/* randist/weibull.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>

/* The Weibull distribution has the form,

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

 */

double

gsl_ran_weibull (const gsl_rng * r, const double a, const double b)

{

  double x = gsl_rng_uniform_pos (r);

  double z = pow (-log (x), 1 / b);

  return a * z;

}

double

gsl_ran_weibull_pdf (const double x, const double a, const double b)

{

  if (x < 0)

    {

      return 0 ;

    }

  else if (x == 0)

    {

      if (b == 1)

        return 1/a ;

      else

        return 0 ;

    }

  else if (b == 1)

    {

      return exp(-x/a)/a ;

    }

  else

    {

      double p = (b/a) * exp (-pow (x/a, b) + (b - 1) * log (x/a));

      return p;

    }

}