POJ 1474 Video Surveillance(半平面交)

题目链接

2Y，模版抄错了一点。

 #include <cstdio>
 #include <cstring>
 #include <string>
 #include <cmath>
 #include <algorithm>
 using namespace std;
 #define eps 1e-8
 #define N 2001
 struct point
 {
     double x,y;
 }p[N],pre[N],temp[N];
 double a,b,c;
 int n,m;
 void getline(point x,point y)
 {
     a = y.y - x.y;
     b = x.x - y.x;
     c = y.x * x.y - x.x * y.y;
 }
 point intersect(point x,point y)
 {
     double u = fabs(a*x.x + b*x.y + c);
     double v = fabs(a*y.x + b*y.y + c);
     point ans;
     ans.x = (x.x*v+y.x*u)/(u+v);
     ans.y = (x.y*v+y.y*u)/(u+v);
     return ans;
 }
 void cut()
 {
     int num = ,i;
     for(i = ;i <= m;i ++)
     {
         if(a*p[i].x + b*p[i].y + c > -eps)
         {
             temp[++num] = p[i];
         }
         else
         {
             if(a*p[i-].x + b*p[i-].y + c > eps)
             temp[++num] = intersect(p[i],p[i-]);
             if(a*p[i+].x + b*p[i+].y + c > eps)
             temp[++num] = intersect(p[i],p[i+]);
         }
     }
     for(i = ;i <= num;i ++)
     p[i] = temp[i];
     p[] = p[num];
     p[num+] = p[];
     m = num;
 }
 void fun()
 {
     int i;
     m = n;
     for(i = ;i <= n;i ++)
     {
         getline(pre[i],pre[i+]);
         cut();
     }
 }
 int main()
 {
     int i,cas = ;
     while(scanf("%d",&n)!=EOF)
     {
         if(!n) break;
         for(i = ;i <= n;i ++)
         {
             scanf("%lf%lf",&pre[i].x,&pre[i].y);
             p[i] = pre[i];
         }
         pre[n+] = pre[];
         p[n+] = p[];
         p[] = p[n];
         fun();
         printf("Floor #%d\n",cas ++);
         if(m)
         printf("Surveillance is possible.\n");
         else
         printf("Surveillance is impossible.\n");
         printf("\n");
     }
     return ;
 }