hdu 1086 You can Solve a Geometry Problem too（计算几何水题）

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<vector>
using namespace std;
double EPS = 1e-10;
double add(double a,double b){
if(abs(a+b)<EPS*(abs(a)+abs(b)))
return 0;
return a+b;
}
struct P{
double x,y;
P() {}
P(double x,double y) : x(x),y(y){
}
P operator +(P p){
return P(add(x,p.x),add(y,p.y));
}
P operator -(P p){
return P(add(x,-p.x),add(y,-p.y));
}
P operator *(double d){
return P(x*d,y*d);
}
double dot(P p){
return add(x*p.x,y*p.y);
}
double det(P p){
return add(x*p.y,-y*p.x);
}
};
bool on_seg(P p1,P p2,P p){
return (p1-p).det(p2-p)==0 && (p1-p).dot(p2-p)<=0;
}
P in(P p1,P p2,P q1,P q2){
return p1+(p2-p1)*( (q2-q1).det(q1-p1)/(q2-q1).det(p2-p1) );
}
P p[105],q[105];
int n;
int main()
{
while(~scanf("%d",&n) && n){
for(int i=0;i<n;i++){
scanf("%lf%lf%lf%lf",&p[i].x,&p[i].y,&q[i].x,&q[i].y);
}
int ans=0;
for(int i=0;i<n;i++){
for(int j=0;j<i;j++){
if((p[i]-q[i]).det(p[j]-q[j])==0){
if(on_seg(p[i],q[i],p[j])+on_seg(p[i],q[i],q[j])+on_seg(p[j],q[j],p[i])+on_seg(p[j],q[j],q[i]))
ans++;
}
else{
P r=in(p[i],q[i],p[j],q[j]);
if(on_seg(p[i],q[i],r) && on_seg(p[j],q[j],r))
ans++;
}
}
}
printf("%d\n",ans);
}
return 0;
}