source: trunk/minix/lib/math/atan.c@ 12

/*
 * (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/atan.c,v 1.1.1.1 2005/04/21 14:56:24 beng Exp $ */

#include        <float.h>
#include        <math.h>
#include        <errno.h>
#include        "localmath.h"

double
atan(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[] = {
                -0.13688768894191926929e+2,
                -0.20505855195861651981e+2,
                -0.84946240351320683534e+1,
                -0.83758299368150059274e+0
        };
        static double q[] = {
                 0.41066306682575781263e+2,
                 0.86157349597130242515e+2,
                 0.59578436142597344465e+2,
                 0.15024001160028576121e+2,
                 1.0
        };
        static double a[] = {
                0.0,
                0.52359877559829887307710723554658381,  /* pi/6 */
                M_PI_2,
                1.04719755119659774615421446109316763   /* pi/3 */
        };

        int     neg = x < 0;
        int     n;
        double  g;

        if (__IsNan(x)) {
                errno = EDOM;
                return x;
        }
        if (neg) {
                x = -x;
        }
        if (x > 1.0) {
                x = 1.0/x;
                n = 2;
        }
        else    n = 0;

        if (x > 0.26794919243112270647) {       /* 2-sqtr(3) */
                n = n + 1;
                x = (((0.73205080756887729353*x-0.5)-0.5)+x)/
                        (1.73205080756887729353+x);
        }

        /* ??? avoid underflow ??? */

        g = x * x;
        x += x * g * POLYNOM3(g, p) / POLYNOM4(g, q);
        if (n > 1) x = -x;
        x += a[n];
        return neg ? -x : x;
}