[ZOJ 1010] Area (计算几何)

 1 #include <cstdio>
 2 #include <cstdlib>
 3 #include <string>
 4 #include <iostream>
 5 #include <cstring>
 6 #include <algorithm>
 7 #include <cctype>
 8 #include <vector>
 9 #include <map>
10 #include <set>
11 #include <iterator>
12 #include <functional>
13 #include <cmath>
14 #include <numeric>
15 #include <ctime>
16 using namespace std;
17 typedef long long LL;
18 typedef pair<int,int> PII;
19 typedef vector<int> VI;
20 #define PB push_back
21 #define MP make_pair
22 #define SZ size()
23 #define CL clear()
24 #define AA first
25 #define BB second
26 #define EPS 1e-8
27 #define ZERO(x) memset((x),0,sizeof(x))
28 const int INF = ~0U>>1;
29 const double PI = acos(-1.0);
30 
31 struct POINT{
32     double x,y;
33     POINT(double ax=0,double ay=0):x(ax),y(ay) {}
34 };
35 typedef POINT VECTOR;
36 
37 POINT pts[1010];
38 
39 VECTOR operator- (POINT A,POINT B){
40     return VECTOR(A.x-B.x,A.y-B.y);
41 }
42 
43 double CROSS(VECTOR A,VECTOR B){
44     return A.x*B.y-A.y*B.x;
45 }
46 
47 double AREA(POINT *p,int n){
48     double area = 0;
49     for(int i=1;i<n-1;i++){
50         area += CROSS(p[i]-p[0],p[i+1]-p[0]);
51     }
52     return area/2;
53 }
54 
55 bool SegmentProperIntersection(POINT a1,POINT a2,POINT b1,POINT b2){
56     double c1 = CROSS(a2-a1,b1-a1) , c2 = CROSS(a2-a1,b2-a1),
57            c3 = CROSS(b2-b1,a1-b1) , c4 = CROSS(b2-b1,a2-b1);
58     return c1*c2<EPS && c3*c4<EPS;
59 }
60 
61 int main(){
62     int n;
63     int kase = 1;
64     while( scanf("%d",&n), n ){
65         if(kase!=1) puts("");
66         bool flag = true;
67         for(int i=0;i<n;i++){
68             double a,b;
69             scanf("%lf%lf",&a,&b);
70             pts[i] = POINT(a,b);
71             for(int j=0;j<i-2;j++){
72                 if( SegmentProperIntersection(pts[j],pts[j+1],pts[i-1],pts[i])) flag = false;
73             }
74         }
75         for(int j=1;j<n-2;j++){
76             if( SegmentProperIntersection(pts[j],pts[j+1],pts[0],pts[n-1])) flag = false;
77         }
78         double area = fabs(AREA(pts,n));
79         if( n==1||n==2 ){
80             printf("Figure %d: Impossible\n",kase++);
81             continue;
82         }
83         if(!flag) printf("Figure %d: Impossible\n",kase++);
84         else printf("Figure %d: %.2f\n",kase++,area);
85     }
86     return 0;
87 }