Quelle s_tanh.cpp Sprache: C

/* @(#)s_tanh.c 5.1 93/09/24 */
/*
* ====================================================
* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
*
* Developed at SunPro, a Sun Microsystems, Inc. business.
* Permission to use, copy, modify, and distribute this
* software is freely granted, provided that this notice
* is preserved.
* ====================================================
*/

//#include <sys/cdefs.h>
//__FBSDID("$FreeBSD$");

/* Tanh(x)
* Return the Hyperbolic Tangent of x
*
* Method :
*        x    -x
*       e  - e
* 0. tanh(x) is defined to be -----------
*        x    -x
*       e  + e
* 1. reduce x to non-negative by tanh(-x) = -tanh(x).
* 2.  0      <= x <  2**-28 : tanh(x) := x with inexact if x != 0
*         -t
*     2**-28 <= x <  1      : tanh(x) := -----; t = expm1(-2x)
*        t + 2
*      2
*     1      <= x <  22     : tanh(x) := 1 - -----; t = expm1(2x)
*    t + 2
*     22     <= x <= INF    : tanh(x) := 1.
*
* Special cases:
* tanh(NaN) is NaN;
* only tanh(0)=0 is exact for finite argument.
*/

#include <float.h>

#include "math_private.h"

static const volatile double tiny = 1.0e-300;
static const double one = 1.0, two = 2.0, huge = 1.0e300;

double
tanh(double x)
{
double t,z;
int32_t jx,ix;

GET_HIGH_WORD(jx,x);
ix = jx&0x7fffffff;

    /* x is INF or NaN */
if(ix>=0x7ff00000) {
     if (jx>=0) return one/x+one;    /* tanh(+-inf)=+-1 */
     else       return one/x-one;    /* tanh(NaN) = NaN */
}

    /* |x| < 22 */
if (ix < 0x40360000) {  /* |x|<22 */
     if (ix<0x3e300000) { /* |x|<2**-28 */
  if(huge+x>one) return x; /* tanh(tiny) = tiny with inexact */
     }
     if (ix>=0x3ff00000) { /* |x|>=1  */
  t = expm1(two*fabs(x));
  z = one - two/(t+two);
     } else {
         t = expm1(-two*fabs(x));
         z= -t/(t+two);
     }
    /* |x| >= 22, return +-1 */
} else {
     z = one - tiny;  /* raise inexact flag */
}
return (jx>=0)? z: -z;
}

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