s_tanh.c

来自「This is a resource based on j2me embedde」· C语言 代码 · 共 97 行

/*
 * @(#)s_tanh.c	1.9 06/10/10
 *
 * Copyright  1990-2008 Sun Microsystems, Inc. All Rights Reserved.  
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER  
 *   
 * This program is free software; you can redistribute it and/or  
 * modify it under the terms of the GNU General Public License version  
 * 2 only, as published by the Free Software Foundation.   
 *   
 * This program is distributed in the hope that it will be useful, but  
 * WITHOUT ANY WARRANTY; without even the implied warranty of  
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU  
 * General Public License version 2 for more details (a copy is  
 * included at /legal/license.txt).   
 *   
 * You should have received a copy of the GNU General Public License  
 * version 2 along with this work; if not, write to the Free Software  
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  
 * 02110-1301 USA   
 *   
 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa  
 * Clara, CA 95054 or visit www.sun.com if you need additional  
 * information or have any questions. 
 *
 */

/* Tanh(x)
 * 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.
 */

#include "fdlibm.h"

#ifdef __STDC__
static const double one=1.0, two=2.0, tiny = 1.0e-300;
#else
static double one=1.0, two=2.0, tiny = 1.0e-300;
#endif

#ifdef __STDC__
	double tanh(double x)
#else
	double tanh(x)
	double x;
#endif
{
	double t,z;
	int jx,ix;

    /* High word of |x|. */
	jx = __HI(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;
}