoid of P.
      : tco(traits), P(first,last), eps(eps),
        has_center(false), has_axes(false)
    {
      CGAL_APPEL_LOG("appel",
                     "Entering Approximate_min_ellpsoid_d." << std::endl);

      // fetch ambient dimension:
      d = tco.dimension(P[0]);
      CGAL_APPEL_ASSERT(d >= 2 || eps >= 0.0);


      // The ellipsoid E produced by Khachiyan's algorithm has the
      // property that E':= sqrt{(1+e_eps) (d+1)} E contains all
      // embedded points e_P and has volume bounded by
      //
      //   vol(E') <= (1+e_eps)^{(d+1)/2} vol(E_emb*),            (*)
      //
      // where E_emb* is the smallest centrally symmetric enclosing
      // ellipsoid of the embedded points e_P.  By requiring that (1+eps)
      // <= (1+e_eps)^{(d+1)/2}, we get
      //
      //   e_eps <= (1+eps)^{2/(d+1)} - 1                         (**)
      //
      // and with this e_eps, we have vol(E') <= (1+eps) vol(E_emb*).
      // An argument by Khachiyan (see "Rounding of polytopes in the
      // real number model of computation", eq. (4.1)) then guarantees
      // that the intersection E_int of E' with the hyperplane { (x,y)
      // in R^{d+1} | y = 1} is an ellipsoid satisfying vol(E_int) <=
      // (1+eps) vol(E*), where E* is the smallest enclosing ellipsoid
      // of the original points P.  According to (**) we thus set e_eps
      // to (a lower bound on) (1+eps)^{2/(d+1)} - 1:
      FPU_CW_t old = FPU_get_and_set_cw(CGAL_FE_TOWARDZERO); // round to zero
      e_eps = std::exp(2.0/(d+1)*std::log(1.0+eps))-1.0;
      FPU_set_cw(old);                                       // restore
                                                             // rounding mode

      // Find e (1+e_eps)-approximation for the embedded points.  This
      // only works when the points affinely span R^{d+1}.
      E = new Khachiyan_approximation<true,Traits>(d, static_cast<int>(P.size()),tco);
      const bool is_deg = !E->add(P.begin(),P.end(),e_eps);

      // debugging:
      CGAL_APPEL_ASSERT(is_deg == E->is_degenerate());
      CGAL_APPEL_LOG("appel",
                     "  Input points are " << (is_deg? "" : "not ") <<
                     "degenerate." << std::endl);

      if (is_deg)
        find_lower_dimensional_approximation();

      CGAL_APPEL_LOG("appel",
                     "Leaving Approximate_min_ellipsoid_d." << std::endl);
    }

    ~Approximate_min_ellipsoid_d()
    {
      // dispose of approximation:
      if (E != static_cast<Khachiyan_approximation<true,Traits> *>(0))
        delete E;
    }

  public: // access:

    unsigned int number_of_points() const
    // Returns the number of points, i.e., |P|.
    {
      return P.size();
    }

    bool is_empty() const
    // Returns true iff the approximate ellipsoid is empty (which
    // implies degeneracy).  This is the case iff the number of input
    // points was zero at construction time.
    {
      return P.size() == 0;
    }

  private: // access:
    bool is_degenerate() const
    // Returns true iff the approximate ellipsoid is degenerate, i.e.,
    // iff the dimension of the affine hull of S doesn't match the
    // dimension of the ambient space.
    {
      return E->is_degenerate();
    }

  public: // access:

    // Here's how the routines defining_matrix(), defining_vector(),
    // and defining_scalar() are implemented. From (***) we know that
    // the ellipsoid E' = sqrt{alpha} E
    //
    //  (a) encloses all embedded points (p,1), p in P,
    //  (b) has defining matrix M/alpha, i.e.,
    //
    //        E' = { x | x^T M/alpha x <= 1 },
    //
    //      where alpha = (1+a_eps)(d+1) with a_eps the return value
    //      of achieved_epsilon().
    //
    // The ellipsoid E* we actuallly want is the intersection of E' with
    // the hyperplane { (y,z) in R^{d+1} | y = 1}.  Writing
    //
    //       [ M'  m  ]                 [ y ]
    //   M = [ m^T nu ]      and    x = [ 1 ]                           (*****)
    //
    // we thus obtain
    //
    //   x^T M/alpha x = y^T y alpha/M + 2/alpha y^Tm + nu/alpha.
    //
    // It follows
    //
    //   E* = { y | y^T M'/alpha y + 2/alpha y^Tm + (nu/alpha-1) <= 0 }. (****)
    //
    // This is what the routines defining_matrix(), defining_vector(),
    // and defining_scalar() implement.

    bool is_full_dimensional() const
    // Returns !is_degenerate().
    {
      return !is_degenerate();
    }

    FT defining_matrix(int i,int j) const
    // Returns the entry M(i,j) of the symmetric matrix M in the
    // representation
    //
    //    E* = { x | x^T M x + x^T m + mu <= 0 }
    //
    // of the computed approximation. More precisely, the routine does not
    // return M(i,j) but the number (1+achieved_epsilon())*(d+1)*M(i,j).
    //
    // Precondition: !is_degenerate() && 0<=i<d && 0<=j<d
    {

      CGAL_APPEL_ASSERT(!is_degenerate() && 0<=i && i<d && 0<=j && j<d);
      return E->matrix(i,j);
    }

    FT defining_vector(int i) const
    // Returns the entry m(i) of the vector m in the representation
    //
    //    E* = { x | x^T M x + x^T m + mu <= 0 }
    //
    // of the computed approximation. More precisely, the routine does not
    // return m(i) but the number (1+achieved_epsilon())*(d+1)*m(i).
    //
    // Precondition: !is_degenerate() && 0<=i<d
    {
      CGAL_APPEL_ASSERT(!is_degenerate() && 0<=i && i<d);
      return FT(2)*E->matrix(d,i); // Note: if FT is double, the
                                   // multiplication by 2.0 is exact.
    }

    FT defining_scalar() const
    // Returns the number mu in the representation
    //
    //    E* = { x | x^T M x + x^T m + mu <= 0}
    //
    // of the computed approximation. More precisely, the routine does not
    // return mu but the number (1+achieved_epsilon())*(d+1)*(mu+1).
    //
    // Precondition: !is_degenerate()
    {
      CGAL_APPEL_ASSERT(!is_degenerate());
      return E->matrix(d,d);
    }

    double achieved_epsilon() const
    // Returns the approximation ratio; more precisely, this returns a
    // number r such that the computed ellipsoid is a (1+r)-approximation
    // of MEL(P).
    //
    // Precondition: !is_degenerate()
    {
      CGAL_APPEL_ASSERT(!is_degenerate());

      // From (*) we known that Khachian's algorithm produces a
      // centrally-symmetric ellipsoid in R^{d+1} fulfilling
      //
      //  vol(E') <= (1+k_eps)^{(d+1)/2} vol(E_emb*),
      //
      // where k_eps = E->exact_epsilon(). The projecting argument
      // mentioned after (**) says that these approximation ratio also
      // applies to the projected (d-dimensional) ellipsoids, and so the
      // actual approximation ratio we obtain is
      //
      //   ratio:= (1+k_eps)^{(d+1)/2}.
      //
      // So all we need to do is compute the smallest (let's say: a small)
      // double number larger or equal to ratio.
      //
      // Todo: make the calculation below more stable, numerically?
      const double k_eps = E->exact_epsilon();
      FPU_CW_t old = FPU_get_and_set_cw(CGAL_FE_UPWARD); // round up
      const double sum = 1.0 + k_eps;
      double tmp = sum;
      for (int i=0; i<d; ++i)
        tmp *= sum;
      double eps = std::sqrt(tmp)-1.0;
      FPU_set_cw(old);                                   // restore

      if (CGAL::is_negative(eps)) {
        CGAL_APPEL_LOG("appel", "Clamp negative approximate eps to zero" << "\n");
        eps = 0;
      }
      return eps;
    }

    Traits traits() const
    {
      return tco;
    }


    int dimension() const
    {
      return d;
    }

  public: // miscellaneous:

    bool is_valid(bool verbose) const
    // Returns true if and only if the computed ellipsoid is indeed an
    // approximate ellipsoid, that is ... Todo.
    {
      return E->is_valid(verbose);
    }

  public: // miscellaneous 2D/3D support:

    typedef std::vector<double>::const_iterator Center_coordinate_iterator;
    typedef std::vector<double>::const_iterator Axes_lengths_iterator;
    typedef std::vector<double>::const_iterator
                                            Axes_direction_coordinate_iterator;

    Center_coordinate_iterator center_cartesian_begin()
    // Returns a STL random-access iterator pointing to the first of the d
    // Cartesian coordinates of the computed ellipsoid's center.  The center
    // described in this way is a floating-point approximation to the
    // ellipsoid's exact center; no guarantee is given w.r.t. the involved
    // relative error.
    //
    // Precondition: !is_degenerate()
    {
      CGAL_APPEL_ASSERT(!is_degenerate());
      if (!has_center)
        compute_center();

      return center_.begin();
    }

    Center_coordinate_iterator center_cartesian_end()
    // Returns the past-the-end iterator corresponding to
    // center_cartesian_begin().
    //
    // Precondition: !is_degenerate()
    {
      CGAL_APPEL_ASSERT(!is_degenerate());
      if (!has_center)
        compute_center();

      return center_.end();
    }

    Axes_lengths_iterator axes_lengths_begin()
    // Returns a STL random-access iterator to the first of the d lengths of
    // the computed ellipsoid's axes. The d lengths are floating-point
    // approximations to the exact axes-lengths of the computed ellipsoid; no
    // guarantee is given w.r.t. the involved relative error. (See also method
    // `axis_direction_cartesian_begin()`.)  The elements of the iterator are
    // sorted descending.
    //
    // Precondition: !is_degenerate() && (d==2 || d==3)
    {
      CGAL_APPEL_ASSERT(!is_degenerate() && (d==2 || d==3));
      if (!has_axes)
        compute_axes_2_3();

      return lengths_.begin();
    }

    Axes_lengths_iterator axes_lengths_end()
    // Returns the past-the-end iterator corresponding to
    // axes_lengths_begin().
    //
    // Precondition: !is_degenerate() && (d==2 || d==3)
    {
      CGAL_APPEL_ASSERT(!is_degenerate() && (d==2 || d==3));
      if (!has_axes)
        compute_axes_2_3();

      return lengths_.end();
    }

    Axes_direction_coordinate_iterator axis_direction_cartesian_begin(int i)
    // Returns a STL random-access iterator pointing to the first of the d
    // Cartesian coordinates of the computed ellipsoid's i-th axis direction
    // (i.e., unit vector in direction of the ellipsoid's i-th axis).  The
    // direction described by this iterator is a floating-point approximation
    // to the exact axis direction of the computed ellipsoid; no guarantee is
    // given w.r.t. the involved relative error.  An approximation to the
    // length of axis i is given by the i-th entry of axes_lengths_begin().
    //
    // Precondition: !is_degenerate() && (d==2 || d==3) && (0 <= i < d)
    {
      CGAL_APPEL_ASSERT(!is_degenerate() && (d==2 || d==3) &&
                        0 <= i && i < d);
      if (!has_axes)
        compute_axes_2_3();

      return directions_[i].begin();
    }

    Axes_direction_coordinate_iterator axis_direction_cartesian_end(int i)
    // Returns the past-the-end iterator corresponding to
    // axis_direction_cartesian_begin().
    //
    // Precondition: !is_degenerate() && (d==2 || d==3) && (0 <= i < d)
    {
      CGAL_APPEL_ASSERT(!is_degenerate() && (d==2 || d==3) &&
                        0 <= i && i < d);
      if (!has_axes)
        compute_axes_2_3();

      return directions_[i].end();
    }

  public: // internal members for 2D/3D axis/center computation:

    bool has_center, has_axes; // true iff the center or axes-directions and
                               // -lengths, respectively, have already been
                               // computed
    std::vector<double>                center_;
    std::vector<double>                lengths_;
    std::vector< std::vector<double> > directions_;
    std::vector<double>                mi; // contains M^{-1} (see (*****)
                                           // above) iff has_center is true;
                                           // mi[i+d*j] is entry (i,j) of
                                           // M^{-1}

    void compute_center();
    void compute_axes_2_3();
    void compute_axes_2(const double alpha, const double factor);
    void compute_axes_3(const double alpha, const double factor);

  public: // "debugging" routines:

    void write_eps(const std::string& name);
    // Writes the and the point set P to an EPS file.  Returned is the
    // id under which the file was stored (filename 'id.eps').
    //
    // Precondition: d==2 && !is_degenerate().

  private: // internal routines:

    void find_lower_dimensional_approximation(); // (does nothing right now)
  };

}

#include <CGAL/Approximate_min_ellipsoid_d/Approximate_min_ellipsoid_d_impl.h>

#endif // CGAL_CGAL_APPROX_MIN_ELL_D_H

