local function is_mapblock_grid_pos(pos)
    return (pos.x % 16 == 0) and (pos.y % 16 == 0) and (pos.z % 16 == 0)
end

minetest.register_on_placenode(function(pos, newnode, placer)
    if newnode.name ~= "protector:protect" then
        return
    end

    if is_mapblock_grid_pos(pos) then
        return
    end

    minetest.remove_node(pos)

    if placer and placer:is_player() then
        local player_name = placer:get_player_name()

        if not minetest.is_creative_enabled(player_name) then
            local refund = ItemStack("protector:protect")
            local inv = placer:get_inventory()

            if inv and inv:room_for_item("main", refund) then
                inv:add_item("main", refund)
            else
                minetest.add_item(vector.offset(pos, 0, 1, 0), refund)
            end
        end

        minetest.chat_send_player(
            player_name,
            "Protector placement failed: place only at coordinates divisible by 16."
        )
    end
end)