/* cdf/geometric.c

 *

 * Copyright (C) 2004 Jason H. Stover.

 * Copyright (C) 2010 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_math.h>

#include <gsl/gsl_errno.h>

#include <gsl/gsl_cdf.h>

#include "error.h"

/* Pr (X <= k), i.e., the probability of n or fewer failures until the

   first success. */

double

gsl_cdf_geometric_P (const unsigned int k, const double p)

{

  double P, a, q;

  if (p > 1.0 || p < 0.0)

    {

      CDF_ERROR ("p < 0 or p > 1", GSL_EDOM);

    }

  if (k < 1)

    {

      return 0.0;

    }

  q = 1.0 - p;

  a = (double) k;

  if (p < 0.5)

    {

      P = -expm1 (a * log1p (-p));

    }

  else

    {

      P = 1.0 - pow (q, a);

    }

  return P;

}

double

gsl_cdf_geometric_Q (const unsigned int k, const double p)

{

  double Q, a, q;

  if (p > 1.0 || p < 0.0)

    {

      CDF_ERROR ("p < 0 or p > 1", GSL_EDOM);

    }

  if (k < 1)

    {

      return 1.0;

    }

  q = 1.0 - p;

  a = (double) k;

  if (p < 0.5)

    {

      Q = exp (a * log1p (-p));

    }

  else

    {

      Q = pow (q, a);

    }

  return Q;

}