poj 3675 Telescope

#include<iostream>#include<cmath>#include<cstdio>#include<algorithm>#include<vector>const  double eps=1e-10;const double PI=acos(-1.0);using namespace std;struct Point{    double x;    double y;    Point(double x=0,double y=0):x(x),y(y){}    void operator<<(Point &A) {cout<<A.x<<' '<<A.y<<endl;}};int dcmp(double x)  {return (x>eps)-(x<-eps); }int sgn(double x)  {return (x>eps)-(x<-eps); }typedef  Point  Vector;Vector  operator +(Vector A,Vector B) { return Vector(A.x+B.x,A.y+B.y);}Vector  operator -(Vector A,Vector B) { return Vector(A.x-B.x,A.y-B.y); }Vector  operator *(Vector A,double p) { return Vector(A.x*p,A.y*p);  }Vector  operator /(Vector A,double p) {return Vector(A.x/p,A.y/p);}ostream &operator<<(ostream & out,Point & P) { out<<P.x<<' '<<P.y<<endl; return out;}//bool  operator< (const Point &A,const Point &B) { return dcmp(A.x-B.x)<0||(dcmp(A.x-B.x)==0&&dcmp(A.y-B.y)<0); }bool  operator== ( const Point &A,const Point &B) { return dcmp(A.x-B.x)==0&&dcmp(A.y-B.y)==0;}double  Dot(Vector A,Vector B) {return A.x*B.x+A.y*B.y;}double  Cross(Vector A,Vector B)  {return A.x*B.y-B.x*A.y; }double  Length(Vector A)  { return sqrt(Dot(A, A));}double  Angle(Vector A,Vector B) {return acos(Dot(A,B)/Length(A)/Length(B));}double  Area2(Point A,Point B,Point C ) {return Cross(B-A, C-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));}Vector Normal(Vector A) {double L=Length(A);return Vector(-A.y/L,A.x/L);}Point GetLineIntersection(Point P,Vector v,Point Q,Vector w){    Vector u=P-Q;    double t=Cross(w, u)/Cross(v,w);    return P+v*t;}double DistanceToLine(Point P,Point A,Point B){    Vector v1=P-A; Vector v2=B-A;    return fabs(Cross(v1,v2))/Length(v2);}double DistanceToSegment(Point P,Point A,Point B){    if(A==B)  return Length(P-A);    Vector v1=B-A;    Vector v2=P-A;    Vector v3=P-B;    if(dcmp(Dot(v1,v2))==-1)    return  Length(v2);    else if(Dot(v1,v3)>0)    return Length(v3);    else return DistanceToLine(P, A, B);}Point GetLineProjection(Point P,Point A,Point B){    Vector v=B-A;    Vector v1=P-A;    double t=Dot(v,v1)/Dot(v,v);    return  A+v*t;}bool  SegmentProperIntersection(Point a1,Point a2,Point b1,Point b2){    double c1=Cross(b1-a1, a2-a1);    double c2=Cross(b2-a1, a2-a1);    double c3=Cross(a1-b1, b2-b1);    double c4=Cross(a2-b1, b2-b1);    return dcmp(c1)*dcmp(c2)<0&&dcmp(c3)*dcmp(c4)<0 ;}bool  onSegment(Point P,Point A,Point B){    return dcmp(Cross(P-A, P-B))==0&&dcmp(Dot(P-A,P-B))<=0;}double PolygonArea(Point *p,int n){    double area=0;    for(int i=1;i<n-1;i++)    {        area+=Cross(p[i]-p[0], p[i+1]-p[0]);    }    return area/2;}Point  read_point(){    Point P;    scanf("%Lf%Lf",&P.x,&P.y);    return  P;}struct Circle{    Point c;    double r;    Circle(Point c=Point(0,0),double r=0):c(c),r(r) {}    Point point(double a)    {        return Point(c.x+r*cos(a),c.y+r*sin(a));    }};struct  Line{    Point p;    Vector v;    Line(Point p=Point(0,0),Vector v=Vector(0,1)):p(p),v(v) {}    Point point(double t)    {        return Point(p+v*t);    }};int getLineCircleIntersection(Line L,Circle C,double &t1,double &t2,vector<Point> &sol){    double a=L.v.x;    double b=L.p.x-C.c.x;    double c=L.v.y;    double d=L.p.y-C.c.y;    double e=a*a+c*c;    double f=2*(a*b+c*d);    double g=b*b+d*d-C.r*C.r;    double delta=f*f-4*e*g;    if(dcmp(delta)<0) return 0;    if(dcmp(delta)==0)    {        t1=t2=-f/(2*e);        sol.push_back(L.point(t1));        return 1;    }    else    {        t1=(-f-sqrt(delta))/(2*e);        t2=(-f+sqrt(delta))/(2*e);        sol.push_back(L.point(t1));        sol.push_back(L.point(t2));        return 2;    }}double angle(Vector v)  {return atan2(v.y,v.x);}int getCircleCircleIntersection(Circle C1,Circle C2,vector<Point> &sol){    double d=Length(C1.c-C2.c);    if(dcmp(d)==0)    {        if(dcmp(C1.r-C2.r)==0)  return -1;  // 重合        else return 0;    //  内含  0 个公共点    }    if(dcmp(C1.r+C2.r-d)<0)  return 0;  // 外离    if(dcmp(fabs(C1.r-C2.r)-d)>0)  return 0;      double a=angle(C2.c-C1.c);    double da=acos((C1.r*C1.r+d*d-C2.r*C2.r)/(2*C1.r*d));    Point p1=C1.point(a-da);    Point p2=C1.point(a+da);    sol.push_back(p1);    if(p1==p2)  return 1;    else    {        sol.push_back(p2);        return 2;    }}int getTangents(Point p,Circle C,Vector *v){    Vector u=C.c-p;    double dist=Length(u);    if(dcmp(dist-C.r)<0)  return 0;    else if(dcmp(dist-C.r)==0)    {        v[0]=Rotate(u,PI/2);        return 1;    }    else    {        double ang=asin(C.r/dist);        v[0]=Rotate(u,-ang);        v[1]=Rotate(u,+ang);        return 2;    }}int getTangents(Circle A,Circle B,Point *a,Point *b){    int cnt=0;    if(A.r<B.r)    {        swap(A,B); swap(a, b);      }    double d=Length(A.c-B.c);    double rdiff=A.r-B.r;    double rsum=A.r+B.r;    if(dcmp(d-rdiff)<0)  return 0;       double base=angle(B.c-A.c);    if(dcmp(d)==0&&dcmp(rdiff)==0)   return -1 ;      if(dcmp(d-rdiff)==0)      {        a[cnt]=A.point(base);        b[cnt]=B.point(base);        cnt++;        return 1;    }    double ang=acos(rdiff/d);    a[cnt]=A.point(base+ang);    b[cnt]=B.point(base+ang);    cnt++;    a[cnt]=A.point(base-ang);    b[cnt]=B.point(base-ang);    cnt++;    if(dcmp(d-rsum)==0)         {        a[cnt]=A.point(base);        b[cnt]=B.point(base+PI);        cnt++;    }    else  if(dcmp(d-rsum)>0)       {        double  ang_in=acos(rsum/d);        a[cnt]=A.point(base+ang_in);        b[cnt]=B.point(base+ang_in+PI);        cnt++;        a[cnt]=A.point(base-ang_in);        b[cnt]=B.point(base-ang_in+PI);        cnt++;    }    return cnt;}Point Zero=Point(0,0);double  common_area(Circle C,Point A,Point B){    if(A==C.c||B==C.c)  return 0;    double  OA=Length(A-C.c),OB=Length(B-C.c);    double  d=DistanceToLine(Zero, A, B);    int sg=sgn(Cross(A,B));    if(sg==0)  return 0;    double angle=Angle(A,B);    if(dcmp(OA-C.r)<=0&&dcmp(OB-C.r)<=0)    {        return Cross(A,B)/2;    }    else if(dcmp(OA-C.r)>=0&&dcmp(OB-C.r)>=0&&dcmp(d-C.r)>=0)    {        return  sg*C.r*C.r*angle/2;    }    else if (dcmp(OA-C.r)>=0&&dcmp(OB-C.r)>=0&&dcmp(d-C.r)<0)    {        Point prj=GetLineProjection(Zero, A, B);        if(onSegment(prj, A, B))        {        vector<Point> p;        Line L=Line(A,B-A);        double t1,t2;        getLineCircleIntersection(L,C, t1, t2, p);        double s1=0;        s1=C.r*C.r*angle/2;        double s2=0;        s2=C.r*C.r*Angle(p[0],p[1])/2;        s2-=fabs(Cross(p[0],p[1])/2);        s1=s1-s2;        return  sg*s1;        }        else        { return sg*C.r*C.r*angle/2;        }    }    else    { if(dcmp(OB-C.r)<0) {     Point temp=A;     A=B;     B=temp; }         Point inter_point; double t1,t2; Line L=Line(A,B-A); vector<Point> inter; getLineCircleIntersection(L, C, t1, t2,inter);         if(onSegment(inter[0], A, B))     inter_point=inter[0]; else {     inter_point=inter[1]; } double s=fabs(Cross(inter_point, A)/2); s+=C.r*C.r*Angle(inter_point,B)/2; return s*sg;    }}int main(){    Point p[60];    double R;    int n;    Circle C(Zero,0);    while(cin>>R)    {        C.r=R;        cin>>n;        for(int i=0;i<n;i++) p[i]=read_point();        p[n]=p[0];        double ans=0;        for(int i=0;i<n;i++) ans+=common_area(C, p[i], p[i+1]);        printf("%.2fn",fabs(ans)+eps);    }     return 0;}