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- /*
- * (c) copyright 1988 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;
- double
- atan(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[] = {
- -0.13688768894191926929e+2,
- -0.20505855195861651981e+2,
- -0.84946240351320683534e+1,
- -0.83758299368150059274e+0
- };
- static double q[] = {
- 0.41066306682575781263e+2,
- 0.86157349597130242515e+2,
- 0.59578436142597344465e+2,
- 0.15024001160028576121e+2,
- 1.0
- };
- static double a[] = {
- 0.0,
- 0.52359877559829887307710723554658381, /* pi/6 */
- M_PI_2,
- 1.04719755119659774615421446109316763 /* pi/3 */
- };
- int neg = x < 0;
- int n;
- double g;
- if (neg) {
- x = -x;
- }
- if (x > 1.0) {
- x = 1.0/x;
- n = 2;
- }
- else n = 0;
- if (x > 0.26794919243112270647) { /* 2-sqtr(3) */
- n = n + 1;
- x = (((0.73205080756887729353*x-0.5)-0.5)+x)/
- (1.73205080756887729353+x);
- }
- /* ??? avoid underflow ??? */
- g = x * x;
- x += x * g * POLYNOM3(g, p) / POLYNOM4(g, q);
- if (n > 1) x = -x;
- x += a[n];
- return neg ? -x : x;
- }
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