Initialize intersectionCount to zero.
    For each edge e of Q, defined by p1 and p2, check the following:
        If p1y < Py and p2y < Py, do nothing (both points are in region R1).
        If p1y >= Py and p2y >= Py, do nothing (both points are in region R2).
        Otherwise, calculate the intersection point S of edge e and the line y = Py. If Sx >= Px, increment intersectionCount.
    After all edges have been checked, P is inside Q, if and only if intersectionCount is odd.