bzoj 2451 Uyuw's Concert

裸的半平面交。感觉这些东西，纯属在考代码能力啊。。

 #include<cstdio>
 #include<algorithm>
 #include<cmath>
 #define eps 1e-8
 using namespace std;
 int n,tot;
 double ans;
 struct point{double x,y;}a[];
 struct line{point a,b; double angle;}l[],q[];
 point operator - (point a, point b){
     point t; t.x=a.x-b.x; t.y=a.y-b.y; return t;
 }
 long double operator * (point a, point b){
     return a.x*b.y-a.y*b.x;
 }
 bool operator < (line a, line b){
     if (a.angle==b.angle) return (b.b-a.a)*(b.a-a.a)<;
     return a.angle<b.angle;
 }
 point intersection_point(line a, line b)
 {
     double k1,k2,t;
     point ans;
     k1=(a.b-b.a)*(b.b-b.a);
     k2=(b.b-b.a)*(a.a-b.a);
     t=k1/(k1+k2);
     ans.x=a.b.x+(a.a.x-a.b.x)*t;
     ans.y=a.b.y+(a.a.y-a.b.y)*t;
     return ans;
 }
 bool judge(line a, line b, line t)
 {
     point p=intersection_point(a,b);
 //    printf("%lf %lf\n",a.b.x,a.b.y);
     return (t.a-p)*(t.b-p)<;
 }
 void half_plane_intersection()
 {
     sort(l+,l+n+);
     int top=,bottom=; tot=;
     for (int i=; i<=n; i++)
         if (l[i].angle!=l[i-].angle) l[++tot]=l[i];
     n=tot; q[]=l[]; q[]=l[];
     //for (int i=1; i<=n; i++)
     //    printf("%.1lf  %.1lf  %.1lf   %.1lf\n",l[i].a.x,l[i].a.y,l[i].b.x,l[i].b.y);
     for (int i=; i<=n; i++)
     {
         while (bottom<top && judge(q[top],q[top-],l[i])) top--;
         while (bottom<top && judge(q[bottom],q[bottom+],l[i])) bottom++;
         q[++top]=l[i];
     }
     while (bottom<top && judge(q[top],q[top-],q[bottom])) top--;
     while (bottom<top && judge(q[bottom],q[bottom+],q[top])) bottom++;
     q[top+]=q[bottom];
     tot=;
     for (int i=bottom; i<=top; i++)
         a[++tot]=intersection_point(q[i],q[i+]);
 }
 void get_area()
 {
     if (tot<) return;
     a[++tot]=a[];
     //for (int i=1; i<=tot; i++)
     //    printf("%lf %lf\n",a[i].x,a[i].y); while (1);
     for (int i=; i<=tot; i++)
         ans+=a[i]*a[i+];
     ans=fabs(ans)/;
 }
 int main()
 {
     while (~scanf("%d",&n))
     {
         for (int i=; i<=n; i++)
             scanf("%lf%lf%lf%lf",&l[i].a.x,&l[i].a.y,&l[i].b.x,&l[i].b.y);
         l[++n].a.x=; l[n].a.y=; l[n].b.x=; l[n].b.y=;
         l[++n].a.x=; l[n].a.y=; l[n].b.x=; l[n].b.y=;
         l[++n].a.x=; l[n].a.y=; l[n].b.x=; l[n].b.y=;
         l[++n].a.x=; l[n].a.y=; l[n].b.x=; l[n].b.y=;
         for (int i=; i<=n; i++)
             l[i].angle=atan2((l[i].b.y-l[i].a.y),(l[i].b.x-l[i].a.x));
         half_plane_intersection();
         ans=; get_area();
         printf("%.1lf",ans);
     }
     return ;
 }