uva 190 Circle Through Three Points

計算幾何的題目，三點確定一個圓。

#include <stdio.h>
#include <math.h>
struct Line {
	int no_slope;
	double k;
	double i_x;
	double i_y;
};
struct Point {
	double x;
	double y;
};
const double E = 1e-5;
bool double_cmp(double a, double b) {
	return fabs(a-b) < E;
}

Line set_line(const Point& p1, const Point& p2) {
	Line tmp;
	if (double_cmp(p1.x, p2.x)) {
		tmp.no_slope = true;
		tmp.i_x = p1.x;
	}
	else {
		tmp.no_slope = false;
		tmp.k = (p2.y-p1.y)/(p2.x-p1.x);
		tmp.i_y = p1.y-tmp.k*p1.x;
	}
	return tmp;
}
Line get_vertical(const Line& l, const Point& p) {
	Line tmp;
	if (l.no_slope == true) {
		tmp.no_slope = false;
		tmp.k = 0.0;
		tmp.i_y = p.y;
	} 
	else if (double_cmp(l.k, 0.0)) {
		tmp.no_slope = true;
		tmp.i_x = p.x;
	}
	else {
		tmp.no_slope = false;
		tmp.k = -1/l.k;
		tmp.i_y = p.y - tmp.k*p.x;
	}
	return tmp;
}
int main() {
	double x,y,r;
	double ax,ay,bx,by,cx,cy;
	Point a,b,c; 
	Point mid1,mid2;
	Line l1,l2,v1,v2;
	int flag=0;
	while (scanf("%lf",&ax)!=EOF) {
		scanf("%lf%lf%lf%lf%lf", &ay,&bx,&by,&cx,&cy);
		a.x=ax;
		a.y=ay;
		b.x=bx;
		b.y=by;
		c.x=cx;
		c.y=cy;
		mid1.x=(a.x+b.x)/2;
		mid1.y=(a.y+b.y)/2;
		mid2.x=(a.x+c.x)/2;
		mid2.y=(a.y+c.y)/2;
		l1=set_line(a,b);
		l2=set_line(a,c);
		v1=get_vertical(l1,mid1);
		v2=get_vertical(l2,mid2);
		if (v1.no_slope) {
    		x = v1.i_x;
   			y = v2.k*x + v2.i_y; 
		}
		else if (v2.no_slope) {
			x = v2.i_x;
    		y = v1.k*x + v1.i_y;
		}
		else {
    		x = (v1.i_y - v2.i_y) / (v2.k - v1.k);
    		y = v1.k*x + v1.i_y;
		}
		r = sqrt((ax-x)*(ax-x)+(ay-y)*(ay-y));
		char c1, c2,c3;
		if (x>0)
			c1='-';
		else 
			c1='+';
		if (y>0)
			c2='-';
		else 
			c2='+';
		printf("(x %c %.3lf)^2 + (y %c %.3lf)^2 = %.3lf^2\n",c1,fabs(x),c2,fabs(y),r);
		double t1=-2.0*x;
		double t2=-2.0*y;
		double t3=x*x+y*y-r*r;
		if (t1>=0)
			c1='+';
		else
			c1='-';
		if (t2>=0)
			c2='+';
		else 
			c2='-';
		if (t3>=0)
			c3='+';
		else
			c3='-';
		printf("x^2 + y^2 %c %.3lfx %c %.3lfy %c %.3lf = 0\n",c1,fabs(t1),c2,fabs(t2),c3,fabs(t3));
		printf("\n");
	//	printf("%.3lf %.3lf %.3lf\n", x, y, r);
	}
	return 0;
}