/* randist/logarithmic.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>

/* Logarithmic distribution 

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

   We use Kemp's second accelerated generator, from Luc Devroye's book

   on "Non-Uniform Random Variate Generation", Springer */

unsigned int

gsl_ran_logarithmic (const gsl_rng * r, const double p)

{

  double c = log (1-p) ;

  double v = gsl_rng_uniform_pos (r);

  if (v >= p)

    {

      return 1 ;

    }

  else

    {

      double u = gsl_rng_uniform_pos (r);      

      double q = 1 - exp (c * u);

      if (v <= q*q)

        {

          double x = 1 + log(v)/log(q) ;

          return x ;

        }

      else if (v <= q)

        {

          return 2;

        }

      else

        {

          return 1 ;

        }

    }

}

double

gsl_ran_logarithmic_pdf (const unsigned int k, const double p)

{

  if (k == 0)

    {

      return 0 ;

    }

  else 

    {

      double P = pow(p, (double)k) / (double) k / log(1/(1-p)) ;

      return P;

    }

}