poisson.h

// Copyright 2009-2020 NTESS. Under the terms
// of Contract DE-NA0003525 with NTESS, the U.S.
// Government retains certain rights in this software.
//
// Copyright (c) 2009-2020, NTESS
// All rights reserved.
//
// This file is part of the SST software package. For license
// information, see the LICENSE file in the top level directory of the
// distribution.

#ifndef _H_SST_CORE_RNG_POISSON
#define _H_SST_CORE_RNG_POISSON

#include "math.h"

#include "distrib.h"
#include "mersenne.h"

using namespace SST::RNG;

namespace SST {
namespace RNG {

/**
    \class SSTPoissonDistribution poisson.h "sst/core/rng/poisson.h"

    Creates an Poisson distribution for use within SST. This distribution is the same across
    platforms and compilers.
*/
class SSTPoissonDistribution : public SSTRandomDistribution {

    public:
        /**
            Creates an Poisson distribution with a specific lambda
            \param mn The lambda of the Poisson distribution
        */
    SSTPoissonDistribution(const double mn)  :
    SSTRandomDistribution(), lambda(mn) {

        baseDistrib = new MersenneRNG();
        deleteDistrib = true;
    }

        /**
            Creates an Poisson distribution with a specific lambda and a base random number generator
            \param lambda The lambda of the Poisson distribution
            \param baseDist The base random number generator to take the distribution from.
        */
    SSTPoissonDistribution(const double mn, SSTRandom* baseDist)  :
    SSTRandomDistribution(), lambda(mn) {

        baseDistrib = baseDist;
        deleteDistrib = false;
    }

        /**
            Destroys the Poisson distribution
        */
    ~SSTPoissonDistribution()  {
        if(deleteDistrib) {
            delete baseDistrib;
        }
    }

        /**
            Gets the next (random) double value in the distribution
            \return The next random double from the distribution
        */
     double getNextDouble() {
        const double L = exp(-lambda);
        double p = 1.0;
        int k = 0;

        do {
            k++;
            p *= baseDistrib->nextUniform();
        } while(p > L);

        return k - 1;
    }

        /**
            Gets the lambda with which the distribution was created
            \return The lambda which the user created the distribution with
        */
    double getLambda()  {
        return lambda;
    }

    protected:
        /**
            Sets the lambda of the Poisson distribution.
        */
        const double lambda;
        /**
            Sets the base random number generator for the distribution.
        */
        SSTRandom* baseDistrib;

        /**
            Controls whether the base distribution should be deleted when this class is destructed.
        */
        bool deleteDistrib;

};

}
}

#endif