[BZOJ4445] [Scoi2015] small convex want to run (half plane turn) __

Topic Description

Transmission Door

The main idea: a convex n-shaped, n vertices are numbered counterclockwise from the 0~n-l. Now a small convex random selection of a position in the polygon, marked as P point. The P-point and N-vertices are connected to each other to form n triangles. If the P-point, number No. 0, and point 1th form the face of the triangle
The product is the smallest of the n triangles, and the small convex is considered to be a correct stance. Now little Cam wants to know what the probability is that he will be right at once. Solving

The selected position coordinates are (x,y) and the equation is made with a fork
(x1−x) (y2−y) − (y1−y) (x2−x) ≤ (xk−x) (yk+1−y) − (xk+1−x) (yk−y) (x_1-x) (y_2-y)-(y_1-y) (x_2-x) \le (x_k-x) (y_{k+1}-y)-(x_{k+1}-x ) (Y_K-Y)
(y1−y2−yk+yk+1) x+ (x2−x1−xk+1+xk) y+ (X1y2−x2y1−xkyk+1+xk+1yk) ≤0 (y_1-y_2-y_k+y_{k+1}) x+ (X_2-x_1-x_{k+1}+x_k) y+ (x_ 1y_2-x_2y_1-x_ky_{k+1}+x_{k+1}y_k) \le 0
This is a linear form of the ax+by+c≤0 Ax+by+c\le 0, which judges the direction of the line according to the ABC notation, and gives the case that AB is 0.
Then all the straight lines hand over a convex shell, and the area of the convex shell/original area is OK.
This problem is a bit accurate, EPS open 1e-15 code

#include <algorithm> #include <iostream> #include <cstring> #include <cstdio> #include <
Cmath> using namespace std;
#define N 200005 const double eps=1e-15;
const double INF=1E18;
    int dcmp (double x) {if (x<=eps&&x>=-eps) return 0;
Return (x>0) 1:-1;
    struct Vector {double x,y;
    Vector (double x=0,double y=0) {x=x,y=y;
}
};
typedef Vector Point; struct line {point P;
    Vector v;
    Double ang;
        Line (Point P=point (0,0), Vector v=vector (0,0)) {p=p,v=v;
    Ang=atan2 (v.y,v.x);
    BOOL operator < (const line &a) Const {return ang<a.ang;
}
};
Vector operator + (vector a,vector b) {return vector (A.X+B.X,A.Y+B.Y);}
Vector operator-(vector a,vector b) {return vector (A.X-B.X,A.Y-B.Y);}

Vector operator * (vector a,double b) {return vector (a.x*b,a.y*b);}
int n,line,cnt,l,r;
Double Area,ans;
Line l[n<<1],q[n<<1]; Point Pt[n],p[n<<1],poly[n<<1]; Double Cross (Vector A,vector b) {