ack/lang/pc/libpc/log.c

/*
 * (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
 */

/* $Id$ */

#define __NO_DEFS
#include <math.h>
#include "pc.h"

#if __STDC__
#include <float.h>
#endif
#undef HUGE
#define HUGE 1e1000

double _log(double x)
{
	/*	Algorithm and coefficients from:
			"Software manual for the elementary functions"
			by W.J. Cody and W. Waite, Prentice-Hall, 1980
	*/
	static double a[] = {
		-0.64124943423745581147e2,
		0.16383943563021534222e2,
		-0.78956112887491257267e0
	};
	static double b[] = {
		-0.76949932108494879777e3,
		0.31203222091924532844e3,
		-0.35667977739034646171e2,
		1.0
	};

	double znum, zden, z, w;
	int exponent;

	if (x <= 0)
	{
		_trp(ELOG);
		return -HUGE;
	}

	x = _fef(x, &exponent);
	if (x > M_1_SQRT2)
	{
		znum = (x - 0.5) - 0.5;
		zden = x * 0.5 + 0.5;
	}
	else
	{
		znum = x - 0.5;
		zden = znum * 0.5 + 0.5;
		exponent--;
	}
	z = znum / zden;
	w = z * z;
	x = z + z * w * (POLYNOM2(w, a) / POLYNOM3(w, b));
	z = exponent;
	x += z * (-2.121944400546905827679e-4);
	return x + z * 0.693359375;
}