A Round Peg in a Ground Hole(圆与凸包)

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

题意：判断所给的点能不能形成凸包，并判断所给的圆是否在凸包内。

改了好几天的一个题，今天才发现是输入顺序弄错了，办过的最脑残的事情。。sad

 #include <stdio.h>
 #include <algorithm>
 #include <iostream>
 #include <string.h>
 #include <math.h>
 using namespace std;
 const int N=;
 const double eps=1e-;
 double pi=acos(-1.0);
 int n;
 struct point
 {
     double x,y;
     point(double x = ,double y = ):x(x),y(y) {}
     double norm()//向量的模
     {
         return sqrt(x*x+y*y);
     }

 };
 point operator-(const point &a,const point &b)
 {
     return point(a.x-b.x,a.y-b.y);
 }
 int cmp(double x)//精度处理
 {
     if (fabs(x) < eps)
         return ;
     if (x > )
         return ;
     return -;

 }
 double det(const point &a,const point &b)//叉乘
 {
     return a.x*b.y-a.y*b.x;
 }

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

 double dist(const point &a,const point &b)//两点间的距离
 {
     return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
 }
 bool PointOnSegment(point p,point s,point t)//判断点是否在线段上
 {
     return cmp(det(p-s,t-s))==&&cmp(dot(p-s,p-t))<=;
 }

 double dis_point_segment(const point p,const point s,const point t)//点到线段的距离
 {
     if(cmp(dot(p-s,t-s))<) return (p-s).norm();
     if(cmp(dot(p-t,s-t))<) return (p-t).norm();
     return fabs(det(s-p,t-p)/dist(s,t));
 }
 bool is_convex(point *p)//判断凸包
 {
     int pre = ;
     p[n] = p[];
     p[n+] = p[];
     for (int i = ; i <= n; i++)
     {
         int dir = cmp(det(p[i-]-p[i-],p[i]-p[i-]));
         if (!pre)
             pre = dir;
         if (pre*dir < ) return false;
     }
     return true;
 }
 int point_in(point t,point *ch)//判断点是否在凸包内
 {
     int num=,d1,d2,k;
     ch[n]=ch[];
     for(int i=; i<n; i++)
     {
         if(PointOnSegment(t,ch[i],ch[i+])) return ;
         k=cmp(det(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()
 {
     double x,y,r;
     while(~scanf("%d",&n))
     {
         if (n < )
             break;
         point p[N];
         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);
         }
         p[n] = p[];//连接首尾的点
         if (!is_convex(p))
         {
             printf("HOLE IS ILL-FORMED\n");
             continue;
         }
         if(point_in(t,p))
         {
             double Min = dis_point_segment(t,p[],p[]);
             for (int i = ; i < n; i++)
             {
                 double dis = dis_point_segment(t,p[i],p[i+]);
                 Min = min(dis,Min);//圆心到所有线段的最小距离
             }
             if (cmp(Min-r)>= )
                 printf("PEG WILL FIT\n");
             else
                 printf("PEG WILL NOT FIT\n");
         }
         else
             printf("PEG WILL NOT FIT\n");

     }
     return ;
 }