【高级技巧】Lua空值缓存：祖玛教主查无此物的负结果缓存

local function find(id)
    local hit = CACHE[id]
    if hit then return hit end       -- 只缓存命中，未命中反复全扫
    local r = scan(id)
    if r then CACHE[id] = r end
    return r
end

local CACHE = {}
local function find(actor, id)
    local hit = CACHE[id]
    if hit ~= nil then return hit or nil end   -- 负缓存直接回
    local r = scan(id)
    CACHE[id] = r or false           -- 查无记false，堵住重扫
    if r then
        sendmsg(actor, 1, "祖玛教主掉落 " .. r.name)
    end
    return r
end