67 lines
1.1 KiB
C
67 lines
1.1 KiB
C
/*
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* (c) copyright 1988 by the Vrije Universiteit, Amsterdam, The Netherlands.
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* See the copyright notice in the ACK home directory, in the file "Copyright".
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*
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* Author: Ceriel J.H. Jacobs
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*/
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/* $Id$ */
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#include <float.h>
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#include <math.h>
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#include <errno.h>
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#include <ack/config.h>
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#include "localmath.h"
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#if ACKCONF_WANT_FLOAT
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double
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tanh(double x)
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{
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/* Algorithm and coefficients from:
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"Software manual for the elementary functions"
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by W.J. Cody and W. Waite, Prentice-Hall, 1980
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*/
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static double p[] = {
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-0.16134119023996228053e+4,
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-0.99225929672236083313e+2,
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-0.96437492777225469787e+0
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};
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static double q[] = {
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0.48402357071988688686e+4,
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0.22337720718962312926e+4,
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0.11274474380534949335e+3,
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1.0
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};
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int negative = x < 0;
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if (__IsNan(x))
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{
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errno = EDOM;
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return x;
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}
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if (negative)
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x = -x;
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if (x >= 0.5 * M_LN_MAX_D)
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{
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x = 1.0;
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}
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#define LN3D2 0.54930614433405484570e+0 /* ln(3)/2 */
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else if (x > LN3D2)
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{
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x = 0.5 - 1.0 / (exp(x + x) + 1.0);
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x += x;
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}
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else
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{
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/* ??? avoid underflow ??? */
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double g = x * x;
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x += x * g * POLYNOM2(g, p) / POLYNOM3(g, q);
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}
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return negative ? -x : x;
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}
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#endif
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