这代码有些不懂

function PointInPoly(pt, poly) {

    for (var c = false, i = -1, l = poly.length, j = l - 1; ++i < l; j = i)

        ((poly[i].y <= pt.y && pt.y < poly[j].y) || (poly[j].y <= pt.y && pt.y < poly[i].y))

        && (pt.x < (poly[j].x - poly[i].x) * (pt.y - poly[i].y) / (poly[j].y - poly[i].y) + poly[i].x)

        && (c = !c);

    return c;

}