ICPC NEAU Programming Contest 2020 桌球 （計算幾何）

思路：

你可以求出白球要移動到哪個點，同時白球和紅球分別在紅線兩邊纔可能進行移動。

#include <cstdio>
#include <cstring>
#include <vector>
#include <algorithm>
#include <queue>
#include <cmath>

using namespace std;

const int maxn = 4e5 + 7;
const double PI = acos(-1);
const double eps = 1e-6;

int sign(double x) {
    if(x > 0) return 1;
    else if(x < 0) return -1;
    else return 0;
}

double Get(double x,double y,double nx,double ny,double k) {
    return k * x - y + ny - k * nx;
}

int main() {
    int T;scanf("%d",&T);
    while(T--) {
        double wx,wy,rx,ry,hx,hy;
        scanf("%lf%lf%lf%lf%lf%lf",&wx,&wy,&rx,&ry,&hx,&hy);
        double k1 = (hy - ry) / (hx - rx);
        double deg1 = atan2(ry - hy,rx - hx);
        double nx,ny;
        
        nx = rx + 2 * cos(deg1);
        ny = ry + 2 * sin(deg1);
        
        double k2 = -1.0 / k1;
        
        if(sign(Get(wx,wy,nx,ny,k2)) == sign(Get(rx,ry,nx,ny,k2))) {
            printf("Impossible\n");
        }
        else {
            double ans = atan2(ny - wy,nx - wx);
            if(ans < 0) ans += 2 * PI;
            printf("%.6f\n",ans / PI);
        }
    }
    return 0;
}