// Copyright (c) 1997  ETH Zurich (Switzerland).
// All rights reserved.
//
// This file is part of CGAL (www.cgal.org).
//
// $URL: https://github.com/CGAL/cgal/blob/v6.1.2/Bounding_volumes/include/CGAL/Min_sphere_of_spheres_d/Min_sphere_of_spheres_d_impl.h $
// $Id: include/CGAL/Min_sphere_of_spheres_d/Min_sphere_of_spheres_d_impl.h 37e2b278910 $
// SPDX-License-Identifier: GPL-3.0-or-later OR LicenseRef-Commercial
//
//
// Author(s)     : Kaspar Fischer


#ifndef CGAL_MINIBALL_MINIBALL_C
#define CGAL_MINIBALL_MINIBALL_C

#include <CGAL/license/Bounding_volumes.h>


#include <numeric>
#include <CGAL/Min_sphere_of_spheres_d.h>

#ifdef CGAL_MINIBALL_DEBUG
#include <iostream>
#endif

#include <cstdlib>
#include <algorithm>
#include <CGAL/Min_sphere_of_spheres_d.h>

namespace CGAL_MINIBALL_NAMESPACE {

namespace  Bounding_volumes {

  template<typename FT>
  inline bool compare(const FT& a,const FT& b,
                      const FT& ap,const FT& bp) {
    const FT u = a-ap, uu = u*u;
    if (u >= FT(0)) {
      if (bp <= uu)
        return false;

      // here (1) holds
      const FT v = uu-b+bp;
      if (v <= 0)
        return false;

      // here (2) holds
      return 4 * uu * bp < sqr(v);
    } else {
      // here (1) holds
      const FT v = uu-b+bp;
      if (v >= FT(0))
        return true;

      // here (3) holds
      return 4 * uu *bp > sqr(v);
    }
  }

} // namespace Bounding_volumes

  template<class Traits>
  void Min_sphere_of_spheres_d<Traits>::update(LP_algorithm) {
    using namespace Min_sphere_of_spheres_d_impl;
    const int n = (int)l.size();
    int i, k = n;
    do {
      CGAL_MINIBALL_ASSERT(k>=e && e>=0);

      // permute:
      for (int j=k-1; j>=e; --j) // todo. theory: is this necessary?
        std::swap(l[j],l[e+rng()%(j+1-e)]);

      for (i=e; i<n; ++i)
        if (!ss.contains(t.center_cartesian_begin(*l[i]),
                         t.radius(*l[i]),Tol<FT>::result(),Is_exact()) && ss.pivot(l,e,i)) {
          k = i+1;
          break;
        }
    } while (i < n);
  }

  template<class Traits>
  bool Min_sphere_of_spheres_d<Traits>::find_farthest(int from,int to,
                                                      int& i,const Tag_true /* use_sqrt */,
                                                      const Tag_false /* is_exact */) {
    using namespace Min_sphere_of_spheres_d_impl;

    // we will compute the excesses w.r.t. to the ball B with
    // center ss.begin() and radius radius:
    const FT radius = ss.radius() * Tol<FT>::result();

    // find ball with largest excess:
    FT maximum = radius;
    for (int k=from; k<to; ++k) {
      // compute the (squared) distance from c1 to c2:
      const FT dist = inner_product_n<D>(ss.begin(),
                                         t.center_cartesian_begin(*l[k]),FT(0),std::plus<FT>(),
        Subtract_and_square<FT>());

      // compute excess:
      using std::sqrt;
      const FT ex = sqrt(dist)+t.radius(*l[k]);

      // compare with current maximum:
      if (ex > maximum) { // (*)
        maximum = ex;
        i = k;
      }
    }

    // return whether B doesn't contain the ball l[i]:
    return maximum > radius;
  }

  template<class Traits>
  bool Min_sphere_of_spheres_d<Traits>::find_farthest(int from, int to,
                                                      int& i, const Tag_true /* use_sqrt */,
                                                      const Tag_true /* is_exact*/) {
    using namespace Min_sphere_of_spheres_d_impl;

    // we will compute the excesses w.r.t. to the ball B with
    // center center and radius radius:
    Pair_to_double<FT> cast(ss.disc());
    const double radius = cast(ss.radius());
    double center[D];
    std::transform(ss.begin(),ss.begin()+D,center,cast);

    // find ball with largest excess:
    double maximum = radius;
    for (int k=from; k<to; ++k) {
      // compute the (squared) distance from c1 to c2:
      const double dist = inner_product_n<D>(center,
         t.center_cartesian_begin(*l[k]),0.0,std::plus<double>(),
         Subtract_and_square_to_double<FT>());

      // compute excess:
      using std::sqrt;
      const double ex = sqrt(dist) +
         CGAL_MINIBALL_NTS to_double(t.radius(*l[k]));

      // compare with current maximum:
      if (ex > maximum) { // (*)
        maximum = ex;
        i = k;
      }
    }

    // return whether B doesn't contain the ball l[i]:
    return maximum > radius &&
           !ss.contains(t.center_cartesian_begin(*l[i]),
                        t.radius(*l[i]),Tol<FT>::result(),Is_exact());
  }

  template<class Traits>
  bool Min_sphere_of_spheres_d<Traits>::find_farthest(int from,int to,
                                                      int& i, const Tag_false /* use_sqrt */,
                                                      const Tag_false /* is_exact*/ ) {
    using namespace Min_sphere_of_spheres_d_impl;

    // we will compute the excesses w.r.t. to the ball with
    // center ss.begin() and radius radius:
    const FT radius = ss.radius() * Tol<FT>::result();

    // find ball with largest excess:
    bool found = false;
    FT max = radius, maxp = 0;
    for (int k=from; k<to; ++k) {
      // compute the (squared) distance from c1 to c2:
      const FT dist = inner_product_n<D>(ss.begin(),
                                         t.center_cartesian_begin(*l[k]),FT(0),std::plus<FT>(),
        Subtract_and_square<FT>());

      if (Bounding_volumes::compare(max,maxp,t.radius(*l[k]),dist)) {
        max   = t.radius(*l[k]);
        maxp  = dist;
        i     = k;
        found = true;
      }
    }

    // return whether B doesn't contain the ball l[i]:
    return found;
  }

  template<class Traits>
  bool Min_sphere_of_spheres_d<Traits>::find_farthest(int from,int to,
                                                      int& i,const Tag_false /* use_sqrt */,
                                                      const Tag_true /* is_exact */) {
    using namespace Min_sphere_of_spheres_d_impl;

    // we will compute the excesses w.r.t. to the ball B with
    // center center and radius radius:
    Pair_to_double<FT> cast(ss.disc());
    const double radius = cast(ss.radius());
    double center[D];
    std::transform(ss.begin(),ss.begin()+D,center,cast);

    // find ball with largest excess:
    bool found = false;
    double max = radius, maxp = 0;
    for (int k=from; k<to; ++k) {
      // compute the (squared) distance from c1 to c2:
      const double dist = inner_product_n<D>(center,
         t.center_cartesian_begin(*l[k]),0.0,std::plus<double>(),
         Subtract_and_square_to_double<FT>());

      const double r = CGAL_MINIBALL_NTS to_double(t.radius(*l[k]));
      if (Bounding_volumes::compare(max,maxp,r,dist)) {
        max   = r;
        maxp  = dist;
        i     = k;
        found = true;
      }
    }

    // return whether B doesn't contain the ball l[i]:
    return found &&
           !ss.contains(t.center_cartesian_begin(*l[i]),
                        t.radius(*l[i]),Tol<FT>::result(),Is_exact());
  }

  template<class Traits>
  void Min_sphere_of_spheres_d<Traits>::update(Farthest_first_heuristic) {
    const int n = (int)l.size();
    int i = e;
    CGAL_MINIBALL_ASSERT(e <= n);

    bool enclosing = (e == 0)? n == 0 :
      !find_farthest(e,n,i,Use_sqrt(),Is_exact());

    while (!enclosing && ss.pivot(l,e,i)) {
      enclosing = !find_farthest(e,n,i,Use_sqrt(),Is_exact());
    }

    if (!is_approximate(Is_exact()))
      update(LP_algorithm());
  }

  template<class Traits>
  bool Min_sphere_of_spheres_d<Traits>::
  is_valid(const Tag_false /* is_exact */) {
    return true;
  }

  template<class Traits>
  bool Min_sphere_of_spheres_d<Traits>::
    is_valid(const Tag_true /* is_exact */) {
    using namespace Min_sphere_of_spheres_d_impl;
    using std::cerr;
    using std::endl;

    // check size of support set:
    if (e > static_cast<int>(l.size()) || e > (D+1)) {
      cerr << "BUG: Min_sphere_of_spheres_d: support set too large." << endl
           << "Refer to the bug-reporting instructions at https://www.cgal.org/bug_report.html" << endl;
      return false;
    } else if (l.size() > 0 && e<=0) {
      cerr << "BUG: Min_sphere_of_spheres_d: support set too small." << endl
           << "Refer to the bug-reporting instructions at https://www.cgal.org/bug_report.html" << endl;
      return false;
    }

    // check case of no balls:
    if (l.size() <= 0) {
      if (!is_empty()) {
        cerr << "BUG: Min_sphere_of_spheres_d: miniball of {} non-empty." << endl
             << "Refer to the bug-reporting instructions at https://www.cgal.org/bug_report.html" << endl;
        return false;
      } else
        return true;
    }

    // check that the miniball is enclosing:
    for (unsigned int i=0; i<l.size(); ++i)
      if (!ss.contains(t.center_cartesian_begin(*l[i]),
                       t.radius(*l[i]),Tol<FT>::result(),Is_exact())) {
        cerr << "Min_sphere_of_spheres_d: miniball not enclosing." << endl
             << "Please contact the author <kf@iaeth.ch>." << endl;
        return false;
      }

    // check that all support balls lie on the boundary:
    typedef Pair<FT> P;
    bool isSupporting = true;
    for (int i=0; i<e; ++i) {
      // check radii:
      const P rd = ss.radius()-t.radius(*l[i]);
      if (is_neg(rd,ss.disc()))
        isSupporting = false;

      // compute the (squared) distance from ss.begin() to l[i]'s center:
      const P dist = inner_product_n<D>(ss.begin(),
        t.center_cartesian_begin(*l[i]),P(0,0),std::plus<P>(),
        Subtract_and_square_pair<FT>(ss.disc()));

      // compute the square of rd:
      const P sqrRd(sqr(rd.first)+sqr(rd.second)*ss.disc(),
                    FT(2)*rd.first*rd.second);

      // check containment:
      if (!is_zero(dist-sqrRd,ss.disc()))
        isSupporting = false;
    }
    if (!isSupporting) {
      cerr << "BUG: Min_sphere_of_spheres_d: support not on boundary." << endl
           << "Refer to the bug-reporting instructions at https://www.cgal.org/bug_report.html" << endl;
      return false;
    }

    // set up initial system's coefficient matrix:
    FT (*m)[D+1] = new FT[e][D+1];
    for (int j=0; j<e; ++j)
      copy_n<D>(t.center_cartesian_begin(*l[j]),m[j]);
    for (int j=0; j<e; ++j)
      m[j][D] = FT(1);

    // set up initial system's right-hand-side:
    Pair<FT> rhs[D+1];
    copy_n<D>(ss.begin(),rhs);
    rhs[D] = FT(1);

    // perform Gaussian elimination:
    for (int j=0; j<e; ++j) {
      // check rank:
      if (m[j][j] == FT(0)) {
        // find row with non-zero entry in column j:
        int i = j;
        while (i<D+1 && m[j][i]==FT(0))
          ++i;
        if (i >= D+1) {
          cerr << "BUG: Min_sphere_of_spheres_d: supp. centers aff. dep." << endl
               << "Refer to the bug-reporting instructions at https://www.cgal.org/bug_report.html" << endl;
          return false;
        }

        // exchange rows:
        for (int k=0; k<e; ++k)
          std::swap(m[k][j],m[k][i]);
        std::swap(rhs[j],rhs[i]);
      }
      CGAL_MINIBALL_ASSERT(m[j][j] != FT(0));

      // eliminate m[j][j+1..D] by subtracting a
      // multiple of row j from row i:
      for (int i=j+1; i<D+1; ++i) {
        // determine factor:
        const FT factor = m[j][i]/m[j][j];

        // subtract row j times factor from row i:
        for (int k=0; k<e; ++k)
          m[k][i] -= m[k][j]*factor;
        rhs[i] -= rhs[j]*factor;
      }
    }

    // check that we now have an upper triangular matrix:
    for (int j=0; j<e; ++j)
      for(int i=j+1; i<D+1; ++i)
        CGAL_MINIBALL_ASSERT(m[j][i] == FT(0));

    // check solvability:
    for (int i=e; i<D+1; ++i)
      if (!is_zero(rhs[i],ss.disc())) {
        cerr << "BUG: Min_sphere_of_spheres_d: center of the miniball" << endl
             << "     not in the span of the support centers." << endl
             << "Refer to the bug-reporting instructions at https://www.cgal.org/bug_report.html" << endl;
        return false;
      }

    // compute coefficients by backsubstitution:
    Pair<FT> lambda[D+1];
    for (int i=e-1; i>=0; --i) {
      lambda[i] = rhs[i];
      for (int j=i+1; j<e; ++j)
        lambda[i] -= lambda[j]*m[j][i];
      lambda[i] = lambda[i]/m[i][i];
    }

    // check coefficients:
    for (int i=0; i<e; ++i)
      if (is_neg_or_zero(lambda[i],ss.disc())) {
        cerr << "BUG: Min_sphere_of_spheres_d: center of miniball not in" << endl
             << "     interior of convex hull of support centers." << endl
             << "Refer to the bug-reporting instructions at https://www.cgal.org/bug_report.html" << endl;
        return false;
      }

    // tidy up:
    delete[] m;

    return true;
  }


} // namespace CGAL_MINIBALL_NAMESPACE

#endif // CGAL_MINIBALL_MINIBALL_CC
