GreedyKCenters.h

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/* Author: Mark Moll */
 
// This file is a slightly modified version of <ompl/datastructures/GreedyKCenters.h>
 
#pragma once
 
#include <functional>
#include <random>
#include <boost/numeric/ublas/matrix.hpp>
 
namespace cached_ik_kinematics_plugin
{
template <typename _T>
class GreedyKCenters
{
public:
  using DistanceFunction = std::function<double(const _T&, const _T&)>;
  using Matrix = boost::numeric::ublas::matrix<double>;
 
  GreedyKCenters() = default;
 
  virtual ~GreedyKCenters() = default;
 
  void setDistanceFunction(const DistanceFunction& distFun)
  {
    distFun_ = distFun;
  }
 
  const DistanceFunction& getDistanceFunction() const
  {
    return distFun_;
  }
 
  void kcenters(const std::vector<_T>& data, unsigned int k, std::vector<unsigned int>& centers, Matrix& dists)
  {
    // array containing the minimum distance between each data point
    // and the centers computed so far
    std::vector<double> min_dist(data.size(), std::numeric_limits<double>::infinity());
 
    centers.clear();
    centers.reserve(k);
    if (dists.size1() < data.size() || dists.size2() < k)
      dists.resize(std::max(2 * dists.size1() + 1, data.size()), k, false);
    // first center is picked randomly
    centers.push_back(std::uniform_int_distribution<size_t>{ 0, data.size() - 1 }(generator_));
    for (unsigned i = 1; i < k; ++i)
    {
      unsigned ind = 0;
      const _T& center = data[centers[i - 1]];
      double max_dist = -std::numeric_limits<double>::infinity();
      for (unsigned j = 0; j < data.size(); ++j)
      {
        if ((dists(j, i - 1) = distFun_(data[j], center)) < min_dist[j])
          min_dist[j] = dists(j, i - 1);
        // the j-th center is the one furthest away from center 0,..,j-1
        if (min_dist[j] > max_dist)
        {
          ind = j;
          max_dist = min_dist[j];
        }
      }
      // no more centers available
      if (max_dist < std::numeric_limits<double>::epsilon())
        break;
      centers.push_back(ind);
    }
 
    const _T& center = data[centers.back()];
    unsigned i = centers.size() - 1;
    for (unsigned j = 0; j < data.size(); ++j)
      dists(j, i) = distFun_(data[j], center);
  }
 
protected:
  DistanceFunction distFun_;
 
  std::mt19937 generator_{ std::random_device{}() };
};
}  // namespace cached_ik_kinematics_plugin