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/* Author: Mark Moll */

#ifndef OMPL_DATASTRUCTURES_GREEDY_K_CENTERS_
#define OMPL_DATASTRUCTURES_GREEDY_K_CENTERS_

#include "ompl/util/RandomNumbers.h"
#include <functional>
#include <boost/numeric/ublas/matrix.hpp>

namespace ompl
{
    /** \brief An instance of this class can be used to greedily select a given
        number of representatives from a set of data points that are all far
        apart from each other. */
    template <typename _T>
    class GreedyKCenters
    {
    public:
        /** \brief The definition of a distance function */
        using DistanceFunction = std::function<double(const _T &, const _T &)>;
        /** \brief A matrix type for storing distances between points and centers */
        using Matrix = boost::numeric::ublas::matrix<double>;

        GreedyKCenters() = default;

        virtual ~GreedyKCenters() = default;

        /** \brief Set the distance function to use */
        void setDistanceFunction(const DistanceFunction &distFun)
        {
            distFun_ = distFun;
        }

        /** \brief Get the distance function used */
        const DistanceFunction &getDistanceFunction() const
        {
            return distFun_;
        }

        /** \brief Greedy algorithm for selecting k centers
            \param data a vector of data points
            \param k the desired number of centers
            \param centers a vector of length k containing the indices into
                data of the k centers
            \param dists a matrix such that dists(i,j) is the distance
                between data[i] and data[center[j]]
        */
        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> minDist(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(rng_.uniformInt(0, data.size() - 1));
            for (unsigned i = 1; i < k; ++i)
            {
                unsigned ind;
                const _T &center = data[centers[i - 1]];
                double maxDist = -std::numeric_limits<double>::infinity();
                for (unsigned j = 0; j < data.size(); ++j)
                {
                    if ((dists(j, i - 1) = distFun_(data[j], center)) < minDist[j])
                        minDist[j] = dists(j, i - 1);
                    // the j-th center is the one furthest away from center 0,..,j-1
                    if (minDist[j] > maxDist)
                    {
                        ind = j;
                        maxDist = minDist[j];
                    }
                }
                // no more centers available
                if (maxDist < 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:
        /** \brief The used distance function */
        DistanceFunction distFun_;

        /** Random number generator used to select first center */
        RNG rng_;
    };
}

#endif
