xref: /third_party/jerryscript/jerry-libm/tanh.c (revision 425bb815) 
1/* Copyright JS Foundation and other contributors, http://js.foundation
2 *
3 * Licensed under the Apache License, Version 2.0 (the "License");
4 * you may not use this file except in compliance with the License.
5 * You may obtain a copy of the License at
6 *
7 *     http://www.apache.org/licenses/LICENSE-2.0
8 *
9 * Unless required by applicable law or agreed to in writing, software
10 * distributed under the License is distributed on an "AS IS" BASIS
11 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 * See the License for the specific language governing permissions and
13 * limitations under the License.
14 *
15 * This file is based on work under the following copyright and permission
16 * notice:
17 *
18 *     Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
19 *
20 *     Developed at SunSoft, a Sun Microsystems, Inc. business.
21 *     Permission to use, copy, modify, and distribute this
22 *     software is freely granted, provided that this notice
23 *     is preserved.
24 *
25 *     @(#)s_tanh.c 1.3 95/01/18
26 */
27
28#include "jerry-libm-internal.h"
29
30/* tanh(x)
31 * Return the Hyperbolic Tangent of x
32 *
33 * Method:
34 *                                 x   -x
35 *                                e -  e
36 *  0. tanh(x) is defined to be -----------
37 *                                 x    -x
38 *                                e  +  e
39 *
40 *  1. reduce x to non-negative by tanh(-x) = -tanh(x).
41 *  2.  0      <= x <= 2**-55 : tanh(x) := x * (one + x)
42 *
43 *                                          -t
44 *      2**-55 <  x <=  1     : tanh(x) := -----; t = expm1(-2x)
45 *                                         t + 2
46 *
47 *                                               2
48 *      1      <= x <=  22.0  : tanh(x) := 1-  ----- ; t = expm1(2x)
49 *                                             t + 2
50 *
51 *      22.0   <  x <= INF    : tanh(x) := 1.
52 *
53 * Special cases:
54 *  tanh(NaN) is NaN;
55 *  only tanh(0) = 0 is exact for finite x.
56 */
57#define one 1.0
58#define two 2.0
59#define tiny 1.0e-300
60
61double
62tanh (double x)
63{
64  double t, z;
65  int jx, ix;
66
67  /* High word of |x|. */
68  jx = __HI (x);
69  ix = jx & 0x7fffffff;
70
71  /* x is INF or NaN */
72  if (ix >= 0x7ff00000)
73  {
74    if (jx >= 0)
75    {
76      /* tanh(+-inf) = +-1 */
77      return one / x + one;
78    }
79    else
80    {
81      /* tanh(NaN) = NaN */
82      return one / x - one;
83    }
84  }
85
86  /* |x| < 22 */
87  if (ix < 0x40360000)
88  {
89    /* |x| < 2**-55 */
90    if (ix < 0x3c800000)
91    {
92      /* tanh(small) = small */
93      return x * (one + x);
94    }
95    if (ix >= 0x3ff00000)
96    {
97      /* |x| >= 1  */
98      t = expm1 (two * fabs (x));
99      z = one - two / (t + two);
100    }
101    else
102    {
103      t = expm1 (-two * fabs (x));
104      z = -t / (t + two);
105    }
106  }
107  else
108  {
109    /* |x| > 22, return +-1 */
110    z = one - tiny; /* raised inexact flag */
111  }
112  return (jx >= 0) ? z : -z;
113} /* tanh */
114
115#undef one
116#undef two
117#undef tiny
118