ref: 8188b4f4f0e07b6669e6ae3c6c1099af917eaab4
dir: /sys/src/libc/port/exp.c/
/*
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
exp(double arg)
{
double fract, temp1, temp2, xsq;
int ent;
if(arg == 0)
return 1;
if(arg < -maxf)
return 0;
if(arg > maxf)
return Inf(1);
arg *= log2e;
ent = 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 ldexp(sqrt2*(temp2+temp1)/(temp2-temp1), ent);
}