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



*/