log.c

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/*
 * (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: /opt/proj/minix/cvsroot/src/lib/math/log.c,v 1.1.1.1 2005/04/21 14:56:26 beng Exp $ */

#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;
}

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