判断线段相交交点

#include<cstdio>
#include<iostream>
#define min(a,b) (a)>(b)?(b):(a)
#define max(a,b) (a)>(b)?(a):(b)
using namespace std;
struct point
{
	double x,y;
};
struct segment
{
	point a,b; 
};
segment s[1000];
double Direction(point i,point j,point k)
{
	double tempx1=k.x-i.x;
	double tempy1=k.y-i.y;
	double tempx2=j.x-i.x;
	double tempy2=j.y-i.y;
	return (tempx1*tempy2-tempx2*tempy1);
}
bool On(point i,point j,point k)
{
	if((min(i.x,j.x)<=k.x && k.x<=max(i.x,j.x)) && (min(i.y,j.y)<=k.y && k.y<=max(i.y,j.y)))
		return true;
	else
		return false;
}
bool intersect(point a,point b,point c,point d)
{
	double d1,d2,d3,d4;
	d1=Direction(c,d,a);
	d2=Direction(c,d,b);
	d3=Direction(a,b,c);
	d4=Direction(a,b,d);
	if(((d1>0&&d2<0)||(d1<0&&d2>0))&&((d3>0&&d4<0)||(d3<0&&d4>0)))
		return true;
	else if(d1==0 && On(c,d,a))
		return true;
	else if(d2==0 && On(c,d,b))
		return true;
	else if(d3==0 && On(a,b,c))
		return true;
	else if(d4==0 && On(a,b,d))
		return true;
	else
		return false;
}
int main()
{
	int i,n,j;
	double x1,y1,x2,y2;
	while(~scanf("%d",&n) &&n)
	{
		int cnt=0;
		for(i=1;i<=n;i++)
		{
			scanf("%lf%lf%lf%lf",&x1,&y1,&x2,&y2);
			s[i].a.x=x1;
			s[i].a.y=y1;
			s[i].b.x=x2;
			s[i].b.y=y2; 
		}
		for(i=1;i<=n;i++)
			for(j=i+1;j<=n;j++)
			{
				if(intersect(s[i].a,s[i].b,s[j].a,s[j].b))
					cnt++;
			}
		printf("%d\n",cnt); 
	}
	return 0;
}