1/*
2 * CDDL HEADER START
3 *
4 * The contents of this file are subject to the terms of the
5 * Common Development and Distribution License (the "License").
6 * You may not use this file except in compliance with the License.
7 *
8 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
9 * or http://www.opensolaris.org/os/licensing.
10 * See the License for the specific language governing permissions
11 * and limitations under the License.
12 *
13 * When distributing Covered Code, include this CDDL HEADER in each
14 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
15 * If applicable, add the following below this CDDL HEADER, with the
16 * fields enclosed by brackets "[]" replaced with your own identifying
17 * information: Portions Copyright [yyyy] [name of copyright owner]
18 *
19 * CDDL HEADER END
20 */
21
22/*
23 * Copyright 2011 Nexenta Systems, Inc.  All rights reserved.
24 */
25/*
26 * Copyright 2006 Sun Microsystems, Inc.  All rights reserved.
27 * Use is subject to license terms.
28 */
29
30#pragma weak __tanh = tanh
31
32/* INDENT OFF */
33/*
34 * TANH(X)
35 * RETURN THE HYPERBOLIC TANGENT OF X
36 * code based on 4.3bsd
37 * Modified by K.C. Ng for sun 4.0, Jan 31, 1987
38 *
39 * Method :
40 *	1. reduce x to non-negative by tanh(-x) = - tanh(x).
41 *	2.
42 *	    0      <  x <=  1.e-10 :  tanh(x) := x
43 *					          -expm1(-2x)
44 *	    1.e-10 <  x <=  1      :  tanh(x) := --------------
45 *					         expm1(-2x) + 2
46 *							  2
47 *	    1      <= x <=  22.0   :  tanh(x) := 1 -  ---------------
48 *						      expm1(2x) + 2
49 *	    22.0   <  x <= INF     :  tanh(x) := 1.
50 *
51 *	Note: 22 was chosen so that fl(1.0+2/(expm1(2*22)+2)) == 1.
52 *
53 * Special cases:
54 *	tanh(NaN) is NaN;
55 *	only tanh(0)=0 is exact for finite argument.
56 */
57
58#include "libm.h"
59#include "libm_protos.h"
60#include <math.h>
61
62static const double
63	one = 1.0,
64	two = 2.0,
65	small = 1.0e-10,
66	big = 1.0e10;
67/* INDENT ON */
68
69double
70tanh(double x)
71{
72	double t, y, z;
73	int signx;
74	volatile double dummy __unused;
75
76	if (isnan(x))
77		return (x * x);	/* + -> * for Cheetah */
78	signx = signbit(x);
79	t = fabs(x);
80	z = one;
81	if (t <= 22.0) {
82		if (t > one)
83			z = one - two / (expm1(t + t) + two);
84		else if (t > small) {
85			y = expm1(-t - t);
86			z = -y / (y + two);
87		} else {
88			/* raise the INEXACT flag for non-zero t */
89			dummy = t + big;
90#ifdef lint
91			dummy = dummy;
92#endif
93			return (x);
94		}
95	} else if (!finite(t))
96		return (copysign(1.0, x));
97	else
98		return ((signx != 0) ? -z + small * small : z - small * small);
99
100	return ((signx != 0) ? -z : z);
101}
102