poj 2007 Scrambled Polygon 极角排序

/**
极角排序输出，，，
主要atan2（y，x） 容易失精度，，用
bool cmp(point a,point b){
 5     if(cross(a-tmp,b-tmp)>0)
 6         return 1;
 7     if(cross(a-tmp,b-tmp)==0)
 8         return length(a-tmp)<length(b-tmp);
 9     return 0;
10 }
**/
#include <iostream>
#include <algorithm>
#include <cmath>
using namespace std;

struct point {
    double x,y;
    point (double x=0,double y=0):x(x),y(y){}
};
point p[100];
point tmp;
typedef point Vector;
Vector operator - (point a,point b){
    return Vector (a.x-b.x,a.y-b.y);
}
double angle(Vector v){
    return atan2(v.y,v.x);
}
double length(Vector v){
    return sqrt(v.x*v.x+v.y*v.y);
}

double cross(Vector a,Vector b){
    return a.x*b.y-a.y*b.x;
}
bool cmp(point a,point b){
    if(cross(a-tmp,b-tmp)>0)
        return 1;
    if(cross(a-tmp,b-tmp)==0)
        return length(a-tmp)<length(b-tmp);
    return 0;
}

int main()
{
    int cnt =0;
    double x1,y1;
    cin>>tmp.x>>tmp.y;
    while(cin>>x1>>y1){
        p[cnt].x = x1;
        p[cnt].y = y1;
        cnt++;
    }
    sort(p,p+cnt,cmp);
    cout<<"("<<tmp.x<<","<<tmp.y<<")"<<endl;
    for(int i=0;i<cnt;i++)
        cout<<"("<<p[i].x<<","<<p[i].y<<")"<<endl;
    return 0;
}