// Copyright (c) 2006,2007,2009,2010,2011 Tel-Aviv University (Israel).
// All rights reserved.
//
// This file is part of CGAL (www.cgal.org).
//
// $URL: https://github.com/CGAL/cgal/blob/v6.0.1/Arrangement_on_surface_2/include/CGAL/Arr_geometry_traits/Conic_intersections_2.h $
// $Id: include/CGAL/Arr_geometry_traits/Conic_intersections_2.h 50cfbde3b84 $
// SPDX-License-Identifier: GPL-3.0-or-later OR LicenseRef-Commercial
//
//
// Author(s): Ron Wein <wein@post.tau.ac.il>

#ifndef CGAL_CONIC_INTERSECTIONS_2_H
#define CGAL_CONIC_INTERSECTIONS_2_H

#include <CGAL/license/Arrangement_on_surface_2.h>


#include <CGAL/number_utils.h>

/*! \file
 * Implementation of functions related to the intersection of conics curves.
 */

namespace CGAL {

/*! Compute the roots of the resultants of the two bivariate polynomials:
 *   C1:  r1*x^2 + s1*y^2 + t1*xy + u1*x + v1*y + w1 = 0
 *   C2:  r2*x^2 + s2*y^2 + t2*xy + u2*x + v2*y + w2 = 0
 * \param deg1 The degree of the first curve.
 * \param deg2 The degree of the second curve.
 * \param xs Output: The real-valued roots of the polynomial, sorted in an
 *                   ascending order.
 * \pre xs must be a vector of size 4.
 * \return The number of distinct roots found.
 */
template <typename Nt_traits>
int compute_resultant_roots(const Nt_traits& nt_traits,
                            const typename Nt_traits::Integer& r1,
                            const typename Nt_traits::Integer& s1,
                            const typename Nt_traits::Integer& t1,
                            const typename Nt_traits::Integer& u1,
                            const typename Nt_traits::Integer& v1,
                            const typename Nt_traits::Integer& w1,
                            const int& deg1,
                            const typename Nt_traits::Integer& r2,
                            const typename Nt_traits::Integer& s2,
                            const typename Nt_traits::Integer& t2,
                            const typename Nt_traits::Integer& u2,
                            const typename Nt_traits::Integer& v2,
                            const typename Nt_traits::Integer& w2,
                            const int& deg2,
                            typename Nt_traits::Algebraic* xs)
{
  if ((deg1 == 2) && (deg2 == 1)) {
    // If necessary, swap roles between the two curves, so that the first
    // curve always has the minimal degree.
    return (compute_resultant_roots(nt_traits,
                                    r2, s2, t2, u2, v2, w2,
                                    deg2,
                                    r1, s1, t1, u1, v1, w1,
                                    deg1,
                                    xs));
  }

  // Act according to the degree of the first conic curve.
  typedef typename Nt_traits::Integer Integer;
  const Integer two = 2;
  Integer c[5];
  unsigned int degree = 4;
  typename Nt_traits::Algebraic* xs_end;

  if (deg1 == 1) {
    // The first curve has no quadratic coefficients, and represents a line.
    if (CGAL::sign (v1) == ZERO) {
      // The first line is u1*x + w1 = 0, therefore:
      xs[0] = nt_traits.convert(Integer(- w1)) / nt_traits.convert(u1);
      return 1;
    }

    // We can write the first curve as: y = -(u1*x + w1) / v1.
    if (deg2 == 1) {
      // The second curve is also a line. We therefore get the linear
      // equation c[1]*x + c[0] = 0:
      c[1] = v1*u2 - u1*v2;
      c[0] = v1*w2 - w1*v2;

      // Return if the two lines are parallel
      if (CGAL::sign (c[1]) == ZERO) return 0;

      xs[0] =  nt_traits.convert(Integer(- c[0])) / nt_traits.convert(c[1]);
      return 1;
    }

    // We substitute this expression into the equation of the second
    // conic, and get the quadratic equation c[2]*x^2 + c[1]*x + c[0] = 0:
    c[2] = u1*u1*s2 - u1*v1*t2 + v1*v1*r2;
    c[1] = two*u1*w1*s2 - u1*v1*v2 - v1*w1*t2 + v1*v1*u2;
    c[0] = w1*w1*s2 - v1*w1*v2 + v1*v1*w2;

    xs_end = nt_traits.solve_quadratic_equation(c[2], c[1], c[0], xs);
    return static_cast<int>(xs_end - xs);
  }

  // At this stage, both curves have degree 2. We obtain a qaurtic polynomial
  // whose roots are the x-coordinates of the intersection points.
  if (CGAL::sign (s1) == ZERO && CGAL::sign (s2) == ZERO) {
    // If both s1 and s2 are zero, we can write the two curves as:
    //   C1: (t1*x + v1)*y + (r1*x^2 + u1*x + w1) = 0
    //   C2: (t2*x + v2)*y + (r2*x^2 + u2*x + w2) = 0
    // By writing the resultant of these two polynomials we get a cubic
    // polynomial, whose coefficients are given by:
    c[3] = r2*t1 - r1*t2;
    c[2] = t1*u2 - t2*u1 + r2*v1 - r1*v2;
    c[1] = t1*w2 - t2*w1 + u2*v1 - u1*v2;
    c[0] = v1*w2 - v2*w1;

    degree = 3;
  }
  else {
    // We can write the two curves as:
    //   C1: (s1)*y^2 + (t1*x + v1)*y + (r1*x^2 + u1*x + w1) = 0
    //   C2: (s2)*y^2 + (t2*x + v2)*y + (r2*x^2 + u2*x + w2) = 0
    // By writing the resultant of these two polynomials we get a quartic
    // polynomial, whose coefficients are given by:
    c[4] = -two*s1*s2*r1*r2 + s1*t2*t2*r1 - s1*t2*t1*r2 +
      s1*s1*r2*r2 - s2*t1*r1*t2 + s2*t1*t1*r2 + s2*s2*r1*r1;

    c[3] = -t2*r1*v1*s2 - u2*t1*t2*s1 - v2*r1*t1*s2 -
      r2*t1*v2*s1 - two*s1*s2*r1*u2 - t2*u1*t1*s2 + u2*t1*t1*s2 -
      r2*v1*t2*s1 + u1*t2*t2*s1 + two*v2*r1*t2*s1 + two*u2*r2*s1*s1 +
      two*r2*v1*t1*s2 + two*u1*r1*s2*s2 - two*s1*s2*u1*r2;

    c[2] = -r2*v1*v2*s1 + u2*u2*s1*s1 + two*w2*r2*s1*s1 +
      two*u2*v1*t1*s2 - u2*v1*t2*s1 + w2*t1*t1*s2 - two*s1*s2*u1*u2 -
      w2*t1*t2*s1 + v2*v2*r1*s1 + u1*u1*s2*s2 - v2*r1*v1*s2 +
      two*w1*r1*s2*s2 - u2*t1*v2*s1 - t2*u1*v1*s2 - two*s1*s2*r1*w2 -
      two*s1*s2*w1*r2 + r2*v1*v1*s2 + w1*t2*t2*s1 - v2*u1*t1*s2 -
      t2*w1*t1*s2 + two*v2*u1*t2*s1;

    c[1] = two*w2*u2*s1*s1 + two*w2*v1*t1*s2 - w2*v1*t2*s1 +
      two*v2*w1*t2*s1 + two*w1*u1*s2*s2 - v2*u1*v1*s2 - two*s1*s2*u1*w2 -
      v2*w1*t1*s2 + u2*v1*v1*s2 - t2*w1*v1*s2 - w2*t1*v2*s1 +
      v2*v2*u1*s1 - u2*v1*v2*s1 - two*s1*s2*w1*u2;

    c[0] = s2*v1*v1*w2 - s1*v2*v1*w2 - s2*v1*w1*v2 + s2*s2*w1*w1 -
      two*s1*s2*w1*w2 + s1*w1*v2*v2 + s1*s1*w2*w2;

    degree = 4;
  }

  // Compute the roots of the resultant polynomial.
  typename Nt_traits::Polynomial poly =
    nt_traits.construct_polynomial(c, degree);

  xs_end = nt_traits.compute_polynomial_roots(poly, xs);
  return static_cast<int>(xs_end - xs);
}

/*! Compute the roots of the resultants of the two bivariate polynomials:
 *   C1:  r*x^2 + s*y^2 + t*xy + u*x + v*y + w = 0
 *   C2:  A*x + B*y + C = 0
 * \param deg1 The degree of the first curve.
 * \param xs Output: The real-valued roots of the polynomial, sorted in an
 *                   ascending order.
 * \pre xs must be a vector of size 4.
 * \return The number of distinct roots found.
 */
template <typename Nt_traits>
int compute_resultant_roots(const Nt_traits& nt_traits,
                            const typename Nt_traits::Algebraic& r,
                            const typename Nt_traits::Algebraic& s,
                            const typename Nt_traits::Algebraic& t,
                            const typename Nt_traits::Algebraic& u,
                            const typename Nt_traits::Algebraic& v,
                            const typename Nt_traits::Algebraic& w,
                            const int& deg1,
                            const typename Nt_traits::Algebraic& A,
                            const typename Nt_traits::Algebraic& B,
                            const typename Nt_traits::Algebraic& C,
                            typename Nt_traits::Algebraic* xs)
{
  if (deg1 == 1) {
    // We should actually compute the intersection of two line:
    // (u*x + v*y + w = 0) and (A*x + B*y + C = 0):
    const typename Nt_traits::Algebraic denom = A*v - B*u;

    // Return if the two lines are parallel and do not intersect.
    if (CGAL::sign(denom) == CGAL::ZERO) return 0;

    xs[0] = (B*w - C*v) / denom;
    return 1;
  }

  if (CGAL::sign(B) == CGAL::ZERO) {
    // The first line is A*x + C = 0, therefore:
    xs[0] = -C / A;
    return 1;
  }

  // We can write the first curve as: y = -(A*x + C) / B.
  // We substitute this expression into the equation of the conic, and get
  // the quadratic equation c[2]*x^2 + c[1]*x + c[0] = 0:
  const typename Nt_traits::Algebraic two = 2;
  typename Nt_traits::Algebraic c[3];
  typename Nt_traits::Algebraic* xs_end;

  c[2] = A*A*s - A*B*t + B*B*r;
  c[1] = two*A*C*s - A*B*v - B*C*t + B*B*u;
  c[0] = C*C*s - B*C*v + B*B*w;

  xs_end = nt_traits.solve_quadratic_equation(c[2], c[1], c[0], xs);
  return static_cast<int>(xs_end - xs);
}

} //namespace CGAL

#endif

