判断两线段相交

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <string.h>
#include <stdlib.h>
using namespace std;
int n;
struct point
{
    double x;
    double y;
};
struct v
{
    point s;
    point e;
} q[102];
int sum;
double multi(struct point p1,struct point p2,struct point p0)
{
    return (p1.x-p0.x)*(p2.y-p0.y)-(p2.x-p0.x)*(p1.y-p0.y);
}
int main()
{
    while(scanf("%d",&n)!=EOF&&n!=0)
    {
        sum=0;
        for(int i=0; i<n; i++)
        {
            scanf("%lf%lf%lf%lf",&q[i].s.x,&q[i].s.y,&q[i].e.x,&q[i].e.y);
        }
        for(int i=0; i<n; i++)
        {
            for(int j=i+1; j<n; j++)
            {
                if(max(q[i].s.x,q[i].e.x)>=min(q[j].s.x,q[j].e.x)&&
                        min(q[i].s.x,q[i].e.x)<=max(q[j].s.x,q[j].e.x)&&
                        max(q[i].s.y,q[i].e.y)>=min(q[j].s.y,q[j].e.y)&&
                        min(q[i].s.y,q[i].e.y)<=max(q[j].s.y,q[j].e.y)&&
                        multi(q[j].s,q[i].e,q[i].s)*multi(q[i].e,q[j].e,q[i].s)>=0&&
                        multi(q[i].s,q[j].e,q[j].s)*multi(q[j].e,q[i].e,q[j].s)>=0)
                    sum++;
            }
        }
        cout<<sum<<endl;
    }
    return 0;
}