【高级技巧】引用计数：副本实例的借用与归还

local instances = {}
local function borrowInstance(key)
    local inst = instances[key]
    if inst == nil then
        inst = { ref = 0, bornAt = os.time() }
        instances[key] = inst
        sendmsg(nil, 1, "副本实例[" .. key .. "]已创建。")
    end
    inst.ref = inst.ref + 1
    return inst
end
local function releaseInstance(key)
    local inst = instances[key]
    if inst == nil then
        return false
    end
    inst.ref = inst.ref - 1
    if inst.ref <= 0 then
        instances[key] = nil
        sendmsg(nil, 1, "副本实例[" .. key .. "]无人使用，已回收。")
    end
    return true
end

local function enterZumaRoom(teamId)
    local inst = borrowInstance("Zumalair_" .. teamId)
    sendmsg(nil, 1, "祖玛阁实例开放，当前使用队伍 " .. inst.ref .. " 支。")
end
local function leaveZumaRoom(teamId)
    releaseInstance("Zumalair_" .. teamId)
end