poj A Round Peg in a Ground Hole

http://poj.org/problem?id=1584

 #include<cstdio>
 #include<cstring>
 #include<cmath>
 #include<algorithm>
 using namespace std;

 const int maxn=;
 const double pi=acos(-1.0);
 const double eps=10e-;

 int cmp(double x)
 {
    if(fabs(x)<eps) return ;
    if(x>) return ;
    return -;
 }

 double sqr(double x)
 {
     return x*x;
 }

 struct point
 {
     double x,y;
     point(){}
     point(double a,double b):x(a),y(b){}
     bool operator <(const point &a)const
     {
         return (x<a.x)||(x==a.x&&y<a.y);
     }
     friend point operator -(const point &a,const point &b){
         return point(a.x-b.x,a.y-b.y);
     }
     double norm(){
         return sqrt(sqr(x)+sqr(y));
     }
 }p[maxn],ch[maxn];

 struct line
 {
    point a,b;
    line(){}
    line(point x,point y):a(x),b(y){}
 };

 double det(point a,point b,point c)
 {
     return ((b.x-a.x)*(c.y-a.y)-(c.x-a.x)*(b.y-a.y));
 }

 double cross(point a,point b,point c)
 {
     return ((b.x-a.x)*(c.y-b.y)-(c.x-b.x)*(b.y-a.y));
 }
 double det1(const point &a,const point &b)
 {
     return a.x*b.y-a.y*b.x;
 }

 double dot(const point &a,const point &b)
 {
     return a.x*b.x+a.y*b.y;
 }

 double dis(point a,point b)
 {
     return sqrt(sqr(a.x-b.x)+sqr(a.y-b.y));
 }

 double dis_point_segment(const point p,const point s,const point t)
 {
     if(cmp(dot(p-s,t-s))<) return (p-s).norm();
     if(cmp(dot(p-t,s-t))<) return (p-t).norm();
     return fabs(det1(s-p,t-p)/dis(s,t));
 }

 bool pointonsegment(point p,point s,point t)
 {
     return cmp(det1(p-s,t-s))==&&cmp(dot(p-s,p-t))<=;
 }

 int convex_hull(point *p,int n,point *ch)
 {
     sort(p,p+n);
     int m=;
     for(int i=; i<n; i++)
     {
         while(m>&&det(ch[m-],ch[m-],p[i])<=) m--;
         ch[m++]=p[i];
     }
     int k=m;
     for(int i=n-; i>=; i--)
     {
         while(m>k&&det(ch[m-],ch[m-],p[i])<=) m--;
         ch[m++]=p[i];
     }
     if(n>) m--;
     return m;
 }

 bool convex_hull1(point *p,int n)
 {
     int flag=;
     p[n]=p[];
     for(int i=; i<=n; i++)
     {
         //printf("%lf%lf %lf%lf %lf%lf\n",p[i-2].x,p[i-2].y,p[i-1].x,p[i-1].y,p[i].x,p[i].y);
         int t=cmp(cross(p[i-],p[i-],p[i]));
         //printf("%d\n",t);
         if(!flag) flag=t;
         if(flag*t<) return false;
     }
     return true;
 }
 int point_in(point t,point *ch,int n)
 {
     int num=,d1,d2,k;
     ch[n]=ch[];
     for(int i=; i<n; i++)
     {
         if(pointonsegment(t,ch[i],ch[i+])) return ;
         k=cmp(det1(ch[i+]-ch[i],t-ch[i]));
         d1=cmp(ch[i].y-t.y);
         d2=cmp(ch[i+].y-t.y);
         if(k>&&d1<=&&d2>) num++;
         if(k<&&d2<=&&d1>) num--;
     }
     return num!=;
 }
 int main()
 {
     int n;
     double r,x,y;
     //freopen("sb.txt","w",stdout);
     while(scanf("%d",&n)!=EOF)
     {
         if(n<) break;
         scanf("%lf%lf%lf",&r,&x,&y);
         point t(x,y);
         for(int i=; i<n; i++)
         {
             scanf("%lf%lf",&p[i].x,&p[i].y);
         }

         if(!convex_hull1(p,n))
         {
             printf("HOLE IS ILL-FORMED\n");
             continue;
         }
         int cn=convex_hull(p,n,ch);
         if(point_in(t,ch,cn))
         {
             double max1=dis_point_segment(t,ch[],ch[]);
             for(int i=; i<cn+; i++)
             {
                 max1=min(max1,dis_point_segment(t,ch[i-],ch[i]));
             }
             if(max1-r>=) printf("PEG WILL FIT\n");
             else printf("PEG WILL NOT FIT\n");
         }
         else printf("PEG WILL NOT FIT\n");
     }
     return ;
 }