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74 lines
1.8 KiB
C
74 lines
1.8 KiB
C
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/* @(#)s_tanh.c 1.3 95/01/18 */
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/*
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* ====================================================
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* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
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*
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* Developed at SunSoft, a Sun Microsystems, Inc. business.
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* Permission to use, copy, modify, and distribute this
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* software is freely granted, provided that this notice
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* is preserved.
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* ====================================================
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*/
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/* Tanh(x)
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* Return the Hyperbolic Tangent of x
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*
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* Method :
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* x -x
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* e - e
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* 0. tanh(x) is defined to be -----------
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* x -x
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* e + e
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* 1. reduce x to non-negative by tanh(-x) = -tanh(x).
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* 2. 0 <= x <= 2**-55 : tanh(x) := x*(one+x)
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* -t
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* 2**-55 < x <= 1 : tanh(x) := -----; t = expm1(-2x)
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* t + 2
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* 2
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* 1 <= x <= 22.0 : tanh(x) := 1- ----- ; t=expm1(2x)
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* t + 2
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* 22.0 < x <= INF : tanh(x) := 1.
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*
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* Special cases:
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* tanh(NaN) is NaN;
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* only tanh(0)=0 is exact for finite argument.
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*/
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#include "math.h"
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static const double one = 1.0, two = 2.0, tinyv = 1.0e-300;
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double tanh(double x) {
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double t, z;
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int jx, ix;
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/* High word of |x|. */
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jx = __HI(x);
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ix = jx & 0x7fffffff;
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/* x is INF or NaN */
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if(ix >= 0x7ff00000) {
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if(jx >= 0) return one / x + one; /* tanh(+-inf)=+-1 */
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else
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return one / x - one; /* tanh(NaN) = NaN */
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}
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/* |x| < 22 */
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if(ix < 0x40360000) { /* |x|<22 */
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if(ix < 0x3c800000) /* |x|<2**-55 */
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return x * (one + x); /* tanh(small) = small */
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if(ix >= 0x3ff00000) { /* |x|>=1 */
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t = expm1(two * fabs(x));
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z = one - two / (t + two);
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} else {
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t = expm1(-two * fabs(x));
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z = -t / (t + two);
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}
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/* |x| > 22, return +-1 */
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} else {
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z = one - tinyv; /* raised inexact flag */
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}
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return (jx >= 0) ? z : -z;
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}
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