poj1031 计算几何

/*
Time:2019.11.2
Author: Goven
type：计算几何 
err:
ref：https://blog.csdn.net/hering_linux/article/details/74510588
    https://www.cnblogs.com/dengeven/p/3229136.html
    注意dl,a的含义 
    怎样理解ref中的dl用da来表示：dl位微分，先求l关于a的表达式，l = s*tan(a),dl即对l求导 
知识点：
    1.atan() -- atan2() 区别：https://blog.csdn.net/chengkaizone/article/details/50558347 
*/
#include<iostream>
#include<cmath> 
#define PI 3.1415926
using namespace std;

double x[105], y[105];
double angle(double x0, double y0, double x1, double y1) {
    double a = atan2(y0, x0);
    double b = atan2(y1, x1);
    if (a - b > PI) b += 2 * PI;//顶点在3,4象限，顺时针往下走
    if (b - a > PI) a += 2 * PI;//顶点在3,4象限，逆时针往上走
    return a - b; 
} 
int main()
{
    double k, h, t = 0, max = 0, min = 0;
    int n;
    cin >> k >> h >> n;
    for (int i = 0; i < n; i++) {
        cin >> x[i] >> y[i];
    }
    x[n] = x[0], y[n] = y[0];
    
    for (int i = 1; i <= n; i++) {
        t += angle(x[i-1], y[i-1], x[i], y[i]);
        if (t > max) max = t;
        if (t < min) min = t;
        if (max - min >= 2 * PI) {
            max = min + 2 * PI;
            break; 
        }
    }
    printf("%.2lf\n", k * h * (max - min));
    return 0;
}