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- /*
- * (c) copyright 1989 by the Vrije Universiteit, Amsterdam, The Netherlands.
- * See the copyright notice in the ACK home directory, in the file "Copyright".
- *
- * Author: Ceriel J.H. Jacobs
- */
- /* $Id$ */
- #include <math.h>
- #include <errno.h>
- extern int errno;
- extern double exp();
- double
- tanh(x)
- double x;
- {
- /* Algorithm and coefficients from:
- "Software manual for the elementary functions"
- by W.J. Cody and W. Waite, Prentice-Hall, 1980
- */
- static double p[] = {
- -0.16134119023996228053e+4,
- -0.99225929672236083313e+2,
- -0.96437492777225469787e+0
- };
- static double q[] = {
- 0.48402357071988688686e+4,
- 0.22337720718962312926e+4,
- 0.11274474380534949335e+3,
- 1.0
- };
- int negative = x < 0;
- if (negative) x = -x;
- if (x >= 0.5*M_LN_MAX_D) {
- x = 1.0;
- }
- #define LN3D2 0.54930614433405484570e+0 /* ln(3)/2 */
- else if (x > LN3D2) {
- x = 0.5 - 1.0/(exp(x+x)+1.0);
- x += x;
- }
- else {
- /* ??? avoid underflow ??? */
- double g = x*x;
- x += x * g * POLYNOM2(g, p)/POLYNOM3(g, q);
- }
- return negative ? -x : x;
- }
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