判断点是否在凸多边形内

struct Point
{
    float x;
    float y;
    Point() :x(0), y(0){}
};

Point SubPoint(const Point& vTarget1, const Point& vTarget2)
{
    Point vPoint;
    vPoint.x = vTarget1.x - vTarget2.x;
    vPoint.y = vTarget1.y - vTarget2.y;
    return vPoint;
}

float CrossProduct(const Point& vTarget1, const Point& vTarget2)
{
    return vTarget1.x * vTarget2.y - vTarget2.x * vTarget1.y;
}

bool IsPointInConvexPolygon(const vector<Point>& aPoints, const Point& vTarget)
{
    if (aPoints.size() == 0)
        return false;

    float nCurCrossProduct = 0, nLastValue = 0;
    for (int i = 0; i < aPoints.size(); i++)
    {
        Point vU = SubPoint(vTarget, aPoints[i]);
        int nNextIndex = (i + 1) % aPoints.size();
        Point vV = SubPoint(aPoints[nNextIndex], aPoints[i]);

        nCurCrossProduct = CrossProduct(vU, vV);
        if (i > 0 && nCurCrossProduct * nLastValue <= 0)
        {
            return false;
        }
        nLastValue = nCurCrossProduct;
    }
    return true;
}