poj2451Uyuw's Concert(半平面交）

链接

逆时针给出线段，如果模板是顺时针的修改下系数的符号进行平面交即可。

 #include <iostream>
 #include<cstdio>
 #include<cstring>
 #include<algorithm>
 #include<stdlib.h>
 #include<vector>
 #include<cmath>
 #include<queue>
 #include<set>
 using namespace std;
 #define N 20100
 #define LL long long
 #define INF 0xfffffff
 const double eps = 1e-;
 const double pi = acos(-1.0);
 const double inf = ~0u>>;
 int m;
 int cCnt,curCnt;
 struct point
 {
     double x,y;
     point(double x=,double y = ):x(x),y(y) {}
 };
 point points[N],p[N],q[N];
 void getline(point x,point y,double &a,double &b,double   &c)
 {
     a = x.y - y.y;
     b = y.x - x.x;
     c = x.x * y.y-y.x * x.y;
 }
 void initial()
 {
     for(int i = ; i <= m; ++i)p[i] = points[i];
     p[m+] = p[];
     p[] = p[m];
     cCnt = m;
 }
 point intersect(point x,point y,double a,double b,double c)
 {
     double u = fabs(a * x.x + b * x.y + c);
     double v = fabs(a * y.x + b * y.y + c);
     point pt;
     pt.x=(x.x * v + y.x * u) / (u + v);
     pt.y=(x.y * v + y.y * u) / (u + v);
     return  pt;
 }
 void cut(double a,double b ,double c)
 {
     curCnt = ;
     for(int i = ; i <= cCnt; ++i)
     {
         if(a*p[i].x + b*p[i].y + c >= )q[++curCnt] = p[i];
         else
         {
             if(a*p[i-].x + b*p[i-].y + c > )
                 q[++curCnt] = intersect(p[i],p[i-],a,b,c);
             if(a*p[i+].x + b*p[i+].y + c > )
                 q[++curCnt] = intersect(p[i],p[i+],a,b,c);
         }
     }
     for(int i = ; i <= curCnt; ++i)p[i] = q[i];
     p[curCnt+] = q[];
     p[] = p[curCnt];
     cCnt = curCnt;
 }
 void solve()
 {
     double area = ;
     for(int i = ; i <= cCnt; ++i)
         area += p[i].x * p[i + ].y - p[i + ].x * p[i].y;
     area = fabs(area / 2.0);
     printf("%.1f\n",area);

 }
 void GuiZhengHua()
 {
     //规整化方向，逆时针变顺时针，顺时针变逆时针
     for(int i = ; i < (m+)/; i ++)
         swap(points[i], points[m-i]);
 }
 int main()
 {
     points[] = point(,);
     points[] = point(,);
     points[] = point(,);
     points[] = point(,);
     points[] = p[];
     int n,i;
     while(scanf("%d",&n)!=EOF)
     {
         m = ;
         initial();
         for(i = ; i <= ; ++i)
         {
             double a,b,c;
             getline(points[i],points[i+],a,b,c);
             cut(a,b,c);
         }
         for(i =  ; i <= *n ; i+=)
         {
             point p1,p2;
             scanf("%lf%lf%lf%lf",&p1.x,&p1.y,&p2.x,&p2.y);
             double a,b,c;
             getline(p1,p2,a,b,c);
             cut(a,b,c);
         }

        // m = 2*n+4;
         //GuiZhengHua();
         //points[m+1] = points[1];
         solve();
     }
     return ;
 }