55 lines
1 KiB
C
55 lines
1 KiB
C
/*
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* (c) copyright 1989 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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/* $Header$ */
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#include <math.h>
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#include <errno.h>
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extern int errno;
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extern double exp();
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double
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tanh(x)
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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 (negative) x = -x;
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if (x >= 0.5*M_LN_MAX_D) {
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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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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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/* ??? 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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