POJ 1912 A highway and the seven dwarfs （凸包）

#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector>
using namespace std;
double EPS=1e-10;
const double PI=acos(-1.0);
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);} //点积
    double det(P p){return add(x*p.y,-y*p.x);}  //叉积
};
bool cmp_x(const P& p,const P& q){
    if(p.x!=q.x)return p.x<q.x;
    return p.y<q.y;  
}
vector<P> convex_hull(P* ps,int n){
    sort(ps,ps+n,cmp_x);
    int k=0;
    vector<P> qs(n*2);
    for(int i=0;i<n;i++){
        while((k>1)&&(qs[k-1]-qs[k-2]).det(ps[i]-qs[k-1])<=0)k--;
        qs[k++]=ps[i];
    }
    for(int i=n-2,t=k;i>=0;i--){
        while(k>t&&(qs[k-1]-qs[k-2]).det(ps[i]-qs[k-1])<=0)k--;
        qs[k++]=ps[i];
    }qs.resize(k-1);
    return qs;
}
double dist(P p,P q){return sqrt((p-q).dot(p-q));}
double normalize(double r){
	  if(r<-PI/2.0+EPS)r+=PI*2;
	  return r;
}
double atan2(const P& p){
    return normalize(atan2(p.y, p.x));
}
bool double_cmp(double a,double b){
    return a+EPS<b;
}
const int MAX_N=100010;
int N,n;
P ps[MAX_N];
double as[MAX_N];
void solve(){
    for(int i=0;i<N;i++)scanf("%lf%lf",&ps[i].x,&ps[i].y);
    vector<P> chs;
    if(N>1){
        chs=convex_hull(ps,N);
        n=chs.size();
        chs.push_back(chs[0]);
    }
    for(int i=0;i<n;i++)as[i]=atan2(chs[i+1]-chs[i]);
    sort(as,as+n,double_cmp);
    P p1,p2;
    while(~scanf("%lf%lf%lf%lf",&p1.x,&p1.y,&p2.x,&p2.y)){
        if(N<2){puts("GOOD");continue;}
        int x=upper_bound(as,as+n,atan2(p2-p1),double_cmp)-as;
        int y=upper_bound(as,as+n,atan2(p1-p2),double_cmp)-as;
        puts((((p2-p1).det(chs[x]-p1)*(p2-p1).det(chs[y]-p1)>-EPS))?"GOOD":"BAD");
    }
}
int main(){
    while(~scanf("%d",&N))solve();
    return 0;
}