#include <u.h>#include <libc.h>#include <libg.h> /*	Form a circle of radius r centered at x1,y1 *	The boundary is a sequence of vertically, horizontally, *	or diagonally adjacent points that minimize  *	abs(x^2+y^2-r^2). * *	The circle is guaranteed to be symmetric about *	the horizontal, vertical, and diagonal axes */ voidcircle(Bitmap *b, Point p, int r, int v, Fcode f){	int x1=p.x;	int y1=p.y;	int eps = 0;		/* x^2 + y^2 - r^2 */	int dxsq = 1;		/* (x+dx)^2-x^2*/	int dysq = 1 - 2*r;	int exy;	int x0 = x1;	int y0 = y1 - r; 	y1 += r;	if(f > 0xF)		f &= 0xF;	if(r == 0){		point(b, Pt(x0, y0), v, f);		return;	}	point(b, Pt(x0, y0), v, f);	point(b, Pt(x0, y1), v, f);	goto advance; 	while(y1 > y0) {		point(b, Pt(x0,y0), v, f);		point(b, Pt(x0,y1), v, f);		point(b, Pt(x1,y0), v, f);		point(b, Pt(x1,y1), v, f);	advance:		exy = eps + dxsq + dysq;		if(-exy <= eps+dxsq) {			y1--;			y0++;			eps += dysq;			dysq += 2;		}		if(exy <= -eps) {			x1++;			x0--;			eps += dxsq;			dxsq += 2;		}	}	point(b, Pt(x0, y0), v, f);	point(b, Pt(x1, y0), v, f);}