source: trunk/minix/lib/ack/libp/log.c@ 15

/*
 * (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: /cvsup/minix/src/lib/ack/libp/log.c,v 1.1 2005/10/10 15:27:47 beng Exp $ */

#define __NO_DEFS
#include <math.h>
#include <pc_err.h>

#if __STDC__
#include <pc_math.h>
#include <float.h>
#endif
#undef HUGE
#define HUGE    1e1000

double
_log(x)
        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
        };

        extern double   _fef();
        double  znum, zden, z, w;
        int     exponent;

        if (x <= 0) {
                _trp(ELOG);
                return -HUGE;
        }

        x = _fef(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;
}