判断两条线段是否相交

include <iostream>

using namespace std;
struct point
{
double x,y;
};
struct segment
{
point begin,end;
};
double min(double x,double y)
{
return x<y?x:y;
}
double max(double x,double y)
{
return x>y?x:y;
}
bool onsegment(point pi,point pj,point pk) //判断点pk是否在线段pi pj上
{
if(min(pi.x,pj.x)<=pk.x&&pk.x<=max(pi.x,pj.x))
{
if(min(pi.y,pj.y)<=pk.y&&pk.y<=max(pi.y,pj.y))
{
return true;
}
}
return false;
}
double direction(point pi,point pj,point pk) //计算向量pkpi和向量pjpi的叉积
{
return (pi.x-pk.x)*(pi.y-pj.y)-(pi.y-pk.y)*(pi.x-pj.x);
}
bool judge(point p1,point p2,point p3,point p4) //判断线段p1p2和p3p4是否相交
{
double d1 = direction(p3,p4,p1);
double d2 = direction(p3,p4,p2);
double d3 = direction(p1,p2,p3);
double d4 = direction(p1,p2,p4);
if(d1*d2<0&&d3*d4<0)
return true;
if(d1==0&&onsegment(p3,p4,p1))
return true;
if(d2==0&&onsegment(p3,p4,p2))
return true;
if(d3==0&&onsegment(p1,p2,p3))
return true;
if(d4==0&&onsegment(p1,p2,p4))
return true;
return false;
}
int main()
{
int n,count;
segment seg[101];
while(cin>>n&&n)
{
count = 0;
for(int i=1; i<=n; i++)
{
cin>>seg[i].begin.x>>seg[i].begin.y>>seg[i].end.x>>seg[i].end.y;
}
for(int i=1; i<n; i++)
for(int j=i+1; j<=n; j++)
{
if(judge(seg[i].begin,seg[i].end,seg[j].begin,seg[j].end))
{
count++;
}
}
cout<<count<<endl;
}
return 0;
}