【HDOJ】1086 You can Solve a Geometry Problem too

#include <stdio.h>

#define MAXNUM 105

typedef struct {
double x1,  y1;
double x2,  y2;
} line_st;

line_st lines[MAXNUM];

int cal(int i, int j) {
double ab_x, ab_y, ac_x, ac_y, ad_x, ad_y;
double a, b;

ab_x = lines[i].x2 - lines[i].x1;
ab_y = lines[i].y2 - lines[i].y1;
ac_x = lines[j].x1 - lines[i].x1;
ac_y = lines[j].y1 - lines[i].y1;
ad_x = lines[j].x2 - lines[i].x1;
ad_y = lines[j].y2 - lines[i].y1;
a = ab_x*ac_y - ab_y*ac_x;
b = ab_x*ad_y - ab_y*ad_x;
if (a*b <= 0)
return 1;
else
return 0;
}

int main() {
int n;
int i, j, k;

while (scanf("%d", &n)!=EOF && n) {
for (i=0; i<n; ++i)
scanf("%lf%lf%lf%lf", &lines[i].x1,&lines[i].y1,&lines[i].x2,&lines[i].y2);
k = 0;
for (i=0; i<n; ++i)
for (j=0; j<i; ++j)
if (cal(i, j) && cal(j, i))
k++;
printf("%d\n", k);
}

return 0;
}