USACO6.5.2 Closed Fences(fence4)

/*
ID:xsy97051
LANG:C++
TASK:fence4
*/
#include <iostream>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <cstring>

using namespace std;

struct POINT
{
double x;
double y; POINT(double a=0, double b=0) { x=a; y=b;} //constructor
};
struct LINESEG {
POINT s;
POINT e;
LINESEG(POINT a, POINT b) { s=a; e=b;}
LINESEG() { }
};
double dist(POINT p1,POINT p2)
{
return(sqrt((p1.x-p2.x)*(p1.x-p2.x)+(p1.y-p2.y)*(p1.y-p2.y)));
}

double multiply(POINT sp,POINT ep,POINT op)
{
return((sp.x-op.x)*(ep.y-op.y)-(ep.x-op.x)*(sp.y-op.y));
}

bool online(LINESEG l,POINT p)
{
return((multiply(l.e,p,l.s)==0) &&( ( (p.x-l.s.x)*(p.x-l.e.x)<=0 )&&( (p.y-l.s.y)*(p.y-l.e.y)<=0 ) ) );
}

POINT midpoint(POINT a, POINT b)
{
return POINT((a.x + b.x) / 2, (a.y + b.y) / 2);
}

bool intersect(LINESEG u,LINESEG v)
{
return ( (max(u.s.x,u.e.x)>=min(v.s.x,v.e.x))&& //排斥实验
(max(v.s.x,v.e.x)>=min(u.s.x,u.e.x))&&
(max(u.s.y,u.e.y)>=min(v.s.y,v.e.y))&&
(max(v.s.y,v.e.y)>=min(u.s.y,u.e.y))&&
(multiply(v.s,u.e,u.s)*multiply(u.e,v.e,u.s)>=0)&& //跨立实验
(multiply(u.s,v.e,v.s)*multiply(v.e,u.e,v.s)>=0));
}

bool intersect_A(LINESEG u,LINESEG v)
{
return ((intersect(u,v))&&
(!online(u,v.s))&&
(!online(u,v.e))&&
(!online(v,u.e))&&
(!online(v,u.s)));
}

int n;
POINT p[2000], e;
LINESEG s[2000];
bool sv[2000];

bool save(int k, LINESEG a)
{
if(dist(a.s,a.e)<1e-5) return 0;
int flag = 0;
for(int v = 1; v <= n; v++)
if(v != k)
{
if(intersect(LINESEG(e,a.s),s[v]) && intersect(LINESEG(e,a.e), s[v]))
{
flag=1;
break;
}
if(intersect_A(LINESEG(e,a.s),s[v]) || intersect_A(LINESEG(e,a.e),s[v]))
flag=2;
}
if(flag==0) return 1;
if(flag==1) return 0;
else return save(k,LINESEG(a.s,midpoint(a.s, a.e))) || save(k,LINESEG(midpoint(a.s,a.e),a.e));
}

int main()
{
freopen("fence4.in", "r", stdin);
freopen("fence4.out", "w", stdout);
scanf("%d",&n);
scanf("%lf%lf",&e.x,&e.y);
for(int i=1;i<=n;i++)
scanf("%lf%lf",&p[i].x,&p[i].y);
for(int i=1;i<n;i++)
s[i]=LINESEG(p[i],p[i+1]);
s
=LINESEG(p[1],p
);
int cnt = 0;
memset(sv,0,sizeof(sv));
for(int i=1;i<=n;i++)
if(save(i,s[i]))
{
cnt++;
sv[i] = 1;
}
printf("%d\n", cnt);
for(int i=1;i<n-1;i++)
if(sv[i])
printf("%d %d %d %d\n",(int)s[i].s.x,(int)s[i].s.y,(int)s[i].e.x,(int)s[i].e.y);
if(sv
)
printf("%d %d %d %d\n",(int)s
.s.x,(int)s
.s.y,(int)s
.e.x,(int)s
.e.y);
if(sv[n-1])
printf("%d %d %d %d\n",(int)s[n-1].s.x,(int)s[n-1].s.y,(int)s[n-1].e.x,(int)s[n-1].e.y);
return 0;
}