Geometric.h

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/*!
 * 
 *
 * \brief       Implements a geometric distribution.
 * 
 * 
 *
 * \author      O. Krause
 * \date        2010-01-01
 *
 *
 * \par Copyright 1995-2017 Shark Development Team
 * 
 * <BR><HR>
 * This file is part of Shark.
 * <http://shark-ml.org/>
 * 
 * Shark is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published 
 * by the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 * 
 * Shark is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Lesser General Public License for more details.
 * 
 * You should have received a copy of the GNU Lesser General Public License
 * along with Shark.  If not, see <http://www.gnu.org/licenses/>.
 *
 */
#ifndef SHARK_RNG_GEOMETRIC_H
#define SHARK_RNG_GEOMETRIC_H



#include <cmath>

namespace shark{

    /**
    * \brief Implements the geometric distribution.
    *
    * Note that a support {1,2,3,...} is assumed here.
    */
    template<typename RngType = shark::DefaultRngType>
    class Geometric:public boost::variate_generator<RngType*,boost::geometric_distribution<long> > {
    private:
        typedef boost::variate_generator<RngType*,boost::geometric_distribution<long> > Base;
    public:

        /**
        * \brief C'tor, initializes the parameter p modelling the probability of success. Associates
        * the distribution with a custom RNG.
        * \param [in,out] rng The RNG to associate the distribution with.
        * \param [in] p Parameter p, the probability of success.
        */
        Geometric( RngType & rng, double p = 0.5 )
            :Base(&rng,boost::geometric_distribution<long>(1.0-p))
        {}

        /** \brief Injects the default sampling operator. */
        using Base::operator();

        /**
        * \brief Reinitializes the distribution with the supplied success probability and samples a random number.
        * \param [in] p The new success probability.
        */
        long operator()(double p) {
            boost::geometric_distribution<long> dist( p );
            return dist(Base::engine());
        }

        /**
        * \brief Accesses the success probability.
        */
        double prob() const {
            return 1-Base::distribution().p();
        }

        /**
        * \brief Adjusts the success probability.
        */
        void prob(double newMean) {
            Base::distribution()=boost::geometric_distribution<long>(1-newMean);
        }

        /**
        * \brief Calculates the probability of x.
        */
        double p( long x ) {
            return( x > 0 ? prob() * std::pow(1 - prob(), (double) x - 1) : 0 );
        }

    };
}
#endif