expm1.c

VXWORKS源代码

/* expm1.c - math routines */

/* Copyright 1992 Wind River Systems, Inc. */

/*
modification history
--------------------
01c,20sep92,smb  documentation additions.
01b,30jul92,kdl  marked routine NOMANUAL.
01a,08jul92,smb  documentation.
*/

/*
DESCRIPTION
*
* This file includes a support routine (expm1()) which is used by
* other portions of the UCB ANSI C library.
*
* Copyright (c) 1985 Regents of the University of California.
* All rights reserved.
*
* Redistribution and use in source and binary forms are permitted
* provided that the above copyright notice and this paragraph are
* duplicated in all such forms and that any documentation,
* advertising materials, and other materials related to such
* distribution and use acknowledge that the software was developed
* by the University of California, Berkeley.  The name of the
* University may not be used to endorse or promote products derived
* from this software without specific prior written permission.
* THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
* IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
* WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.
*
* All recipients should regard themselves as participants in an ongoing
* research project and hence should feel obligated to report their
* experiences (good or bad) with these elementary function codes, using
* the sendbug(8) program, to the authors.
*
* SEE ALSO: American National Standard X3.159-1989
*
* NOMANUAL
*/

#include "vxWorks.h"
#include "math.h"

#if defined(vax)||defined(tahoe)	/* VAX D format */
#ifdef vax
#define _0x(A,B)	0x/**/A/**/B
#else	/* vax */
#define _0x(A,B)	0x/**/B/**/A
#endif	/* vax */
/* static double */
/* ln2hi  =  6.9314718055829871446E-1    , Hex  2^  0   *  .B17217F7D00000 */
/* ln2lo  =  1.6465949582897081279E-12   , Hex  2^-39   *  .E7BCD5E4F1D9CC */
/* lnhuge =  9.4961163736712506989E1     , Hex  2^  7   *  .BDEC1DA73E9010 */
/* invln2 =  1.4426950408889634148E0     ; Hex  2^  1   *  .B8AA3B295C17F1 */
static long     ln2hix[] = { _0x(7217,4031), _0x(0000,f7d0)};
static long     ln2lox[] = { _0x(bcd5,2ce7), _0x(d9cc,e4f1)};
static long    lnhugex[] = { _0x(ec1d,43bd), _0x(9010,a73e)};
static long    invln2x[] = { _0x(aa3b,40b8), _0x(17f1,295c)};
#define    ln2hi    (*(double*)ln2hix)
#define    ln2lo    (*(double*)ln2lox)
#define   lnhuge    (*(double*)lnhugex)
#define   invln2    (*(double*)invln2x)
#else	/* defined(vax)||defined(tahoe) */
static double
ln2hi  =  6.9314718036912381649E-1    , /*Hex  2^ -1   *  1.62E42FEE00000 */
ln2lo  =  1.9082149292705877000E-10   , /*Hex  2^-33   *  1.A39EF35793C76 */
lnhuge =  7.1602103751842355450E2     , /*Hex  2^  9   *  1.6602B15B7ECF2 */
invln2 =  1.4426950408889633870E0     ; /*Hex  2^  0   *  1.71547652B82FE */
#endif	/* defined(vax)||defined(tahoe) */

/*****************************************************************************
* expm1 -
*
* EXPM1(X)
* RETURN THE EXPONENTIAL OF X MINUS ONE
* DOUBLE PRECISION (IEEE 53 BITS, VAX D FORMAT 56 BITS)
* CODED IN C BY K.C. NG, 1/19/85;
* REVISED BY K.C. NG on 2/6/85, 3/7/85, 3/21/85, 4/16/85.
*
* Required system supported functions:
*	scalb(x,n)
*	copysign(x,y)
*	finite(x)
*
* Kernel function:
*	exp__E(x,c)
*
* Method:
*	1. Argument Reduction: given the input x, find r and integer k such
*	   that
*	                   x = k*ln2 + r,  |r| <= 0.5*ln2 .
*	   r will be represented as r := z+c for better accuracy.
*
*	2. Compute EXPM1(r)=exp(r)-1 by
*
*			EXPM1(r=z+c) := z + exp__E(z,c)
*
*	3. EXPM1(x) =  2^k * ( EXPM1(r) + 1-2^-k ).
*
* 	Remarks:
*	   1. When k=1 and z < -0.25, we use the following formula for
*	      better accuracy:
*			EXPM1(x) = 2 * ( (z+0.5) + exp__E(z,c) )
*	   2. To avoid rounding error in 1-2^-k where k is large, we use
*			EXPM1(x) = 2^k * { [z+(exp__E(z,c)-2^-k )] + 1 }
*	      when k>56.
*
* Special cases:
*	EXPM1(INF) is INF, EXPM1(NaN) is NaN;
*	EXPM1(-INF)= -1;
*	for finite argument, only EXPM1(0)=0 is exact.
*
* Accuracy:
*	EXPM1(x) returns the exact (exp(x)-1) nearly rounded. In a test run with
*	1,166,000 random arguments on a VAX, the maximum observed error was
*	.872 ulps (units of the last place).
*
* Constants:
* The hexadecimal values are the intended ones for the following constants.
* The decimal values may be used, provided that the compiler will convert
* from decimal to binary accurately enough to produce the hexadecimal values
* shown.
*
* NOMANUAL
*/

double expm1(x)
double x;
{
	static double one=1.0, half=1.0/2.0;
	double scalb(), copysign(), exp__E(), z,hi,lo,c;
	int k,finite();
#if defined(vax)||defined(tahoe)
	static prec=56;
#else	/* defined(vax)||defined(tahoe) */
	static prec=53;
#endif	/* defined(vax)||defined(tahoe) */
#if !defined(vax)&&!defined(tahoe)
	if(x!=x) return(x);	/* x is NaN */
#endif	/* !defined(vax)&&!defined(tahoe) */

	if( x <= lnhuge ) {
		if( x >= -40.0 ) {

		    /* argument reduction : x - k*ln2 */
			k= invln2 *x+copysign(0.5,x);	/* k=NINT(x/ln2) */
			hi=x-k*ln2hi ;
			z=hi-(lo=k*ln2lo);
			c=(hi-z)-lo;

			if(k==0) return(z+exp__E(z,c));
			if(k==1)
			    if(z< -0.25)
				{x=z+half;x +=exp__E(z,c); return(x+x);}
			    else
				{z+=exp__E(z,c); x=half+z; return(x+x);}
		    /* end of k=1 */

			else {
			    if(k<=prec)
			      { x=one-scalb(one,-k); z += exp__E(z,c);}
			    else if(k<100)
			      { x = exp__E(z,c)-scalb(one,-k); x+=z; z=one;}
			    else
			      { x = exp__E(z,c)+z; z=one;}

			    return (scalb(x+z,k));
			}
		}
		/* end of x > lnunfl */

		else
		     /* expm1(-big#) rounded to -1 (inexact) */
		     if(finite(x))
			 { ln2hi+ln2lo; return(-one);}

		     /* expm1(-INF) is -1 */
		     else return(-one);
	}
	/* end of x < lnhuge */

	else
	/*  expm1(INF) is INF, expm1(+big#) overflows to INF */
	    return( finite(x) ?  scalb(one,5000) : x);
}