w_gamma.c

Newlib 嵌入式 C库 标准实现代码

/* @(#)w_gamma.c 5.1 93/09/24 */
/*
 * ====================================================
 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
 *
 * Developed at SunPro, a Sun Microsystems, Inc. business.
 * Permission to use, copy, modify, and distribute this
 * software is freely granted, provided that this notice 
 * is preserved.
 * ====================================================
 *
 */

/*
FUNCTION
        <<gamma>>, <<gammaf>>, <<lgamma>>, <<lgammaf>>, <<gamma_r>>,
	<<gammaf_r>>, <<lgamma_r>>, <<lgammaf_r>>---logarithmic gamma
	function
INDEX
gamma
INDEX
gammaf
INDEX
lgamma
INDEX
lgammaf
INDEX
gamma_r
INDEX
gammaf_r
INDEX
lgamma_r
INDEX
lgammaf_r

ANSI_SYNOPSIS
#include <math.h>
double gamma(double <[x]>);
float gammaf(float <[x]>);
double lgamma(double <[x]>);
float lgammaf(float <[x]>);
double gamma_r(double <[x]>, int *<[signgamp]>);
float gammaf_r(float <[x]>, int *<[signgamp]>);
double lgamma_r(double <[x]>, int *<[signgamp]>);
float lgammaf_r(float <[x]>, int *<[signgamp]>);

TRAD_SYNOPSIS
#include <math.h>
double gamma(<[x]>)
double <[x]>;
float gammaf(<[x]>)
float <[x]>;
double lgamma(<[x]>)
double <[x]>;
float lgammaf(<[x]>)
float <[x]>;
double gamma_r(<[x]>, <[signgamp]>)
double <[x]>;
int <[signgamp]>;
float gammaf_r(<[x]>, <[signgamp]>)
float <[x]>;
int <[signgamp]>;
double lgamma_r(<[x]>, <[signgamp]>)
double <[x]>;
int <[signgamp]>;
float lgammaf_r(<[x]>, <[signgamp]>)
float <[x]>;
int <[signgamp]>;

DESCRIPTION
<<gamma>> calculates 
@tex
$\mit ln\bigl(\Gamma(x)\bigr)$, 
@end tex
the natural logarithm of the gamma function of <[x]>.  The gamma function
(<<exp(gamma(<[x]>))>>) is a generalization of factorial, and retains
the property that  
@ifnottex
<<exp(gamma(N))>> is equivalent to <<N*exp(gamma(N-1))>>.
@end ifnottex
@tex
$\mit \Gamma(N)\equiv N\times\Gamma(N-1)$.
@end tex
Accordingly, the results of the gamma function itself grow very
quickly.  <<gamma>> is defined as 
@tex
$\mit ln\bigl(\Gamma(x)\bigr)$ rather than simply $\mit \Gamma(x)$
@end tex
@ifnottex
the natural log of the gamma function, rather than the gamma function
itself, 
@end ifnottex
to extend the useful range of results representable.

The sign of the result is returned in the global variable <<signgam>>,
which is declared in math.h.

<<gammaf>> performs the same calculation as <<gamma>>, but uses and
returns <<float>> values.

<<lgamma>> and <<lgammaf>> are alternate names for <<gamma>> and
<<gammaf>>.  The use of <<lgamma>> instead of <<gamma>> is a reminder
that these functions compute the log of the gamma function, rather
than the gamma function itself.

The functions <<gamma_r>>, <<gammaf_r>>, <<lgamma_r>>, and
<<lgammaf_r>> are just like <<gamma>>, <<gammaf>>, <<lgamma>>, and
<<lgammaf>>, respectively, but take an additional argument.  This
additional argument is a pointer to an integer.  This additional
argument is used to return the sign of the result, and the global
variable <<signgam>> is not used.  These functions may be used for
reentrant calls (but they will still set the global variable <<errno>>
if an error occurs).

RETURNS
Normally, the computed result is returned.  

When <[x]> is a nonpositive integer, <<gamma>> returns <<HUGE_VAL>>
and <<errno>> is set to <<EDOM>>.  If the result overflows, <<gamma>>
returns <<HUGE_VAL>> and <<errno>> is set to <<ERANGE>>.

You can modify this error treatment using <<matherr>>.

PORTABILITY
Neither <<gamma>> nor <<gammaf>> is ANSI C.  */

/* double gamma(double x)
 * Return the logarithm of the Gamma function of x.
 *
 * Method: call gamma_r
 */

#include "fdlibm.h"
#include <reent.h>
#include <errno.h>

#ifndef _DOUBLE_IS_32BITS

#ifdef __STDC__
	double gamma(double x)
#else
	double gamma(x)
	double x;
#endif
{
#ifdef _IEEE_LIBM
	return __ieee754_gamma_r(x,&(_REENT_SIGNGAM(_REENT)));
#else
        double y;
	struct exception exc;
        y = __ieee754_gamma_r(x,&(_REENT_SIGNGAM(_REENT)));
        if(_LIB_VERSION == _IEEE_) return y;
        if(!finite(y)&&finite(x)) {
#ifndef HUGE_VAL 
#define HUGE_VAL inf
	    double inf = 0.0;

	    SET_HIGH_WORD(inf,0x7ff00000);	/* set inf to infinite */
#endif
	    exc.name = "gamma";
	    exc.err = 0;
	    exc.arg1 = exc.arg2 = x;
            if (_LIB_VERSION == _SVID_)
                exc.retval = HUGE;
            else
                exc.retval = HUGE_VAL;
	    if(floor(x)==x&&x<=0.0) {
		/* gamma(-integer) or gamma(0) */
		exc.type = SING;
		if (_LIB_VERSION == _POSIX_)
		  errno = EDOM;
		else if (!matherr(&exc)) {
		  errno = EDOM;
		}
            } else {
		/* gamma(finite) overflow */
		exc.type = OVERFLOW;
                if (_LIB_VERSION == _POSIX_)
		  errno = ERANGE;
                else if (!matherr(&exc)) {
                  errno = ERANGE;
                }
            }
	    if (exc.err != 0)
	       errno = exc.err;
	    return exc.retval; 
        } else
            return y;
#endif
}             

#endif /* defined(_DOUBLE_IS_32BITS) */