poj 1474 Video Surveillance （半平面交）

 1 #include <stdio.h>
 2 #include <math.h>
 3 #include <string.h>
 4 #include <stdlib.h>
 5 #include <iostream>
 6 #include <algorithm>
 7 
 8 #define eps 1e-8
 9 #define MAXX 105
10 using namespace std;
11 typedef struct point
12 {
13     double x;
14     double y;
15 }point;
16 
17 point p[MAXX],s[MAXX];
18 
19 bool  dy(double x,double y){ return x>y+eps; }
20 bool  xy(double x,double y){ return x<y-eps; }
21 bool dyd(double x,double y){ return x>y-eps; }
22 bool xyd(double x,double y){ return x<y+eps; }
23 bool  dd(double x,double y){ return fabs(x-y)<eps; }
24 
25 double crossProduct(point a,point b,point c)
26 {
27     return (c.x-a.x)*(b.y-a.y)-(c.y-a.y)*(b.x-a.x);
28 }
29 point IntersectPoint(point u1,point u2,point v1,point v2)
30 {
31     point ans=u1;
32     double t=((u1.x-v1.x)*(v1.y-v2.y)-(u1.y-v1.y)*(v1.x-v2.x))/
33         ((u1.x-u2.x)*(v1.y-v2.y)-(u1.y-u2.y)*(v1.x-v2.x));
34     ans.x+=(u2.x - u1.x)*t;
35     ans.y+=(u2.y - u1.y)*t;
36     return ans;
37 }
38 
39 void cut(point p[],point s[],int n,int &len)
40 {
41     point tp[MAXX];
42     p[n]=p[0];
43     for(int i=0; i<=n; i++)
44     {
45         tp[i] = p[i];
46     }
47     int cp=n,tc;
48     for(int i=0; i<n; i++)
49     {
50         tc=0;
51         for(int k=0; k<cp; k++)
52         {
53             if(dyd(crossProduct(p[i],p[i+1],tp[k]),0.0))//clock-wise
54                 s[tc++]=tp[k];
55             if(xy(crossProduct(p[i],p[i+1],tp[k])*
56                   crossProduct(p[i],p[i+1],tp[k+1]),0.0))
57                 s[tc++]=IntersectPoint(p[i],p[i+1],tp[k],tp[k+1]);
58         }
59         s[tc]=s[0];
60         for(int k=0; k<=tc; k++)
61             tp[k]=s[k];
62         cp=tc;
63     }
64     len=cp;
65 }
66 
67 int main()
68 {
69     int n,i,j;
70     int cas=1;
71     while(scanf("%d",&n)!=EOF && n)
72     {
73         for(i=0 ;i<n; i++)
74         {
75             scanf("%lf%lf",&p[i].x,&p[i].y);
76         }
77         int len;
78         cut(p,s,n,len);
79         printf("Floor #%d\n",cas++);
80         if(len)
81             printf("Surveillance is possible.\n\n");
82         else printf("Surveillance is impossible.\n\n");
83     }
84     return 0;
85 }