random_point_selection.hpp

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#ifndef MLPACK_METHODS_DBSCAN_RANDOM_POINT_SELECTION_HPP
#define MLPACK_METHODS_DBSCAN_RANDOM_POINT_SELECTION_HPP

#include <mlpack/prereqs.hpp>

namespace mlpack {
namespace dbscan {

class RandomPointSelection
{
 public:
  template<typename MatType>
  size_t Select(const size_t /* point */,
                const MatType& data)
  {
    // Initialize the length of the unvisited bitset.
    size_t size = data.n_cols; // Get the size of points.
    if (unvisited.size() != size)
      unvisited.resize(size, true); // Resize & Set bitset to one.

    // Count the unvisited points and generate nth index randomly.
    const size_t max = std::count(unvisited.begin(), unvisited.end(), true);
    const size_t index = math::RandInt(max);

    // Select the index'th unvisited point.
    size_t found = 0;
    for (size_t i = 0; i < unvisited.size(); ++i)
    {
      if (unvisited[i])
        ++found;

      if (found > index)
      {
        unvisited[i].flip(); // Set unvisited point to visited point.
        return i;
      }
    }
    return 0; // Not sure if it is possible to get here.
  }

 private:
  // Bitset for unvisited points. If true, mean unvisited.
  std::vector<bool> unvisited;
};

} // namespace dbscan
} // namespace mlpack

#endif