/* randist/geometric.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_rng.h>

#include <gsl/gsl_randist.h>

/* Geometric distribution (bernoulli trial with probability p) 

   prob(k) =  p (1 - p)^(k-1) for n = 1, 2, 3, ...

   It gives the distribution of "waiting times" for an event that

   occurs with probability p. */

unsigned int

gsl_ran_geometric (const gsl_rng * r, const double p)

{

  double u = gsl_rng_uniform_pos (r);

  unsigned int k;

  if (p == 1)

    {

      k = 1;

    }

  else

    {

      k = log (u) / log (1 - p) + 1;

    }

  return k;

}

double

gsl_ran_geometric_pdf (const unsigned int k, const double p)

{

  if (k == 0)

    {

      return 0 ;

    }

  else if (k == 1)

    {

      return p ;

    }

  else

    {

      double P = p * pow (1 - p, k - 1.0);

      return P;

    }

}