Bzoj 4445 [Scoi2015] small convex want to run half plane to turn __ac Road

A semilinear equation for solving inequalities in a plane

#include <cstdio> #include <cstring> #include <iostream> #include <algorithm> #include <
Cmath> #define EPS 1e-10 #define MAXN 500021 using namespace std;
int N,cnt,tot;
Double all;
	struct p{double x,y;
	void Init () {scanf ("%lf%lf", &x,&y);}
P (Double a=0,double b=0): X (a), Y (b) {}}P[MAXN];
typedef P VEC;
Double operator* (P a,p b) {return a.x*b.y-a.y*b.x;} vec operator-(P a,p b) {return VEC (A.X-B.X,A.Y-B.Y);} P operator+ (P A,vec b) {return P (A.X+B.X,A.Y+B.Y);} vec operator/(VEC a,double t) {return VEC (a.x/t,a.y/t);} VEC operator* ( 
	VEC a,double t) {return VEC (a.x*t,a.y*t);} bool dcmp (double a,double b) {return fabs (a-b) <=eps;} struct line{P A;vec b;
	Double slop;
		BOOL operator< (const line& y) const{if (Fabs (slop-y.slop) >eps) return slop<y.slop;
	Return b* (y.a-a) <0;

}}L[MAXN],Q[MAXN];
	void Build () {scanf ("%d", &n);
	P[1].init (), P[2].init ();
	Double a,b,c,x,y;p[n+1]=p[1];
		for (int j,i=3;i<=n+1;i++) {if (i<=n) p[i].init (); J=i-1;
		A=P[1].Y-P[2].Y-P[J].Y+P[I].Y;
 		b=p[2].x-p[1].x-p[i].x+p[j].x;
		C= (P[2].X*P[1].Y-P[1].X*P[2].Y+P[J].X*P[I].Y-P[I].X*P[J].Y) *-1; L[++cnt].a= Fabs (a) >eps?
		P (-c/a,0): P (0,-c/b);
	L[cnt].b=vec (-b,a);
	
	for (int i=1;i<=n;i++) all+=p[i]*p[i+1];
		for (int i=1;i<=n;i++) {l[++cnt].a=p[i];
	L[cnt].b=p[i+1]-p[i];
	for (int i=1;i<=cnt;i++) l[i].slop=atan2 (l[i].b.y,l[i].b.x);
	Sort (l+1,l+1+cnt); for (int i=1;i<=cnt;i++) {if!dcmp (l[i].slop,l[i-1].slop) | |
	i==1) L[++tot]=l[i];
}cnt=tot;
	P Q (line aa,line bb) {p A=AA.A,B=BB.A;
	VEC a=aa.b,b=bb.b,c=a-b;
	Double k= (b*c)/(A*B);
return a+a*k;
	BOOL Judge (line A,line B,line c) {P x=q (a,b);
Return (X-C.A) *c.b>0;
	} void Solve () {q[1]=l[1];
	int l=1,r=1;
		for (int i=2;i<=cnt;i++) {while (L<r&&judge (q[r-1],q[r],l[i)) r--;
		while (L<r&&judge (Q[l],q[l+1],l[i])) l++;
	Q[++r]=l[i];
	while (L<r&&judge (Q[r-1],q[r],q[l])) r--;
	q[r+1]=q[l];tot=0; for (int i=l;i<=r;i++) {p[++tot]=q (q[I],q[i+1]);
}P[TOT+1]=P[1];
	int main () {build ();
	Solve ();
	Double ans=0;
	for (int i=1;i<=tot;i++) ans+=p[i]*p[i+1];
	if (tot<3) {puts ("0.0000"); return 0;}
	All=fabs (All), ans=fabs (ans);
	Ans=ans/all;
	printf ("%.4lf", ans);
return 0; }