HDU_1086 You can Solve a Geometry Problem too（几何题）

#include <stdio.h>
struct node
{
double x1, y1;
double x2, y2;
}l[110];

double X(double a, double b, double c, double d)
{
return a*d - b*c;
}

int judge(int i, int j)
{
double a = X(l[i].x1-l[j].x1, l[i].y1-l[j].y1, l[j].x2-l[j].x1, l[j].y2-l[j].y1);
double b = X(l[i].x2-l[j].x1, l[i].y2-l[j].y1, l[j].x2-l[j].x1, l[j].y2-l[j].y1);
double c = a*b;
double d = X(l[j].x1-l[i].x1, l[j].y1-l[i].y1, l[i].x2-l[i].x1, l[i].y2-l[i].y1);
double e = X(l[j].x2-l[i].x1, l[j].y2-l[i].y1, l[i].x2-l[i].x1, l[i].y2-l[i].y1);
double f = d*e;
if(c <= 0 && f <= 0)
return 1;
return 0;
}

int main()
{
int n, i, j;
//freopen("data.in", "r", stdin);
while(scanf("%d", &n), n)
{
int ans = 0;
for(i = 1; i <= n; i++)
scanf("%lf%lf%lf%lf", &l[i].x1, &l[i].y1, &l[i].x2, &l[i].y2);
for(i = 1; i < n; i++)
for(j = i+1; j <= n; j++)
{
if(judge(i, j))
ans++;
}
printf("%d\n", ans);
}
return 0;
}