function Maze:_process_neighbor_cells(ox, oy)
    local cellList = { {ox,oy} }
    while #cellList > 0 do
        os.queueEvent("go")
        coroutine.yield()
        local cell = table.remove(cellList,1)
        local x,y = cell[1],cell[2]
        local parent = cell[3]

        if not self.cells[y][x].is_visited then
            self.cells[y][x].is_visited = true

            local neighbor_cells = self:_shuffle(self:_get_neighbor_cells(x, y))
            if parent then
                if parent.x > x     then     -- open wall with right neighbor
                    self.cells[y][x].right_wall                              = false
                    self.cells[parent.y][parent.x].left_wall   = false
                elseif parent.x < x then     -- open wall with left neighbor
                    self.cells[y][x].left_wall                               = false
                    self.cells[parent.y][parent.x].right_wall  = false
                elseif parent.y > y then     -- open wall with bottom neighbor
                    self.cells[y][x].bottom_wall                             = false
                    self.cells[parent.y][parent.x].top_wall    = false
                elseif parent.y < y then     -- open wall with top neighbor
                    self.cells[y][x].top_wall                                = false
                    self.cells[parent.y][parent.x].bottom_wall = false
                end

            end
            for index, neighbor_cell in ipairs(neighbor_cells) do
                if self.cells[neighbor_cell.y][neighbor_cell.x].is_visited == false then
                    table.insert(cellList,1,{neighbor_cell.x,neighbor_cell.y,{x=x,y=y}})
                end
                --self:_process_neighbor_cells(neighbor_cell.x, neighbor_cell.y)
            end
        end
    end

    return
end