/* randist/erlang.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 sum of N samples from an exponential distribution gives an

   Erlang distribution

   p(x) dx = x^(n-1) exp (-x/a) / ((n-1)!a^n) dx

   for x = 0 ... +infty */

double

gsl_ran_erlang (const gsl_rng * r, const double a, const double n)

{

  return gsl_ran_gamma (r, n, a);

}

double

gsl_ran_erlang_pdf (const double x, const double a, const double n)

{

  if (x <= 0) 

    {

      return 0 ;

    }

  else

    {

      double p;

      double lngamma = gsl_sf_lngamma (n);

      p = exp ((n - 1) * log (x/a) - x/a - lngamma) / a;

      return p;

    }

}