ref: bd6c2aad586814b091ce5aca9d41cf2c51adb37b
dir: /libkern/log.c/
/*
log returns the natural logarithm of its floating
point argument.
The coefficients are #2705 from Hart & Cheney. (19.38D)
It calls frexp.
*/
#include <lib9.h>
#define log2 0.693147180559945309e0
#define ln10o1 .4342944819032518276511
#define sqrto2 0.707106781186547524e0
#define p0 -.240139179559210510e2
#define p1 0.309572928215376501e2
#define p2 -.963769093377840513e1
#define p3 0.421087371217979714e0
#define q0 -.120069589779605255e2
#define q1 0.194809660700889731e2
#define q2 -.891110902798312337e1
double
log(double arg)
{
double x, z, zsq, temp;
int exp;
if(arg <= 0)
return NaN();
x = frexp(arg, &exp);
while(x < 0.5) {
x *= 2;
exp--;
}
if(x < sqrto2) {
x *= 2;
exp--;
}
z = (x-1) / (x+1);
zsq = z*z;
temp = ((p3*zsq + p2)*zsq + p1)*zsq + p0;
temp = temp/(((zsq + q2)*zsq + q1)*zsq + q0);
temp = temp*z + exp*log2;
return temp;
}
double
log10(double arg)
{
if(arg <= 0)
return NaN();
return log(arg) * ln10o1;
}