poj1584A Round Peg in a Ground Hole

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题意甚是难懂！这是第二遍做这道题了，依旧无法理解题意，搜了下题意。。。

首先需要判断是不是为凸多边形。（从一个顶点走一遍即可，要注意顺逆时针，题目中没有指明）

其次看一下圆是不是能够放入多边形内。（首先判断一下圆心是否在圆内，然后枚举圆心到所有边的距离与半径r进行比较）

#include <iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<stdlib.h>
#include<vector>
#include<cmath>
#include<queue>
#include<set>
using namespace std;
#define N 10000
#define LL long long
#define INF 0xfffffff
const double eps = 1e-8;
const double pi = acos(-1.0);
const double inf = ~0u>>2;
#define _sign(x) ((x) > eps?1:((x) < -eps ? 2 :0))
struct Point
{
    double x,y;
    Point(double x=0,double y=0):x(x),y(y) {}
}p[N],ch[N];
int top;
struct Circle
{
    Point c;
    double r;
    //Circle(Point c,double r):c(c),r(r){}
    Point point (double a)
    {
        return Point(c.x+cos(a)*r,c.y+sin(a)*r);
    }
};
typedef Point pointt;
pointt operator + (Point a,Point b)
{
    return Point(a.x+b.x,a.y+b.y);
}
pointt operator - (Point a,Point b)
{
    return Point(a.x-b.x,a.y-b.y);
}
double cross(Point a,Point b)
{
    return a.x*b.y-a.y*b.x;
}
double mul(Point p0,Point p1,Point p2)
{
    return cross(p1-p0,p2-p0);
}
double dis(Point a)
{
    return sqrt(a.x*a.x+a.y*a.y);
}
//精度判正负
int dcmp(double x)
{
    if(fabs(x)<eps) return 0;
    else return x<0?-1:1;
}
int Graham(int n)
{
    int i,s[3] = {1,1,1};
    double t;
    for(i = 0; i < n&&s[1]|s[2]; i ++)
    {
        t = mul(p[(i+1)%n],p[(i+2)%n],p[i]);
        s[_sign(t)] = 0;
    }
    return s[1]|s[2];
}
double distoline(Point p,Point a,Point b)
{
    Point v1 = b-a,v2 = p-a;
    return fabs(cross(v1,v2)/dis(v1));
}
int inside(Point po,int n)
{
    int i,s[3] = {1,1,1};
    double t;
    for(i = 0;i < n&&s[1]|s[2];i ++)
    {
        t = mul(p[(i+1)%n],po,p[i]);
        s[_sign(t)] = 0;
    }
    return s[1]|s[2];
}
int main()
{
    int n,i;
    Circle cc;
    while(scanf("%d",&n)!=EOF)
    {
        if(n<3) break;
        scanf("%lf%lf%lf",&cc.r,&cc.c.x,&cc.c.y);
        for(i  = 0; i < n; i++)
        scanf("%lf%lf",&p[i].x,&p[i].y);
        top = 0;
        int m = Graham(n);
        if(!m)
        {
            puts("HOLE IS ILL-FORMED");
            continue;
        }
        int flag = 1;
        if(!inside(cc.c,n))
        {
            puts("PEG WILL NOT FIT");
            continue;
        }
        for(i = 1; i < n; i ++)
        {
            if(dcmp(distoline(cc.c,p[i],p[i-1])-cc.r)<0)
            {
                flag = 0;
                break;
            }
        }
        if(!flag)
        puts("PEG WILL NOT FIT");
        else
        puts("PEG WILL FIT");
    }
    return 0;
}