source: trunk/minix/lib/math/exp.c@ 9

/*
 * (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/math/exp.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
exp(double x)
{
        /*      Algorithm and coefficients from:
                        "Software manual for the elementary functions"
                        by W.J. Cody and W. Waite, Prentice-Hall, 1980
        */

        static double p[] = {
                0.25000000000000000000e+0,
                0.75753180159422776666e-2,
                0.31555192765684646356e-4
        };

        static double q[] = {
                0.50000000000000000000e+0,
                0.56817302698551221787e-1,
                0.63121894374398503557e-3,
                0.75104028399870046114e-6
        };
        double  xn, g;
        int     n;
        int     negative = x < 0;

        if (__IsNan(x)) {
                errno = EDOM;
                return x;
        }
        if (x < M_LN_MIN_D) {
                errno = ERANGE;
                return 0.0;
        }
        if (x > M_LN_MAX_D) {
                errno = ERANGE;
                return HUGE_VAL;
        }

        if (negative) x = -x;
 
        /* ??? avoid underflow ??? */

        n = x * M_LOG2E + 0.5;  /* 1/ln(2) = log2(e), 0.5 added for rounding */
        xn = n;
        {
                double  x1 = (long) x;
                double  x2 = x - x1;

                g = ((x1-xn*0.693359375)+x2) - xn*(-2.1219444005469058277e-4);
        }
        if (negative) {
                g = -g;
                n = -n;
        }
        xn = g * g;
        x = g * POLYNOM2(xn, p);
        n += 1;
        return (ldexp(0.5 + x/(POLYNOM3(xn, q) - x), n));
}