MEB_filtration.h

/*    This file is part of the Gudhi Library - https://gudhi.inria.fr/ - which is released under MIT.
 *    See file LICENSE or go to https://gudhi.inria.fr/licensing/ for full license details.
 *    Author(s):       Marc Glisse
 *
 *    Copyright (C) 2023 Inria
 *
 *    Modification(s):
 *      - 2024/10 Vincent Rouvreau: Add Output_squared_values argument to enable/disable squared radii computation
 *      - YYYY/MM Author: Description of the modification
 */
 
#ifndef MEB_FILTRATION_H_
#define MEB_FILTRATION_H_
 
#include <CGAL/NT_converter.h>
 
#include <vector>
#include <utility>  // for std::pair
#include <cmath>  // for std::sqrt
 
namespace Gudhi::cech_complex {

template<bool Output_squared_values = true, typename Kernel, typename SimplicialComplexForMEB, typename PointRange>
void assign_MEB_filtration(Kernel&&k, SimplicialComplexForMEB& complex, PointRange const& points, bool exact = false) {
  using Point_d = typename Kernel::Point_d;
  using FT = typename Kernel::FT;
  using Sphere = std::pair<Point_d, FT>;
 
  using Vertex_handle = typename SimplicialComplexForMEB::Vertex_handle;
  using Simplex_handle = typename SimplicialComplexForMEB::Simplex_handle;
  using Filtration_value = typename SimplicialComplexForMEB::Filtration_value;
 
  // For users to be able to define their own sqrt function on their desired Filtration_value type
  using std::sqrt;
 
  std::vector<Sphere> cache_;
  std::vector<Point_d> pts;
  CGAL::NT_converter<FT, Filtration_value> cvt;
 
  // This block is only needed to get ambient_dim
  if(std::begin(points) == std::end(points)) {
    // assert(complex.is_empty());
    return;
  }
  int ambient_dim = k.point_dimension_d_object()(*std::begin(points));
 
  auto fun = [&](Simplex_handle sh, int dim){
    using std::max;
    if (dim == 0) complex.assign_filtration(sh, 0);
    else if (dim == 1) {
      // For a Simplex_tree, this would be a bit faster, but that's probably negligible
      // Vertex_handle u = sh->first; Vertex_handle v = self_siblings(sh)->parent();
      auto verts = complex.simplex_vertex_range(sh);
      auto vert_it = verts.begin();
      Vertex_handle u = *vert_it;
      Vertex_handle v = *++vert_it;
      auto&& pu = points[u];
      Point_d m = k.midpoint_d_object()(pu, points[v]);
      FT r = k.squared_distance_d_object()(m, pu);
      if (exact) CGAL::exact(r);
      complex.assign_key(sh, cache_.size());
      Filtration_value filt{max(cvt(r), Filtration_value(0))};
      if constexpr (!Output_squared_values)
        filt = sqrt(filt);
      complex.assign_filtration(sh, filt);
      cache_.emplace_back(std::move(m), std::move(r));
    } else if (dim > ambient_dim) {
      // The sphere is always defined by at most d+1 points
      Filtration_value maxf = 0; // max filtration of the faces
      for (auto face : complex.boundary_simplex_range(sh)) {
        maxf = max(maxf, complex.filtration(face));
      }
      complex.assign_filtration(sh, maxf);
    } else {
      Filtration_value maxf = 0; // max filtration of the faces
      bool found = false;
      for (auto face_opposite_vertex : complex.boundary_opposite_vertex_simplex_range(sh)) {
        maxf = max(maxf, complex.filtration(face_opposite_vertex.first));
        if (!found) {
          auto key = complex.key(face_opposite_vertex.first);
          Sphere const& sph = cache_[key];
          if (k.squared_distance_d_object()(sph.first, points[face_opposite_vertex.second]) > sph.second) continue;
          found = true;
          complex.assign_key(sh, key);
          // With exact computations, we could stop here
          // complex.assign_filtration(sh, complex.filtration(face_opposite_vertex.first)); return;
          // but because of possible rounding errors, we continue with the equivalent of make_filtration_non_decreasing
        }
      }
      if (!found) {
        // None of the faces are good enough, MEB must be the circumsphere.
        pts.clear();
        for (auto vertex : complex.simplex_vertex_range(sh))
          pts.push_back(points[vertex]);
        Point_d c = k.construct_circumcenter_d_object()(pts.begin(), pts.end());
        FT r = k.squared_distance_d_object()(c, pts.front());
        if (exact) CGAL::exact(r);
        // For Epick_d, if the circumcenter computation is too unstable, we could compute
        //   int d2 = dim * dim;
        //   Filtration_value max_sanity = maxf * d2 / (d2 - 1);
        // and use min(max_sanity, ...), which would limit how bad numerical errors can be.
        Filtration_value filt{cvt(r)};
        if constexpr (!Output_squared_values)
          filt = sqrt(max(filt, Filtration_value(0)));
        // maxf = filt except for rounding errors
        maxf = max(maxf, filt);
        complex.assign_key(sh, cache_.size());
        // We could check if the simplex is maximal and avoiding adding it to the cache in that case.
        cache_.emplace_back(std::move(c), std::move(r));
      }
      complex.assign_filtration(sh, maxf);
    }
  };
  complex.for_each_simplex(fun);
 
  // We could avoid computing maxf, but when !exact rounding errors may cause
  // the filtration values to be non-monotonous, so we would need to call
  // if (!exact) complex.make_filtration_non_decreasing();
  // which is way more costly than computing maxf. The exact case is already so
  // costly that it isn't worth maintaining code without maxf just for it.
  // Cech_complex has "free" access to the max of the faces, because
  // expansion_with_blockers computes it before the callback.
 
  // TODO: use a map if complex does not provide key?
}
}  // namespace Gudhi::cech_complex
 
#endif  // MEB_FILTRATION_H_