【模板】线段相交

#include <stdio.h>
#include <iostream>
#include <math.h>
#include <algorithm>
using namespace std;

struct point
{
    double x,y;
};
point a[105][2];//a[i][0]代表第i条线段的头，a[i][1]代表尾

double fan(double x,double y)
{
    return x>y?x:y;
}

double fin(double c,double d)
{
    return c<d?c:d;
}

double cnt(point a,point b)
{
    return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
}

int is(point a,point b,point c,point d)
{
    if(a.x==b.x&&c.x==d.x)
    {
        return 0;
    }
    if(a.x==b.x&&c.x!=d.x)
    {
        double m1=a.x;
        double m2=(a.x-c.x)*(d.y-c.y)/(d.x-c.x)+c.y;
        if(m1<=fan(a.x,b.x)&&m1>=fin(a.x,b.x)&&m2>=fin(a.y,b.y)&&m2<=fan(a.y,b.y)&&m1<=fan(c.x,d.x)&&m1>=fin(c.x,d.x)&&m2>=fin(c.y,d.y)&&m2<=fan(c.y,d.y))
            return 1;
    }
    if(c.x==d.x&&a.x!=b.x)
    {
        double m1=c.x;
        double m2=a.y+(b.y-a.y)*(c.x-a.x)/(b.x-a.x);
        if(m1<=fan(a.x,b.x)&&m1>=fin(a.x,b.x)&&m2>=fin(a.y,b.y)&&m2<=fan(a.y,b.y)&&m1<=fan(c.x,d.x)&&m1>=fin(c.x,d.x)&&m2>=fin(c.y,d.y)&&m2<=fan(c.y,d.y))
            return 1;
    }
    double k1=(b.y-a.y)/(b.x-a.x);
    double k2=(d.y-c.y)/(d.x-c.x);
    double m1,m2,x,y;
    if(k1==k2)  return 0;
    else
    {
        m1=a.y-k1*a.x;
        m2=c.y-k2*c.x;
        x=(m1-m2)/(k2-k1);
        y=k1*x+m1;
        if(x<=fan(a.x,b.x)&&x>=fin(a.x,b.x)&&y>=fin(a.y,b.y)&&y<=fan(a.y,b.y)&&x<=fan(c.x,d.x)&&x>=fin(c.x,d.x)&&y>=fin(c.y,d.y)&&y<=fan(c.y,d.y))
            return 1;
    }
    return 0;
}

int main()
{
    int cas = 1;
    int n,i,j;
    while(~scanf("%d",&n),n)
    {
        int cnt = 0;
        for(i = 0;i<n;i++)
        scanf("%lf%lf%lf%lf",&a[i][0].x,&a[i][0].y,&a[i][1].x,&a[i][1].y);//线段的首尾坐标
        for(i = 0;i<n;i++)
        {
            for(j = i+1;j<n;j++)
            {
                if(is(a[i][0],a[i][1],a[j][0],a[j][1]))
                cnt++;
            }
        }
        printf("%d\n",cnt);
    }

    return 0;
}