minix/lib/math/log.c
2005-04-21 14:53:53 +00:00

67 lines
1.3 KiB
C
Executable file

/*
* (c) copyright 1988 by the Vrije Universiteit, Amsterdam, The Netherlands.
* See the copyright notice in the ACK home directory, in the file "Copyright".
*
* Author: Ceriel J.H. Jacobs
*/
/* $Header$ */
#include <math.h>
#include <float.h>
#include <errno.h>
#include "localmath.h"
double
log(double x)
{
/* Algorithm and coefficients from:
"Software manual for the elementary functions"
by W.J. Cody and W. Waite, Prentice-Hall, 1980
*/
static double a[] = {
-0.64124943423745581147e2,
0.16383943563021534222e2,
-0.78956112887491257267e0
};
static double b[] = {
-0.76949932108494879777e3,
0.31203222091924532844e3,
-0.35667977739034646171e2,
1.0
};
double znum, zden, z, w;
int exponent;
if (__IsNan(x)) {
errno = EDOM;
return x;
}
if (x < 0) {
errno = EDOM;
return -HUGE_VAL;
}
else if (x == 0) {
errno = ERANGE;
return -HUGE_VAL;
}
if (x <= DBL_MAX) {
}
else return x; /* for infinity and Nan */
x = frexp(x, &exponent);
if (x > M_1_SQRT2) {
znum = (x - 0.5) - 0.5;
zden = x * 0.5 + 0.5;
}
else {
znum = x - 0.5;
zden = znum * 0.5 + 0.5;
exponent--;
}
z = znum/zden; w = z * z;
x = z + z * w * (POLYNOM2(w,a)/POLYNOM3(w,b));
z = exponent;
x += z * (-2.121944400546905827679e-4);
return x + z * 0.693359375;
}