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#include "mplot.h"
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/*		arc plotting routine		*/
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/*		from x1,y1 to x2,y2		*/
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/*	with center xc,yc and radius rr	*/
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/*	integrates difference equation		*/
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/*	negative rr draws counterclockwise	*/
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#define PI4 0.7854
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void rarc(double x1, double y1, double x2, double y2, double xc, double yc, double rr){
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	register double dx, dy, a, b;
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	double	ph, dph, rd, xnext;
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	register int	n;
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	dx = x1 - xc;
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	dy = y1 - yc;
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	rd = sqrt(dx * dx + dy * dy);
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	if (rd / e1->quantum < 1.0) {
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		move(xc, yc);
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		vec(xc, yc);
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		return;
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	}
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	dph = acos(1.0 - (e1->quantum / rd));
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	if (dph > PI4)
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		dph = PI4;
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	ph=atan2((y2-yc),(x2 - xc)) - atan2(dy, dx);
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	if (ph < 0)
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		ph += 6.2832;
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	if (rr < 0)
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		ph = 6.2832 - ph;
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	if (ph < dph)
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		plotline(x1, y1, x2, y2);
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	else {
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		n = ph / dph;
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		a = cos(dph);
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		b = sin(dph);
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		if (rr < 0)
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			b = -b;
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		move(x1, y1);
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		while ((n--) >= 0) {
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			xnext = dx * a - dy * b;
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			dy = dx * b + dy * a;
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			dx = xnext;
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			vec(dx + xc, dy + yc);
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		}
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	}
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}