USACO6.5.2 Closed Fences(fence4)

思路简单，难写，耐性子写吧

/*
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[n]=LINESEG(p[1],p[n]);
    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[n])
    	printf("%d %d %d %d\n",(int)s[n].s.x,(int)s[n].s.y,(int)s[n].e.x,(int)s[n].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;
}