rnchisq.h

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// -*- mode: C++; c-indent-level: 4; c-basic-offset: 4; tab-width: 4 -*-
//
// rnchisq.h: Rcpp R/C++ interface class library -- 
//
// Copyright (C) 2010 - 2011 Douglas Bates, Dirk Eddelbuettel and Romain Francois
//
// This file is part of Rcpp.
//
// Rcpp is free software: you can redistribute it and/or modify it
// under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 2 of the License, or
// (at your option) any later version.
//
// Rcpp 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 General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with Rcpp.  If not, see <http://www.gnu.org/licenses/>.

#ifndef Rcpp__stats__random_rnchisq_h
#define Rcpp__stats__random_rnchisq_h

namespace Rcpp {
        namespace stats {

                class NChisqGenerator : public ::Rcpp::Generator<false,double> {
                public:
        
                        NChisqGenerator( double df_, double lambda_ ) : 
                                df(df_), df_2(df_ / 2.0), lambda_2(lambda_ / 2.0 ) {}
        
                        inline double operator()() const {
                                double r = ::Rf_rpois( lambda_2 ) ;
                                // if( r > 0.0 ) r = Rf_rchisq( 2. * r ) ;
                                // replace by so that we can skip the tests in rchisq
                                // because there is no point in doing them as we know the 
                                // outcome for sure
                                if( r > 0.0 ) r = ::Rf_rgamma( r, 2. ) ;
                                if (df > 0.) r += ::Rf_rgamma( df_2, 2.);
                                return r;
                        }
        
                private:
                        double df ;
                        double df_2 ;
                        double lambda_2 ;
                } ;
        } // stats

        // Please make sure you to read Section 6.3 of "Writing R Extensions"
        // about the need to call GetRNGstate() and PutRNGstate() when using 
        // the random number generators provided by R.
        inline NumericVector rnchisq( int n, double df, double lambda ){
                if (!R_FINITE(df) || !R_FINITE(lambda) || df < 0. || lambda < 0.) 
                        return NumericVector(n, R_NaN) ;
                if( lambda == 0.0 ){
                        // using the central generator, see rchisq.h
                        return NumericVector( n, stats::ChisqGenerator( df ) ) ;
                }
                return NumericVector( n, stats::NChisqGenerator( df, lambda ) ) ;
        }

        // Please make sure you to read Section 6.3 of "Writing R Extensions"
        // about the need to call GetRNGstate() and PutRNGstate() when using 
        // the random number generators provided by R.
        inline NumericVector rnchisq( int n, double df /*, double lambda = 0.0 */ ){
                if (!R_FINITE(df) || df < 0. ) 
                        return NumericVector(n, R_NaN) ;
                return NumericVector( n, stats::ChisqGenerator( df ) ) ;
        }

} // Rcpp

#endif