/* -*- mode: c++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */

/*
 Copyright (C) 2015 Johannes Göttker-Schnetmann
 Copyright (C) 2015 Klaus Spanderen

 This file is part of QuantLib, a free-software/open-source library
 for financial quantitative analysts and developers - http://quantlib.org/

 QuantLib is free software: you can redistribute it and/or modify it
 under the terms of the QuantLib license.  You should have received a
 copy of the license along with this program; if not, please email
 <quantlib-dev@lists.sf.net>. The license is also available online at
 <https://www.quantlib.org/license.shtml>.

 This program 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 license for more details.
*/


#include <ql/methods/finitedifferences/utilities/squarerootprocessrndcalculator.hpp>

#include <boost/math/distributions/non_central_chi_squared.hpp>

namespace QuantLib {

    SquareRootProcessRNDCalculator::SquareRootProcessRNDCalculator(
        Real v0, Real kappa, Real theta, Real sigma)
    : v0_(v0), kappa_(kappa), theta_(theta),
      d_(4*kappa/(sigma*sigma)), df_(d_*theta) {    }


    Real SquareRootProcessRNDCalculator::pdf(Real v, Time t) const {
        const Real e   = std::exp(-kappa_*t);
        const Real k   = d_/(1-e);
        const Real ncp = k*v0_*e;

        const boost::math::non_central_chi_squared_distribution<Real>
            dist(df_, ncp);

        return boost::math::pdf(dist, v*k) * k;
    }

    Real SquareRootProcessRNDCalculator::cdf(Real v, Time t) const {
        const Real e   = std::exp(-kappa_*t);
        const Real k   = d_/(1-e);
        const Real ncp = k*v0_*e;

        const boost::math::non_central_chi_squared_distribution<Real>
            dist(df_, ncp);

        return boost::math::cdf(dist, v*k);
    }

    Real SquareRootProcessRNDCalculator::invcdf(Real q, Time t) const {
        const Real e   = std::exp(-kappa_*t);
        const Real k   = d_/(1-e);
        const Real ncp = k*v0_*e;

        const boost::math::non_central_chi_squared_distribution<Real>
            dist(df_, ncp);

        return boost::math::quantile(dist, q) / k;
    }

    Real SquareRootProcessRNDCalculator::stationary_pdf(Real v) const {
        const Real alpha = 0.5*df_;
        const Real beta = alpha/theta_;

        return std::pow(beta, alpha)*std::pow(v, alpha-1)
                *std::exp(-beta*v-boost::math::lgamma(alpha));
    }

    Real SquareRootProcessRNDCalculator::stationary_cdf(Real v) const {
        const Real alpha = 0.5*df_;
        const Real beta = alpha/theta_;

        return boost::math::gamma_p(alpha, beta*v);
    }

    Real SquareRootProcessRNDCalculator::stationary_invcdf(Real q) const {
        const Real alpha = 0.5*df_;
        const Real beta = alpha/theta_;

        return boost::math::gamma_p_inv(alpha, q)/beta;
    }
}
