判断线段相交

#include <iostream>
#include <cmath>
#include <cstdio>
using namespace std;
struct line{
    double x1,y1;
    double x2,y2;
    bool operator < (const line &a) const
    {
       return x1 < a.x1;
    }
}arr[110];
bool cross(line a,line b)
{
    double fa=((a.x2-a.x1)*(b.y1-a.y1)-(a.y2-a.y1)*(b.x1-a.x1))*
        ((a.x2-a.x1)*(b.y2-a.y1)-(a.y2-a.y1)*(b.x2-a.x1));
    if(fa>0) return false;

    double fb=((b.x2-b.x1)*(a.y1-b.y1)-(b.y2-b.y1)*(a.x1-b.x1))*
        ((b.x2-b.x1)*(a.y2-b.y1)-(b.y2-b.y1)*(a.x2-b.x1));
    if(fb>0) return false;
    if(fa==0&&fb==0)
    if( (a.x1-b.x1)*(a.x2-b.x1)+(a.y1-b.y1)*(a.y2-b.y1)>0 &&
        (a.x1-b.x2)*(a.x2-b.x2)+(a.y1-b.y2)*(a.y2-b.y2)>0 )
        return false;
    return true;
}
int main()
{
    int n;
    while(cin>>n,n)
    {
        for(int i=0;i<n;i++)
        {
            scanf("%lf%lf%lf%lf",&arr[i].x1,&arr[i].y1,&arr[i].x2,&arr[i].y2);
        }
        int ans=0;
        for(int i=0;i<n;i++)
        for(int j=i+1;j<n;j++)
        if(cross(arr[i],arr[j])) ans++;
        cout<<ans<<endl;
    }
    return 0;
}
/*
2
0 0 0 2
0 3 0 5
2
0 0 0 2
0 2 4 0

*/