72 lines
1.3 KiB
C
72 lines
1.3 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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/* $Header$ */
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#define __NO_DEFS
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#include <math.h>
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#if __STDC__
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#include <pc_math.h>
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#endif
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double
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_atn(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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-0.13688768894191926929e+2,
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-0.20505855195861651981e+2,
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-0.84946240351320683534e+1,
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-0.83758299368150059274e+0
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};
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static double q[] = {
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0.41066306682575781263e+2,
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0.86157349597130242515e+2,
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0.59578436142597344465e+2,
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0.15024001160028576121e+2,
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1.0
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};
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static double a[] = {
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0.0,
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0.52359877559829887307710723554658381, /* pi/6 */
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M_PI_2,
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1.04719755119659774615421446109316763 /* pi/3 */
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};
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int neg = x < 0;
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int n;
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double g;
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if (neg) {
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x = -x;
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}
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if (x > 1.0) {
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x = 1.0/x;
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n = 2;
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}
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else n = 0;
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if (x > 0.26794919243112270647) { /* 2-sqtr(3) */
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n = n + 1;
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x = (((0.73205080756887729353*x-0.5)-0.5)+x)/
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(1.73205080756887729353+x);
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}
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/* ??? avoid underflow ??? */
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g = x * x;
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x += x * g * POLYNOM3(g, p) / POLYNOM4(g, q);
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if (n > 1) x = -x;
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x += a[n];
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return neg ? -x : x;
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}
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