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/* $OpenBSD: n_expm1.c,v 1.12 2009/10/27 23:59:29 deraadt Exp $ */
/* $NetBSD: n_expm1.c,v 1.1 1995/10/10 23:36:46 ragge Exp $ */
/*
* Copyright (c) 1985, 1993
* The Regents of the University of California. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. Neither the name of the University nor the names of its contributors
* may be used to endorse or promote products derived from this software
* without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/
/* 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:
* scalbn(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.
*/
#include "math.h"
#include "mathimpl.h"
static const double ln2hi = 6.9314718055829871446E-1;
static const double ln2lo = 1.6465949582897081279E-12;
static const double lnhuge = 9.4961163736712506989E1;
static const double invln2 = 1.4426950408889634148E0;
double
expm1(double x)
{
static const double one=1.0, half=1.0/2.0, tiny=1e-300;
double z,hi,lo,c;
int k;
#if defined(__vax__)
static prec=56;
#else /* defined(__vax__) */
static prec=53;
#endif /* defined(__vax__) */
if (isnan(x))
return (x);
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-scalbn(one,-k); z += __exp__E(z,c);}
else if(k<100)
{ x = __exp__E(z,c)-scalbn(one,-k); x+=z; z=one;}
else
{ x = __exp__E(z,c)+z; z=one;}
return (scalbn(x+z,k));
}
}
/* end of x > lnunfl */
else
/* expm1(-big#) rounded to -1 (inexact) */
if(finite(x))
return(tiny-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) ? scalbn(one,5000) : x);
}
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