zoj 1670 Jewels from Heaven

题意：三个人，在给定正方形内，求第一个人拿到珠宝的概率。珠宝随机出现在正方形内。

思路：中垂线+半平面相交。

 #include<cstdio>
 #include<cstring>
 #include<algorithm>
 #include<iostream>
 #include<cstdlib>
 #include<string>
 #include<cmath>
 #include<vector>
 using namespace std;
 const int maxn=1e5+;
 const double eps=1e-;
 const double pi=acos(-);

 double dcmp(double x)
 {
     if(fabs(x) < eps) return ;
     else return x <  ? - : ;
 }

 struct Point
 {
     double x, y;
     Point(double x=, double y=):x(x),y(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 Point& A, const Point& B)
 {
     return Vector(A.x-B.x, A.y-B.y);
 }

 Vector operator * (const Point& A, double v)
 {
     return Vector(A.x*v, A.y*v);
 }

 Vector operator / (const Point& A, double v)
 {
     return Vector(A.x/v, A.y/v);
 }

 double Cross(const Vector& A, const Vector& B)
 {
     return A.x*B.y - A.y*B.x;
 }

 double Dot(const Vector& A, const Vector& B)
 {
     return A.x*B.x + A.y*B.y;
 }

 double Length(const Vector& A)
 {
     return sqrt(Dot(A,A));
 }

 Vector Rotate(Vector A,double rad)
 {
     return Vector(A.x*cos(rad)-A.y*sin(rad),A.x*sin(rad)+A.y*cos(rad));
 }

 bool operator < (const Point& p1, const Point& p2)
 {
     return p1.x < p2.x || (p1.x == p2.x && p1.y < p2.y);
 }

 bool operator == (const Point& p1, const Point& p2)
 {
     return p1.x == p2.x && p1.y == p2.y;
 }

 Vector Normal(Vector A)
 {
     double L=Length(A);
     return Vector(-A.y/L,A.x/L);
 }
 struct Line
 {
     Point P;
     Vector v;
     double ang;
     Line() {}
     Line(Point P, Vector v):P(P),v(v)
     {
         ang = atan2(v.y, v.x);
     }
     bool operator < (const Line& L) const
     {
         return ang < L.ang;
     }
 };

 bool OnLeft(const Line& L, const Point& p)
 {
     return Cross(L.v, p-L.P) > ;
 }

 Point GetLineIntersection(const Line& a, const Line& b)
 {
     Vector u = a.P-b.P;
     double t = Cross(b.v, u) / Cross(a.v, b.v);
     return a.P+a.v*t;
 }

 const double INF = 1e8;

 Point ansPoly[maxn];
 int HalfplaneIntersection(vector<Line> L)     //L为切割平面的直线集合，求半平面交，返回点的个数，点存在anspoly数组中
 {
     int n = L.size();
     sort(L.begin(), L.end()); // 按极角排序
     int first, last;         // 双端队列的第一个元素和最后一个元素的下标
     vector<Point> p(n);      // p[i]为q[i]和q[i+1]的交点
     vector<Line> q(n);       //
     q[first=last=] = L[];  //
     for(int i = ; i < n; i++)
     {
         while(first < last && !OnLeft(L[i], p[last-])) last--;
         while(first < last && !OnLeft(L[i], p[first])) first++;
         q[++last] = L[i];
         if(fabs(Cross(q[last].v, q[last-].v)) < eps)   //
         {
             last--;
             if(OnLeft(q[last], L[i].P)) q[last] = L[i];
         }
         if(first < last) p[last-] = GetLineIntersection(q[last-], q[last]);
     }
     while(first < last && !OnLeft(q[first], p[last-])) last--; //
     if(last - first <= ) return ; //
     p[last] = GetLineIntersection(q[last], q[first]); //
     // 从deque复制到输出中
     int index=;
     for(int i = first; i <= last; i++) ansPoly[index++]=p[i];
     return index;
 }

 double PolygonArea(int n,Point *p)
 {
     double area=;
     for(int i=; i<n-; i++)
         area+=Cross(p[i]-p[],p[i+]-p[]);
     return area/;
 }

 Point p[];
 int main()
 {
 //  freopen("in.txt","r",stdin);
     while(cin>>p[].x>>p[].y>>p[].x>>p[].y>>p[].x>>p[].y)
     {
         if(p[].x==p[].x&&p[].y==p[].y&&p[].x==)break;
 //        Point zd1,zd2;
         vector<Line>  vec;
         vec.push_back(Line(Point(,),Point(,)));
         vec.push_back(Line(Point(,),Point(,)));
         vec.push_back(Line(Point(,),Point(-,)));
         vec.push_back(Line(Point(,),Point(,-)));
         Vector v=(p[]-p[]);
         vec.push_back(Line((p[]+p[])*0.5,Normal(v)));
         v=(p[]-p[]);
         vec.push_back(Line((p[]+p[])*0.5,Normal(v)));
         int m=HalfplaneIntersection(vec);
         double ans=PolygonArea(m,ansPoly);
         printf("%.3f\n",ans/(1.0**));
     }
     return ;
 }