ack/lang/pc/libpc/exp.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>
#define M_MIN_D DBL_MIN
#define M_MAX_D DBL_MAX
#define M_DMINEXP DBL_MIN_EXP
#endif
#undef HUGE
#define HUGE 1e1000

static double Ldexp(double fl, int exp)
{
	int sign = 1;
	int currexp;

	if (fl < 0)
	{
		fl = -fl;
		sign = -1;
	}
	fl = _fef(fl, &currexp);
	exp += currexp;
	if (exp > 0)
	{
		while (exp > 30)
		{
			fl *= (double)(1L << 30);
			exp -= 30;
		}
		fl *= (double)(1L << exp);
	}
	else
	{
		while (exp < -30)
		{
			fl /= (double)(1L << 30);
			exp += 30;
		}
		fl /= (double)(1L << -exp);
	}
	return sign * fl;
}

double _exp(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[] = {
		0.25000000000000000000e+0,
		0.75753180159422776666e-2,
		0.31555192765684646356e-4
	};

	static double q[] = {
		0.50000000000000000000e+0,
		0.56817302698551221787e-1,
		0.63121894374398503557e-3,
		0.75104028399870046114e-6
	};
	double xn, g;
	int n;
	int negative = x < 0;

	if (x <= M_LN_MIN_D)
	{
		g = M_MIN_D / 4.0;

		if (g != 0.0)
		{
			/* unnormalized numbers apparently exist */
			if (x < (M_LN2 * (M_DMINEXP - 53)))
				return 0.0;
		}
		else
		{
			if (x < M_LN_MIN_D)
				return 0.0;
			return M_MIN_D;
		}
	}
	if (x >= M_LN_MAX_D)
	{
		if (x > M_LN_MAX_D)
		{
			_trp(EEXP);
			return HUGE;
		}
		return M_MAX_D;
	}
	if (negative)
		x = -x;

	n = x * M_LOG2E + 0.5; /* 1/ln(2) = log2(e), 0.5 added for rounding */
	xn = n;
	{
		double x1 = (long)x;
		double x2 = x - x1;

		g = ((x1 - xn * 0.693359375) + x2) - xn * (-2.1219444005469058277e-4);
	}
	if (negative)
	{
		g = -g;
		n = -n;
	}
	xn = g * g;
	x = g * POLYNOM2(xn, p);
	n += 1;
	return (Ldexp(0.5 + x / (POLYNOM3(xn, q) - x), n));
}