jehanne/sys/src/lib/jehanne/port/exp.c

50 lines
1.3 KiB
C

/*
* This file is part of the UCB release of Plan 9. It is subject to the license
* terms in the LICENSE file found in the top-level directory of this
* distribution and at http://akaros.cs.berkeley.edu/files/Plan9License. No
* part of the UCB release of Plan 9, including this file, may be copied,
* modified, propagated, or distributed except according to the terms contained
* in the LICENSE file.
*/
/*
exp returns the exponential function of its
floating-point argument.
The coefficients are #1069 from Hart and Cheney. (22.35D)
*/
#include <u.h>
#include <libc.h>
#define p0 .2080384346694663001443843411e7
#define p1 .3028697169744036299076048876e5
#define p2 .6061485330061080841615584556e2
#define q0 .6002720360238832528230907598e7
#define q1 .3277251518082914423057964422e6
#define q2 .1749287689093076403844945335e4
#define log2e 1.4426950408889634073599247
#define sqrt2 1.4142135623730950488016887
#define maxf 10000
double
jehanne_exp(double arg)
{
double fract, temp1, temp2, xsq;
int ent;
if(arg == 0)
return 1;
if(arg < -maxf)
return 0;
if(arg > maxf)
return jehanne_Inf(1);
arg *= log2e;
ent = jehanne_floor(arg);
fract = (arg-ent) - 0.5;
xsq = fract*fract;
temp1 = ((p2*xsq+p1)*xsq+p0)*fract;
temp2 = ((xsq+q2)*xsq+q1)*xsq + q0;
return jehanne_ldexp(sqrt2*(temp2+temp1)/(temp2-temp1), ent);
}