The R Project SVN R

/*
 *  Mathlib : A C Library of Special Functions
 *  Copyright (C) 1998 Ross Ihaka
 *  Copyright (C) 2000 The R Development Core Team
 *
 *  This program 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.
 *
 *  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
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program; if not, write to the Free Software
 *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307 USA.
 *
 *  SYNOPSIS
 *
 *    #include "Mathlib.h"
 *    double dnbinom(double x, double n, double p);
 *
 *  DESCRIPTION
 *
 *    Random variates from the negative binomial distribution.
 *
 *  NOTES
 *
 *    x = the number of failures before the n-th success
 *
 *  REFERENCE
 *
 *    Devroye, L. (1986).
 *    Non-Uniform Random Variate Generation.
 *    New York:Springer-Verlag. Page 480.
 *
 *  METHOD
 *
 *    Generate lambda as gamma with shape parameter n and scale
 *    parameter p/(1-p).  Return a Poisson deviate with mean lambda.
 */

#include "nmath.h"

double rnbinom(double n, double p)
{
    if(!R_FINITE(n) || !R_FINITE(p) || n <= 0 || p <= 0 || p >= 1)
    ML_ERR_return_NAN;

    return rpois(rgamma(n, (1 - p) / p));
}