/* @(#)s_tanh.c 1.3 95/01/18 */

/*
 * Copyright (c) 2015-2017 Carsten Sonne Larsen  <cs@innolan.dk>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * The origin source code can be obtained from:
 * http://www.netlib.org/fdlibm/s_tanh.c
 * 
 */

/*
 * ====================================================
 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
 *
 * Developed at SunSoft, a Sun Microsystems, Inc. business.
 * Permission to use, copy, modify, and distribute this
 * software is freely granted, provided that this notice
 * is preserved.
 * ====================================================
 *
 */

#include "prim.h"
#include "math.h"

static const double one=1.0, two=2.0, tiny = 1.0e-300;

/**
 * @brief   Hyperbolic tangent function.
 * @version 1.3
 * @date    95/01/18
 * @details
 * <pre>
 * Return the Hyperbolic Tangent of x
 *
 * Method :
 *				       x    -x
 *				      e  - e
 *	0. tanh(x) is defined to be -----------
 *				       x    -x
 *				      e  + e
 *	1. reduce x to non-negative by tanh(-x) = -tanh(x).
 *	2.  0      <= x <= 2**-55 : tanh(x) := x*(one+x)
 *					        -t
 *	    2**-55 <  x <=  1     : tanh(x) := -----; t = expm1(-2x)
 *					       t + 2
 *						     2
 *	    1      <= x <=  22.0  : tanh(x) := 1-  ----- ; t=expm1(2x)
 *						   t + 2
 *	    22.0   <  x <= INF    : tanh(x) := 1.
 *
 * Special cases:
 *	tanh(NaN) is NaN;
 *	only tanh(0)=0 is exact for finite argument.
 * </pre>
 * @copyright Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
 * @license   Developed at SunSoft, a Sun Microsystems, Inc. business. Permission
 *            to use, copy, modify, and distribute this software is freely granted,
 *            provided that this notice is preserved.
 */

double tanh(double x)
{
    double t,z;
    sword jx,ix;

    /* High word of |x|. */
    GET_HIGH_WORD(jx,x);
    ix = jx&0x7fffffff;

    /* x is INF or NaN */
    if(ix>=0x7ff00000) {
        if (jx>=0) return one/x+one;    /* tanh(+-inf)=+-1 */
        else       return one/x-one;    /* tanh(NaN) = NaN */
    }

    /* |x| < 22 */
    if (ix < 0x40360000) {		/* |x|<22 */
        if (ix<0x3c800000) 		/* |x|<2**-55 */
            return x*(one+x);    	/* tanh(small) = small */
        if (ix>=0x3ff00000) {	/* |x|>=1  */
            t = expm1(two*fabs(x));
            z = one - two/(t+two);
        } else {
            t = expm1(-two*fabs(x));
            z= -t/(t+two);
        }
        /* |x| > 22, return +-1 */
    } else {
        z = one - tiny;		/* raised inexact flag */
    }
    return (jx>=0)? z: -z;
}
