tanh.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/tanh.c,v 1.1.1.1 2005/04/21 14:56:26 beng Exp $ */

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

double
tanh(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[] = {
                -0.16134119023996228053e+4,
                -0.99225929672236083313e+2,
                -0.96437492777225469787e+0
        };
        static double q[] = {
                 0.48402357071988688686e+4,
                 0.22337720718962312926e+4,
                 0.11274474380534949335e+3,
                 1.0
        };
        int     negative = x < 0;

        if (__IsNan(x)) {
                errno = EDOM;
                return x;
        }
        if (negative) x = -x;

        if (x >= 0.5*M_LN_MAX_D) {
                x = 1.0;
        }
#define LN3D2   0.54930614433405484570e+0       /* ln(3)/2 */
        else if (x > LN3D2) {
                x = 0.5 - 1.0/(exp(x+x)+1.0);
                x += x;
        }
        else {
                /* ??? avoid underflow ??? */
                double g = x*x;
                x += x * g * POLYNOM2(g, p)/POLYNOM3(g, q);
        }
        return negative ? -x : x;
}

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