高效算法设计专项：UVa 10535

#include <iostream>
#include <cstdio>
#include <cmath>
#include <algorithm>
using namespace std;
#define M 510
const double eps=1e-8;
int dcmp(double x)
{
if(fabs(x)<eps) return 0;
else return x<0?-1:1;
}
struct point
{
double x,y;
point(double x=0,double y=0):x(x),y(y){}
};
point operator-(point a,point b){return point(a.x-b.x,a.y-b.y);}
point operator+(point a,point b){return point(a.x+b.x,a.y+b.y);}
point operator*(point a,double p){return point(a.x*p,a.y*p);}
double cross(point a,point b){return a.x*b.y-a.y*b.x;}
double dot(point a,point b){return a.x*b.x+a.y*b.y;}
bool seginter(point a1,point a2,point b1,point b2)
{
double c1=cross(a2-a1,b1-a1),c2=cross(a2-a1,b2-a1);
double c3=cross(b2-b1,a1-b1),c4=cross(b2-b1,a2-b1);
return dcmp(c1)*dcmp(c2)<0&&dcmp(c3)*dcmp(c4)<0;
}
bool onseg(point p,point a1,point a2)
{
return dcmp(cross(a1-p,a2-p))==0&&dcmp(dot(a1-p,a2-p))<0;
}
int n;
point s[M],e[M],q;
int main()
{
freopen("in.txt","r",stdin);
while(cin>>n&&n)
{
for(int i=0;i<n;i++) cin>>s[i].x>>s[i].y>>e[i].x>>e[i].y;
cin>>q.x>>q.y;
int ans=0;
for(int i=0;i<n;i++)
{
point tmp;int cnt;
tmp=q+(s[i]-q)*1e9;
cnt=0;
for(int j=0;j<n;j++)
{
if(seginter(q,tmp,s[j],e[j])||onseg(s[j],q,tmp)||onseg(e[j],q,tmp))
cnt++;
}
ans=max(ans,cnt);
tmp=q+(e[i]-q)*1e9;
cnt=0;
for(int j=0;j<n;j++)
{
if(seginter(q,tmp,s[j],e[j])||onseg(s[j],q,tmp)||onseg(e[j],q,tmp))
cnt++;
}
ans=max(ans,cnt);
}
cout<<ans<<endl;
}
return 0;
}