66 lines
1.1 KiB
C
66 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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#define __NO_DEFS
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#include <math.h>
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#include "pc.h"
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#if __STDC__
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#include <float.h>
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#endif
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#undef HUGE
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#define HUGE 1e1000
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double _log(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 a[] = {
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-0.64124943423745581147e2,
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0.16383943563021534222e2,
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-0.78956112887491257267e0
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};
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static double b[] = {
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-0.76949932108494879777e3,
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0.31203222091924532844e3,
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-0.35667977739034646171e2,
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1.0
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};
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double znum, zden, z, w;
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int exponent;
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if (x <= 0)
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{
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_trp(ELOG);
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return -HUGE;
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}
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x = _fef(x, &exponent);
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if (x > M_1_SQRT2)
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{
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znum = (x - 0.5) - 0.5;
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zden = x * 0.5 + 0.5;
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}
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else
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{
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znum = x - 0.5;
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zden = znum * 0.5 + 0.5;
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exponent--;
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}
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z = znum / zden;
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w = z * z;
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x = z + z * w * (POLYNOM2(w, a) / POLYNOM3(w, b));
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z = exponent;
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x += z * (-2.121944400546905827679e-4);
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return x + z * 0.693359375;
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}
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