判断点与多边形的位置关系

版权声明：2018/4/10重启blog；转载请注明出处 https://blog.csdn.net/zhaiqiming2010/article/details/82228340

struct point
{
    double x,y;
    point() {}
    point(double x_, double y_)
    {
        x = x_;
        y = y_;
    }

    friend point operator - (const point &a,const point &b)
    {
        return point(a.x-b.x,a.y-b.y);
    }

    friend point operator + (const point &a,const point &b)
    {
        return point(a.x+b.x,a.y+b.y);
    }

};
int cmp(double x)
{
    if(fabs(x) < eps)
        return 0;
    if(x > 0)
        return 1;
    return -1;
}
double det(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;
}
bool PointOnSegment(point p1, point s, point t)
{
    return cmp(det(p1-s,t-s)) == 0
    && cmp(dot(p1-s,p1-t)) <= 0;
}
struct polygon
{
    int n ;
    point a[maxn];
    polygon() {}
    int point_in(point t)
    {
        int num = 0;
        int i, d1,d2,k;
        a[n]=a[0];
        for(i = 0 ; i < n ; i++)
        {
            if(PointOnSegment(t,a[i],a[i+1]))
                return 2;
            k = cmp(det(a[i+1]-a[i],t-a[i]));
            d1 = cmp(a[i].y-t.y);
            d2 = cmp(a[i+1].y-t.y);
            if(k > 0 && d1 <= 0 && d2 > 0)
                num++;
            if(k < 0 && d2 <= 0 && d1 > 0)
                num--;
        }
        return num!=0;
    }
} poly;