libjava/classpath/native/fdlibm/s_tanh.c

   1
   2 /* @(#)s_tanh.c 1.3 95/01/18 */
   3 /*
   4  * ====================================================
   5  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
   6  *
   7  * Developed at SunSoft, a Sun Microsystems, Inc. business.
   8  * Permission to use, copy, modify, and distribute this
   9  * software is freely granted, provided that this notice
  10  * is preserved.
  11  * ====================================================
  12  */
  13
  14 /* Tanh(x)
  15  * Return the Hyperbolic Tangent of x
  16  *
  17  * Method :
  18  *                                     x    -x
  19  *                                    e  - e
  20  *      0. tanh(x) is defined to be -----------
  21  *                                     x    -x
  22  *                                    e  + e
  23  *      1. reduce x to non-negative by tanh(-x) = -tanh(x).
  24  *      2.  0      <= x <= 2**-55 : tanh(x) := x*(one+x)
  25  *                                              -t
  26  *          2**-55 <  x <=  1     : tanh(x) := -----; t = expm1(-2x)
  27  *                                             t + 2
  28  *                                                   2
  29  *          1      <= x <=  22.0  : tanh(x) := 1-  ----- ; t=expm1(2x)
  30  *                                                 t + 2
  31  *          22.0   <  x <= INF    : tanh(x) := 1.
  32  *
  33  * Special cases:
  34  *      tanh(NaN) is NaN;
  35  *      only tanh(0)=0 is exact for finite argument.
  36  */
  37
  38 #include "fdlibm.h"
  39
  40 #ifndef _DOUBLE_IS_32BITS
  41
  42 #ifdef __STDC__
  43 static const double one=1.0, two=2.0, tiny = 1.0e-300;
  44 #else
  45 static double one=1.0, two=2.0, tiny = 1.0e-300;
  46 #endif
  47
  48 #ifdef __STDC__
  49         double tanh(double x)
  50 #else
  51         double tanh(x)
  52         double x;
  53 #endif
  54 {
  55         double t,z;
  56         int32_t jx,ix;
  57
  58     /* High word of |x|. */
  59         GET_HIGH_WORD(jx,x);
  60         ix = jx&0x7fffffff;
  61
  62     /* x is INF or NaN */
  63         if(ix>=0x7ff00000) {
  64             if (jx>=0) return one/x+one;    /* tanh(+-inf)=+-1 */
  65             else       return one/x-one;    /* tanh(NaN) = NaN */
  66         }
  67
  68     /* |x| < 22 */
  69         if (ix < 0x40360000) {          /* |x|<22 */
  70             if (ix<0x3c800000)          /* |x|<2**-55 */
  71                 return x*(one+x);       /* tanh(small) = small */
  72             if (ix>=0x3ff00000) {       /* |x|>=1  */
  73                 t = expm1(two*fabs(x));
  74                 z = one - two/(t+two);
  75             } else {
  76                 t = expm1(-two*fabs(x));
  77                 z= -t/(t+two);
  78             }
  79     /* |x| > 22, return +-1 */
  80         } else {
  81             z = one - tiny;             /* raised inexact flag */
  82         }
  83         return (jx>=0)? z: -z;
  84 }
  85 #endif /* _DOUBLE_IS_32BITS */