【半平面交】bzoj2618 [Cqoi2006]凸多边形

#include<cstdio>
#include<cmath>
#include<algorithm>
using namespace std;
#define EPS 0.0000001
#define N 511
typedef double db;
const db PI=acos(-1.0);
struct Point{db x,y;};
typedef Point Vector;
Vector operator - (const Point &a,const Point &b){return (Vector){a.x-b.x,a.y-b.y};}
Vector operator * (const Vector &a,const db &k){return (Vector){a.x*k,a.y*k};}
Vector operator + (const Vector &a,const Vector &b){return (Vector){a.x+b.x,a.y+b.y};}
db Cross(Vector a,Vector b){return a.x*b.y-a.y*b.x;}
struct Line
{
    Point p; Vector v; db ang;
    Line(){}
    Line(const Point &a,const Point &b)
      {
        v=b-a;
        p=a;
        ang=atan2(v.y,v.x);
        if(ang<0) ang+=2.0*PI; 
      }
};
bool operator < (const Line &a,const Line &b){return a.ang<b.ang;}
bool OnLeft(Line l,Point a){return Cross(l.v,a-l.p)>0;}
Point GetJiaodian(Line a,Line b){return a.p+a.v*(Cross(b.v,a.p-b.p)/Cross(a.v,b.v));}
int n;
Point ps[N];
Line q[N];
int head=1,tail=1;
Line ls[N];
void BPMJ()
{
    sort(ls+1,ls+n+1);
    q[1]=ls[1];
    for(int i=2;i<=n;++i)
      {
        while(head<tail&&(!OnLeft(ls[i],ps[tail-1]))) --tail;
        while(head<tail&&(!OnLeft(ls[i],ps[head]))) ++head;
        q[++tail]=ls[i];
        if(fabs(Cross(q[tail].v,q[tail-1].v))<EPS)
          {
            --tail;
            if(OnLeft(q[tail],ls[i].p))
              q[tail]=ls[i];
          }
        if(head<tail)
          ps[tail-1]=GetJiaodian(q[tail-1],q[tail]);
      }
    while(head<tail&&(!OnLeft(q[head],ps[tail-1]))) --tail;
    ps[tail]=GetJiaodian(q[tail],q[head]);
}
int nn,mm;
Point tmp[51];
db area;
int main()
{
    scanf("%d",&nn);
    for(int i=1;i<=nn;++i)
      {
        scanf("%d",&mm);
        for(int j=1;j<=mm;++j) scanf("%lf%lf",&tmp[j].x,&tmp[j].y);
        for(int j=1;j<mm;++j) ls[++n]=Line(tmp[j],tmp[j+1]);
        ls[++n]=Line(tmp[mm],tmp[1]);
      }
    BPMJ();
    for(int i=head+1;i<tail;++i)
      area+=Cross(ps[i]-ps[head],ps[i+1]-ps[head]);
    printf("%.3lf\n",area/2.0);
    return 0;
}