【性能调优】循环内查询外提：重复查找的缓存消除

---改造前：循环内反复查配置与读变量
function Loop.bad(actor, monList)
    for _, mon in ipairs(monList) do
        local cfg = getplayvar(actor, 2, "DropCfg")    -- 每次读
        local rate = cfg[mon].rate                     -- 每次索引
        if math.random(1, 10000) <= rate then
            Drop.give(actor, mon)
        end
    end
end

---改造后：不变项外提一次
function Loop.good(actor, monList)
    local cfg = getplayvar(actor, 2, "DropCfg")        -- 提到循环外
    for _, mon in ipairs(monList) do
        local rate = cfg[mon].rate
        if math.random(1, 10000) <= rate then
            Drop.give(actor, mon)
        end
    end
end

---不变性成立的三个条件
function Loop.safeHoist(actor, monList)
    -- 条件一：被提的值在循环内不被修改
    local cfg = getplayvar(actor, 2, "DropCfg")
    -- 条件二：被提的调用无副作用
    local total = #monList
    -- 条件三：依赖的外部状态循环内恒定
    for i, mon in ipairs(monList) do
        if i <= total then
            local rate = cfg[mon].rate
            if math.random(1, 10000) <= rate then
                Drop.give(actor, mon)
            end
        end
    end
end