/* randist/chisq.c

 * 

 * Copyright (C) 1996, 1997, 1998, 1999, 2000, 2007 James Theiler, Brian Gough

 * 

 * 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 3 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., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.

 */

#include <config.h>

#include <math.h>

#include <gsl/gsl_sf_gamma.h>

#include <gsl/gsl_rng.h>

#include <gsl/gsl_randist.h>

/* The chisq distribution has the form

   p(x) dx = (1/(2*Gamma(nu/2))) (x/2)^(nu/2 - 1) exp(-x/2) dx

   for x = 0 ... +infty */

double

gsl_ran_chisq (const gsl_rng * r, const double nu)

{

  double chisq = 2 * gsl_ran_gamma (r, nu / 2, 1.0);

  return chisq;

}

double

gsl_ran_chisq_pdf (const double x, const double nu)

{

  if (x < 0)

    {

      return 0 ;

    }

  else

    {

      if(nu == 2.0)

        {

          return exp(-x/2.0) / 2.0;

        }

      else

        {

          double p;

          double lngamma = gsl_sf_lngamma (nu / 2);

          p = exp ((nu / 2 - 1) * log (x/2) - x/2 - lngamma) / 2;

          return p;

        }

    }

}