// Copyright (c) 2019-2022 Google LLC (USA).
// Copyright (c) 2025 GeometryFactory (France)
// All rights reserved.
//
// This file is part of CGAL (www.cgal.org).
//
// $URL: https://github.com/CGAL/cgal/blob/v6.2-beta1/Alpha_wrap_2/include/CGAL/Alpha_wrap_2/internal/offset_intersection.h $
// $Id: include/CGAL/Alpha_wrap_2/internal/offset_intersection.h 431cfb5357d $
// SPDX-License-Identifier: GPL-3.0-or-later OR LicenseRef-Commercial
//
// Author(s)     : Mael Rouxel-Labbé
//
#ifndef CGAL_ALPHA_WRAP_2_INTERNAL_OFFSET_INTERSECTION_H
#define CGAL_ALPHA_WRAP_2_INTERNAL_OFFSET_INTERSECTION_H

#include <CGAL/license/Alpha_wrap_2.h>

#include <CGAL/number_utils.h>

namespace CGAL {
namespace Alpha_wraps_2 {
namespace internal {

template <typename AABBTree,
          typename AABBTraversalTraits>
struct AABB_tree_oracle_helper;

template <typename AABBTree, typename AABBTraversalTraits>
struct AABB_distance_oracle
{
  using FT = typename AABBTree::FT;
  using Point_2 = typename AABBTree::Point;

  using AABB_helper = AABB_tree_oracle_helper<AABBTree, AABBTraversalTraits>;

  AABB_distance_oracle(const AABBTree& tree) : tree(tree) { }

  FT operator()(const Point_2& p) const
  {
    return approximate_sqrt(AABB_helper::squared_distance(p, tree));
  }

public:
  const AABBTree& tree;
};

// @todo even with EPECK, the precision cannot be 0 (otherwise it will not converge),
// thus exactness is pointless. Might as well use a cheap kernel (e.g. SC<double>), as long
// as there exists a mechanism to catch when the cheap kernel fails to converge (iterations?)
template <class Kernel, class DistanceOracle>
class Offset_intersection
{
  using FT = typename Kernel::FT;
  using Point_2 = typename Kernel::Point_2;
  using Vector_2 = typename Kernel::Vector_2;

public:
  Offset_intersection(const DistanceOracle& oracle,
                      const FT& off,
                      const FT& prec)
    : dist_oracle(oracle), offset(off), precision(prec)
  {
    CGAL_assertion(offset > precision);
  }

  bool first_intersection(const Point_2& s,
                          const Point_2& t,
                          Point_2& output_pt)
  {
    return sphere_marching_search(s, t, output_pt);
  }

private:
  Point_2 source;
  Point_2 target;
  FT seg_length;
  Vector_2 seg_unit_v;
  DistanceOracle dist_oracle;
  FT offset;
  FT precision;

  bool sphere_marching_search(const Point_2& s,
                              const Point_2& t,
                              Point_2& output_pt)
  {
#ifdef CGAL_AW2_DEBUG_SPHERE_MARCHING
    std::cout << "Sphere march between " << s << " and " << t << std::endl;
#endif

    Point_2 current_pt = s;
    Point_2 closest_point = dist_oracle.tree.closest_point(current_pt);
    FT current_dist = approximate_sqrt(squared_distance(current_pt, closest_point)) - offset;

    const FT sq_seg_length = squared_distance(s, t);
    const FT seg_length = approximate_sqrt(sq_seg_length);
    if(is_zero(seg_length))
    {
      closest_point = dist_oracle.tree.closest_point(t);
      current_dist = approximate_sqrt(squared_distance(t, closest_point)) - offset;
      output_pt = t;
      return (CGAL::abs(current_dist) < precision);
    }

    const Vector_2 seg_unit_v = (t - s) / seg_length;

    for(;;)
    {
#ifdef CGAL_AW2_DEBUG_SPHERE_MARCHING
      std::cout << "current point " << current_pt << std::endl;
      std::cout << "current dist " << current_dist << std::endl;
#endif

      if(CGAL::abs(current_dist) < precision)
      {
        output_pt = current_pt;
        return true;
      }

      // use the previous closest point as a hint: it's an upper bound
      current_pt = current_pt + (current_dist * seg_unit_v);

      if(squared_distance(s, current_pt) > sq_seg_length)
        return false;

      closest_point = dist_oracle.tree.closest_point(current_pt, closest_point /*hint*/);
      current_dist = approximate_sqrt(squared_distance(current_pt, closest_point)) - offset;
    }

    return false;
  }

  // @fixme somehow this is slower than the naive version...
  bool sphere_marching_search_pp(const Point_2& s,
                                const Point_2& t,
                                Point_2& output_pt)
  {
    // Initial evaluation at s
    Point_2 closest_point = dist_oracle.tree.closest_point(s);
    FT initial_dist = approximate_sqrt(squared_distance(s, closest_point)) - offset;

    const FT sq_seg_length = squared_distance(s, t);
    const FT seg_length = approximate_sqrt(sq_seg_length);
    if(is_zero(seg_length))
    {
      closest_point = dist_oracle.tree.closest_point(t);
      FT dist = approximate_sqrt(squared_distance(t, closest_point)) - offset;
      output_pt = t;
      return (CGAL::abs(dist) < precision);
    }

    const Vector_2 seg_unit_v = (t - s) / seg_length;

    const FT function_sign = initial_dist < 0 ? -1 : 1;

    FT w = 1.6;
    FT step = 0;
    FT step_length = 0;

    FT candidate_error = std::numeric_limits<FT>::infinity();
    FT candidate_step = 0;
    Point_2 candidate_pt = s;
    FT previous_radius = 0;

    const FT t_max = 2 * seg_length;

    for (;;)
    {
      Point_2 current_pt = s + (step * seg_unit_v);
#ifdef CGAL_AW2_DEBUG_SPHERE_MARCHING
      std::cout << "current_pt = " << current_pt << std::endl;
#endif

      closest_point = dist_oracle.tree.closest_point(current_pt, closest_point);
      FT dist = approximate_sqrt(squared_distance(current_pt, closest_point)) - offset;

      FT signed_radius = function_sign * dist;
      FT radius = CGAL::abs(signed_radius);

      const bool sor_fail = w > 1 && (radius + previous_radius) < step_length;

      if(sor_fail)
      {
        step_length -= w * step_length;
        w = 1;
      }
      else
      {
        step_length = signed_radius * w;
      }

      previous_radius = radius;

      if(step <= seg_length)
      {
        FT error = radius;
        if(!sor_fail && error < candidate_error)
        {
          candidate_step = step;
          candidate_error = error;
          candidate_pt = current_pt;
        }
      }

      if(!sor_fail && candidate_error < precision)
        break;

      step += step_length;
    }

    if(candidate_error < precision)
    {
      output_pt = candidate_pt;
      return true;
    }

    return false;
  }
};

} // namespace internal
} // namespace Alpha_wraps_2
} // namespace CGAL

#endif // CGAL_ALPHA_WRAP_2_INTERNAL_OFFSET_INTERSECTION_H
