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 atanh = __atanh
  31 
  32 /* INDENT OFF */
  33 /* atanh(x)
  34  * Code originated from 4.3bsd.
  35  * Modified by K.C. Ng for SUN 4.0 libm.
  36  * Method :
  37  *                  1              2x                          x
  38  *      atanh(x) = --- * log(1 + -------) = 0.5 * log1p(2 * --------)
  39  *                  2             1 - x                      1 - x
  40  * Note: to guarantee atanh(-x) = -atanh(x), we use
  41  *                 sign(x)             |x|
  42  *      atanh(x) = ------- * log1p(2*-------).
  43  *                    2              1 - |x|
  44  *
  45  * Special cases:
  46  *      atanh(x) is NaN if |x| > 1 with signal;
  47  *      atanh(NaN) is that NaN with no signal;
  48  *      atanh(+-1) is +-INF with signal.
  49  */
  50 /* INDENT ON */
  51 
  52 #include "libm.h"
  53 #include "libm_synonyms.h"
  54 #include "libm_protos.h"
  55 #include <math.h>
  56 
  57 double
  58 atanh(double x) {
  59         double t;
  60 
  61         if (isnan(x))
  62                 return x * x;           /* switched from x + x for Cheetah */
  63         t = fabs(x);
  64         if (t > 1.0)
  65                 return _SVID_libm_err(x, x, 30);        /* sNaN */
  66         if (t == 1.0)
  67                 return _SVID_libm_err(x, x, 31);        /* x/0; */
  68         t = t / (1.0 - t);
  69         return copysign(0.5, x) * log1p(t + t);
  70 }