判断一个点是否在一个polygon内部

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struct PolyPoint
{
    double x;
    double y;
    PolyPoint(const double &_x, const double &_y) : x(_x), y(_y) {}
};

struct Polygon
{
    std::deque<PolyPoint> points;
};

bool IsInPolygon(const Polygon &polygon, const PolyPoint &p)
{
    int count = polygon.points.size();

    if (count < 3) {
        return false;
    }

    bool result = false;
    for (int i = 0, j = count - 1; i < count; i++) {
        PolyPoint p1 = polygon.points[i];
        PolyPoint p2 = polygon.points[j];

        if ((p1.x < p.x && p2.x >= p.x) || (p2.x < p.x && p1.x >= p.x)) {
            if (p1.y + (p.x - p1.x) / (p2.x - p1.x) * (p2.y - p1.y) < p.y) {
                result = !result;
            }
        }
        j = i;
    }
    return result;
}