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/*

 *  Mathlib : A C Library of Special Functions

 *  Copyright (C) 1998 Ross Ihaka

 *  Copyright (C) 2000 The R Development 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

 *  http://www.r-project.org/Licenses/

 *

 *  DESCRIPTION

 *

 *	The density of the exponential distribution.

 */

#include "nmath.h"

#include "dpq.h"

double dexp(double x, double scale, int give_log)

{

#ifdef IEEE_754

    /* NaNs propagated correctly */

    if (ISNAN(x) || ISNAN(scale)) return x + scale;

#endif

    if (scale <= 0.0) ML_ERR_return_NAN;

    if (x < 0.)

	return R_D__0;

    return (give_log ?

	    (-x / scale) - log(scale) :

	    exp(-x / scale) / scale);

}