bgeot_kdtree.h

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/* -*- c++ -*- (enables emacs c++ mode) */
/*===========================================================================
 
 Copyright (C) 2004-2026 Julien Pommier
 
 This file is a part of GetFEM
 
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 under  the  terms  of the  GNU  Lesser General Public License as published
 by  the  Free Software Foundation;  either version 3 of the License,  or
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 License and GCC Runtime Library Exception for more details.
 You  should  have received a copy of the GNU Lesser General Public License
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 As a special exception, you  may use  this file  as it is a part of a free
 software  library  without  restriction.  Specifically,  if   other  files
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===========================================================================*/
 
#ifndef BGEOT_KDTREE_H
#define BGEOT_KDTREE_H
 
/** @file bgeot_kdtree.h
    @author  Julien Pommier <Julien.Pommier@insa-toulouse.fr>
    @date January 2004.
    @brief Simple implementation of a KD-tree.
 
    Basically, a KD-tree is a balanced N-dimensional tree.
*/
#include "bgeot_small_vector.h"
 
namespace bgeot {
 
  /* generic node for the kdtree */
  struct kdtree_elt_base {
    enum { PTS_PER_LEAF=8 };
    unsigned n; /* 0 => is a tree node, != 0 => tree leaf storing n points */
    bool isleaf() const { return (n != 0); }
    kdtree_elt_base(unsigned n_) : n(n_) {}
    virtual ~kdtree_elt_base() {}
  };
 
  /// store a point and the associated index for the kdtree.
  /* std::pair<size_type,base_node> is not ok since it does not
     have a suitable overloaded swap function ...
  */
  struct index_node_pair {
    size_type i;
    base_node n;
    index_node_pair() {}
    index_node_pair(size_type i_, base_node n_) : i(i_), n(n_) {}
    void swap(index_node_pair& other) { std::swap(i,other.i); n.swap(other.n);}
  };
 
  /// store a set of points with associated indexes.
  typedef std::vector<index_node_pair> kdtree_tab_type;
 
  /** Balanced tree over a set of points.
 
  Once the tree have been built, it is possible to query very
  quickly for the list of points lying in a given box. Note that
  this is not a dynamic structure: once you start to call
  kdtree::points_in_box, you should not use anymore kdtree::add_point.
 
  Here is an example of use (which tries to find the mapping between
  the dof of the mesh_fem and the node numbers of its mesh):
 
  @code
  void dof_to_nodes(const getfem::mesh_fem &mf) {
    const getfem::getfem_mesh &m = mf.linked_mesh();
    bgeot::kdtree tree;
    for (dal::bv_visitor ip(m.points().index()); !ip.finished(); ++ip) {
      tree.add_point_with_id(m.points()[ip], ip);
    }
    bgeot::kdtree_tab_type t;
    for (size_type d = 0; d < mf.nb_dof(); ++d) {
      getfem::base_node P(mf.point_of_dof(d)), P2(P);
      for (unsigned i=0; i < P.size(); ++i) {
        P[i] -= 1e-12; P2[i]+= 1e-12;
      }
      tree.points_in_box(t, P, P2);
      if (t.size()) {
        assert(t.size() == 1);
        cout << " dof " << d << " maps to mesh node " << t[0].i << "\n";
      }
    }
  }
  @endcode
  */
  class kdtree {
    dim_type N; /* dimension of points */
    std::unique_ptr<kdtree_elt_base> tree;
    kdtree_tab_type pts;
  public:
    kdtree() : N(0) {}
 
    kdtree(const kdtree&) = delete;
    kdtree &operator = (const kdtree&) = delete;
 
    /// reset the tree, remove all points
    void clear() { clear_tree(); pts = kdtree_tab_type(); N = 0; }
    void reserve(size_type n) { pts.reserve(n); }
    /// insert a new point
    size_type add_point(const base_node& n) {
      size_type i = pts.size(); add_point_with_id(n,i); return i;
    }
    /// insert a new point, with an associated number.
    void add_point_with_id(const base_node& n, size_type i) {
      if (pts.size() == 0)
        N = n.size();
      else
        GMM_ASSERT2(N == n.size(), "invalid dimension");
      if (tree) clear_tree();
      pts.push_back(index_node_pair(i, n));
    }
    size_type nb_points() const { return pts.size(); }
    const kdtree_tab_type &points() const { return pts; }
    /* fills ipts with the indexes of points in the box
       [min,max] */
    void points_in_box(kdtree_tab_type &ipts,
                       const base_node &min,
                       const base_node &max);
    /* assigns at ipt the index of the nearest neighbor at location
       pos and returns the square of the distance to this point*/
    scalar_type nearest_neighbor(index_node_pair &ipt,
                                 const base_node &pos);
  private:
    typedef std::vector<size_type>::const_iterator ITER;
    void clear_tree();
  };
}
 
#endif