lua判断点在任意多边形内部

function IsPtInPoly(pt, pts)
    local count  = 0
    local x, y   = pt.x, pt.y
    local x1, y1 = pts[1].x, pts[1].y
    local x1Part = (y1 > y) or ((x1 - x > 0) and (y1 == y))
    local x2, y2
    for i = 1, #pts do
        local point  = pts[i]
        local x2, y2 = point.x, point.y
        local x2Part = (y2 > y) or ((x2 > x) and (y2 == y))
        if x2Part == x1Part then
            x1, y1 = x2, y2
        else
            local mul = (x1 - x)*(y2 - y) - (x2 - x)*(y1 - y)
            if mul > 0 then
                count = count + 1
            elseif mul < 0 then
                count = count - 1
            end
            x1, y1 = x2, y2
            x1Part = x2Part
        end
    end
    if count == 2 or count == -2 then
        return true
    end
    return false
end