Transmitters POJ - 1106（叉积）

#include <iostream>
#include <cmath>

using namespace std;

const int maxn=1111;
const double eps=1e-8;

int sgn(double x)
{
    if(fabs(x)<eps)
        return 0;
    if(x<0)
        return -1;
    else
        return 1;
}
struct Point
{
    double x,y;
    Point(){}
    Point(double _x,double _y)
    {
        x=_x;
        y=_y;
    }
    Point operator -(const Point &b)const
    {
        return Point(x-b.x,y-b.y);
    }
    double operator ^(const Point &b)const
    {
        return x*b.y-y*b.x;
    }
    double operator *(const Point &b)const
    {
        return x*b.x+y*b.y;
    }
};


Point p[maxn],ans[maxn];
double dist(Point a,Point b)
{
    return sqrt((a-b)*(a-b));
}
int main()
{
    double x1,y1,r;
    while(1)
    {
        cin>>x1>>y1>>r;
        Point p1=Point(x1,y1);
        if(r<0)
            break;
        int n;
        cin>>n;
        int cont=0;
        for(int i=0;i<n;i++)
        {
            cin>>p[i].x>>p[i].y;
            double d=dist(p[i],p1);
            if(r-d>=0)
            {
                ans[cont++]=p[i];
            }
        }
        int max1=0;
        for(int i=0;i<cont;i++)
        {
            int cont_max=1;
            for(int j=0;j<cont;j++)
            {
                if(j==i)
                    continue;
                if(sgn((ans[i]-ans[j])^(p1-ans[j]))>=0)
                {
                    cont_max++;
                }
            }
            if(cont_max>max1)
                max1=cont_max;
        }
        cout<<max1<<endl;
    }
    return 0;
}