【高级技巧】软删除：先标记后清理的安全模式

local mails = {}
local KEEP_DAYS = 30
local function softDelete(id)
    local m = mails[id]
    if m ~= nil and m.delAt == nil then
        m.delAt = os.time()
        return true
    end
    return false
end
local function restore(id)
    local m = mails[id]
    if m ~= nil and m.delAt ~= nil then
        m.delAt = nil
        return true
    end
    return false
end
local function purge()
    local now = os.time()
    for id, m in pairs(mails) do
        if m.delAt ~= nil and now - m.delAt > KEEP_DAYS * 86400 then
            mails[id] = nil
        end
    end
end

local function listMail(actor)
    actor = getplayerbyname(actor)
    local shown = 0
    for id, m in pairs(mails) do
        if m.delAt == nil then
            shown = shown + 1
            sendmsg(actor, 1, id .. "：" .. m.title)
        end
    end
    sendmsg(actor, 1, "共 " .. shown .. " 封有效邮件。")
end