/*

 *  Mathlib : A C Library of Special Functions

 *  Copyright (C) 1998 Ross Ihaka

 *  Copyright (C) 2000-2015 The R Core Team

 *

 *  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 2 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, a copy is available at

 *  https://www.R-project.org/Licenses/

 *

 *  DESCRIPTION

 *

 *  The distribution function of the exponential distribution.

 */

#include "nmath.h"

#include "dpq.h"

double pexp(double x, double scale, int lower_tail, int log_p)

{

#ifdef IEEE_754

    if (ISNAN(x) || ISNAN(scale))

  return x + scale;

    if (scale < 0) ML_WARN_return_NAN;

#else

    if (scale <= 0) ML_WARN_return_NAN;

#endif

    if (x <= 0.)

  return R_DT_0;

    /* same as weibull( shape = 1): */

    x = -(x / scale);

    return lower_tail

  ? (log_p ? R_Log1_Exp(x) : -expm1(x))

  : R_D_exp(x);

}