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 /*
  34  * atanh(x)
  35  * Code originated from 4.3bsd.
  36  * Modified by K.C. Ng for SUN 4.0 libm.
  37  * Method :
  38  *                  1              2x                          x
  39  *      atanh(x) = --- * log(1 + -------) = 0.5 * log1p(2 * --------)
  40  *                  2             1 - x                      1 - x
  41  * Note: to guarantee atanh(-x) = -atanh(x), we use
  42  *                 sign(x)             |x|
  43  *      atanh(x) = ------- * log1p(2*-------).
  44  *                    2              1 - |x|
  45  *
  46  * Special cases:
  47  *      atanh(x) is NaN if |x| > 1 with signal;
  48  *      atanh(NaN) is that NaN with no signal;
  49  *      atanh(+-1) is +-INF with signal.
  50  */
  51 /* INDENT ON */
  52 
  53 #include "libm.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 }