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/* Author: Ioan Sucan */

#ifndef FCL_KNN_NEAREST_NEIGHBORS_SQRT_APPROX_H
#define FCL_KNN_NEAREST_NEIGHBORS_SQRT_APPROX_H

#include "fcl/knn/nearest_neighbors_linear.h"
#include "fcl/exception.h"
#include <algorithm>
#include <cmath>

namespace fcl
{
/** \brief A nearest neighbors datastructure that uses linear
    search. The linear search is done over sqrt(n) elements
    only. (Every sqrt(n) elements are skipped).

    \li Search for nearest neighbor is O(sqrt(n)).
    \li Search for k-nearest neighbors is  O(n log(k)).
    \li Search for neighbors within a range is O(n log(n)).
    \li Adding an element to the datastructure is O(1).
    \li Removing an element from the datastructure O(n).
*/
template<typename _T>
class NearestNeighborsSqrtApprox : public NearestNeighborsLinear<_T>
{
public:
  NearestNeighborsSqrtApprox(void) : NearestNeighborsLinear<_T>(), checks_(0), offset_(0)
  {
  }

  virtual ~NearestNeighborsSqrtApprox(void)
  {
  }

  virtual void clear(void)
  {
    NearestNeighborsLinear<_T>::clear();
    checks_ = 0;
    offset_ = 0;
  }

  virtual void add(const _T &data)
  {
    NearestNeighborsLinear<_T>::add(data);
    updateCheckCount();
  }

  virtual void add(const std::vector<_T> &data)
  {
    NearestNeighborsLinear<_T>::add(data);
    updateCheckCount();
  }

  virtual bool remove(const _T &data)
  {
    bool result = NearestNeighborsLinear<_T>::remove(data);
    if (result)
      updateCheckCount();
    return result;
  }

  virtual _T nearest(const _T &data) const
  {
    const std::size_t n = NearestNeighborsLinear<_T>::data_.size();
    std::size_t pos = n;

    if (checks_ > 0 && n > 0)
    {
      double dmin = 0.0;
      for (std::size_t j = 0 ; j < checks_ ; ++j)
      {
        std::size_t i = (j * checks_ + offset_) % n;

        double distance = NearestNeighbors<_T>::distFun_(NearestNeighborsLinear<_T>::data_[i], data);
        if (pos == n || dmin > distance)
        {
          pos = i;
          dmin = distance;
        }
      }
      offset_ = (offset_ + 1) % checks_;
    }
    if (pos != n)
      return NearestNeighborsLinear<_T>::data_[pos];

    throw Exception("No elements found in nearest neighbors data structure");
  }

protected:

  /** \brief The maximum number of checks to perform when searching for a nearest neighbor */
  inline void updateCheckCount(void)
  {
    checks_ = 1 + (std::size_t)floor(sqrt((double)NearestNeighborsLinear<_T>::data_.size()));
  }

  /** \brief The number of checks to be performed when looking for a nearest neighbor */
  std::size_t checks_;

  /** \brief The offset to start checking at (between 0 and \e checks_) */
  mutable std::size_t offset_;

};

}

#endif
