poj1106

极坐标排序+叉积判断夹角

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <algorithm>
using namespace std;

#define maxn 200
#define eps 1.0e-8

struct Point
{
    int x, y;
}s, point[maxn];

double r;
int n;

bool operator < (const Point &a, const Point &b)
{
    return atan2((double)a.x, (double)a.y) < atan2((double)b.x, (double)b.y);
}

void input()
{
    int cnt = 0;
    scanf("%d", &n);
    for (int i = 0; i < n; i++)
    {
        scanf("%d%d", &point[cnt].x, &point[cnt].y);
        point[cnt].x -= s.x;
        point[cnt].y -= s.y;
        if (r * r > point[cnt].x * point[cnt].x + point[cnt].y * point[cnt].y - eps)
            cnt++;
    }
    n = cnt;
}

double xmulti(Point a, Point b)
{
    return a.x * b.y - a.y * b.x;
}

bool within(Point a, Point b)
{
    return xmulti(b, a) > -eps;
}

int work()
{
    int r = 0;
    int ans = 0;
    for (int i = 0; i < n; i++)
    {
        while (r - i < n && within(point[i], point[r % n]))
            r++;
        ans = max(ans, r - i);
    }
    return ans;
}

int main()
{
    //freopen("t.txt", "r", stdin);
    while (scanf("%d%d%lf", &s.x, &s.y, &r), r >= 0)
    {
        input();
        sort(point, point + n);
        printf("%d\n", work());
    }
    return 0;
}