【计算几何】CDOJ1720 几何几何

#include<cstdio>
#include<algorithm>
#include<cmath>
using namespace std;
#define EPS 0.00000001
struct Point{
	double x,y;
}a[2],b[2];
typedef Point Vector;
Vector operator * (const double &a,const Vector &v){
	return (Vector){a*v.x,a*v.y};
}
Vector operator - (const Point &a,const Point &b){
	return (Vector){a.x-b.x,a.y-b.y};
}
Vector operator + (const Vector &a,const Vector &b){
	return (Vector){a.x+b.x,a.y+b.y};
}
double Cross(const Vector &a,const Vector &b){
	return a.x*b.y-a.y*b.x;
}
Point GetIntersection(Point P,Vector v,Point Q,Vector w){
	return P+(Cross(w,P-Q)/Cross(v,w))*v;
}
bool inLine(Point p,Point a,Point b){
	if(a.x>b.x){
		swap(a,b);
	}
	if(p.x-a.x>-EPS && b.x-p.x>-EPS){
		return 1;
	}
	return 0;
}
int main(){
	while(scanf("%lf%lf%lf%lf%lf%lf%lf%lf",&a[0].x,&a[0].y,&a[1].x,&a[1].y,&b[0].x,&b[0].y,&b[1].x,&b[1].y)!=EOF){
		if(fabs(a[0].y-a[1].y)<EPS || fabs(b[0].y-b[1].y)<EPS){
			puts("0.00");
			continue;
		}
		if(fabs(Cross(a[0]-a[1],b[0]-b[1]))<EPS){
			puts("0.00");
			continue;
		}
		if(a[0].y<a[1].y){
			swap(a[0],a[1]);
		}
		if(b[0].y<b[1].y){
			swap(b[0],b[1]);
		}
		Point p=GetIntersection(a[0],a[1]-a[0],b[0],b[1]-b[0]);
		if((!inLine(p,a[0],a[1])) || (!inLine(p,b[0],b[1]))){
			puts("0.00");
			continue;
		}
		if(a[0].y<b[0].y){
			swap(a[0],b[0]);
			swap(a[1],b[1]);
		}
		if(fabs(a[0].y-b[0].y)<EPS){
			printf("%.2f\n",fabs(a[0].x-b[0].x)*0.5*fabs(p.y-a[0].y));
			continue;
		}
		if((!fabs(a[0].x-a[1].x)<EPS) && (!fabs(b[0].x-b[1].x)<EPS)){
			double ka=(a[0].y-a[1].y)/(a[0].x-a[1].x);
			double kb=(b[0].y-b[1].y)/(b[0].x-b[1].x);
			if((ka>EPS && kb>EPS) || (ka<EPS && kb<EPS)){
				if(fabs(ka)-fabs(kb)>EPS){
					if((a[0].x-b[0].x<EPS && p.x-b[0].x>-EPS) || (a[0].x-b[0].x>-EPS && p.x-b[0].x<EPS)){
						puts("0.00");
						continue;
					}
				}
			}
		}
		double rr=(b[0].y-p.y)/(a[0].y-p.y);
		Point bc=p+rr*(a[0]-p);
		printf("%.2f\n",0.5*(b[0].y-p.y)*fabs(bc.x-b[0].x));
	}
	return 0;
}