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lightning-sunbird  0.9+nobinonly
s_tanh.c
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00001 /* -*- Mode: C; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
00002  *
00003  * ***** BEGIN LICENSE BLOCK *****
00004  * Version: MPL 1.1/GPL 2.0/LGPL 2.1
00005  *
00006  * The contents of this file are subject to the Mozilla Public License Version
00007  * 1.1 (the "License"); you may not use this file except in compliance with
00008  * the License. You may obtain a copy of the License at
00009  * http://www.mozilla.org/MPL/
00010  *
00011  * Software distributed under the License is distributed on an "AS IS" basis,
00012  * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
00013  * for the specific language governing rights and limitations under the
00014  * License.
00015  *
00016  * The Original Code is Mozilla Communicator client code, released
00017  * March 31, 1998.
00018  *
00019  * The Initial Developer of the Original Code is
00020  * Sun Microsystems, Inc.
00021  * Portions created by the Initial Developer are Copyright (C) 1998
00022  * the Initial Developer. All Rights Reserved.
00023  *
00024  * Contributor(s):
00025  *
00026  * Alternatively, the contents of this file may be used under the terms of
00027  * either of the GNU General Public License Version 2 or later (the "GPL"),
00028  * or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
00029  * in which case the provisions of the GPL or the LGPL are applicable instead
00030  * of those above. If you wish to allow use of your version of this file only
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00035  * the provisions above, a recipient may use your version of this file under
00036  * the terms of any one of the MPL, the GPL or the LGPL.
00037  *
00038  * ***** END LICENSE BLOCK ***** */
00039 
00040 /* @(#)s_tanh.c 1.3 95/01/18 */
00041 /*
00042  * ====================================================
00043  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
00044  *
00045  * Developed at SunSoft, a Sun Microsystems, Inc. business.
00046  * Permission to use, copy, modify, and distribute this
00047  * software is freely granted, provided that this notice 
00048  * is preserved.
00049  * ====================================================
00050  */
00051 
00052 /* Tanh(x)
00053  * Return the Hyperbolic Tangent of x
00054  *
00055  * Method :
00056  *                                 x    -x
00057  *                                e  - e
00058  *     0. tanh(x) is defined to be -----------
00059  *                                 x    -x
00060  *                                e  + e
00061  *     1. reduce x to non-negative by tanh(-x) = -tanh(x).
00062  *     2.  0      <= x <= 2**-55 : tanh(x) := x*(one+x)
00063  *                                         -t
00064  *         2**-55 <  x <=  1     : tanh(x) := -----; t = expm1(-2x)
00065  *                                        t + 2
00066  *                                             2
00067  *         1      <= x <=  22.0  : tanh(x) := 1-  ----- ; t=expm1(2x)
00068  *                                           t + 2
00069  *         22.0   <  x <= INF    : tanh(x) := 1.
00070  *
00071  * Special cases:
00072  *     tanh(NaN) is NaN;
00073  *     only tanh(0)=0 is exact for finite argument.
00074  */
00075 
00076 #include "fdlibm.h"
00077 
00078 #ifdef __STDC__
00079 static const double one=1.0, two=2.0, tiny = 1.0e-300;
00080 #else
00081 static double one=1.0, two=2.0, tiny = 1.0e-300;
00082 #endif
00083 
00084 #ifdef __STDC__
00085        double fd_tanh(double x)
00086 #else
00087        double fd_tanh(x)
00088        double x;
00089 #endif
00090 {
00091        double t,z;
00092        int jx,ix;
00093         fd_twoints u;
00094 
00095     /* High word of |x|. */
00096         u.d = x;
00097        jx = __HI(u);
00098        ix = jx&0x7fffffff;
00099 
00100     /* x is INF or NaN */
00101        if(ix>=0x7ff00000) { 
00102            if (jx>=0) return one/x+one;    /* tanh(+-inf)=+-1 */
00103            else       return one/x-one;    /* tanh(NaN) = NaN */
00104        }
00105 
00106     /* |x| < 22 */
00107        if (ix < 0x40360000) {             /* |x|<22 */
00108            if (ix<0x3c800000)             /* |x|<2**-55 */
00109               return x*(one+x);           /* tanh(small) = small */
00110            if (ix>=0x3ff00000) {   /* |x|>=1  */
00111               t = fd_expm1(two*fd_fabs(x));
00112               z = one - two/(t+two);
00113            } else {
00114                t = fd_expm1(-two*fd_fabs(x));
00115                z= -t/(t+two);
00116            }
00117     /* |x| > 22, return +-1 */
00118        } else {
00119            z = one - tiny;         /* raised inexact flag */
00120        }
00121        return (jx>=0)? z: -z;
00122 }