#ifndef lint
static const char	RCSid[] = "$Id: tcos.c,v 3.3 2003/02/25 02:47:22 greg Exp $";
#endif
/*
 * Table-based cosine approximation.
 *
 * Use doubles in table even though we're not nearly that accurate just
 * to avoid conversion and guarantee that tsin(x)^2 + tcos(x)^2 == 1.
 *
 * No interpolation in this version.
 *
 * External symbols declared in standard.h
 */

#include "copyright.h"

#include <math.h>

#ifndef NCOSENTRY
#define NCOSENTRY	256
#endif

#ifdef M_PI
#define PI		((double)M_PI)
#else
#define PI		3.14159265358979323846
#endif


double
tcos(x)				/* approximate cosine */
register double	x;
{
	static double	costab[NCOSENTRY+1];
	register int	i;

	if (costab[0] < 0.5)		/* initialize table */
		for (i = 0; i <= NCOSENTRY; i++)
			costab[i] = cos((PI/2./NCOSENTRY)*i);
					/* normalize angle */
	if (x < 0.)
		x = -x;
	i = (NCOSENTRY*2./PI) * x  +  0.5;
	if (i >= 4*NCOSENTRY)
		i %= 4*NCOSENTRY;
	switch (i / NCOSENTRY) {
	case 0:
		return(costab[i]);
	case 1:
		return(-costab[(2*NCOSENTRY)-i]);
	case 2:
		return(-costab[i-(2*NCOSENTRY)]);
	case 3:
		return(costab[(4*NCOSENTRY)-i]);
	}
	return(0.);		/* should never be reached */
}