Lua函数记忆化：相同入参直接命中缓存

-- 错误：相同入参每次都全量重算
function calcScore(a, b)
    return heavyCalc(a, b)
end

local cache = {}
local function calcScore(a, b)
    local key = a .. "_" .. b
    local hit = cache[key]
    if hit ~= nil then
        return hit
    end
    local result = heavyCalc(a, b)
    cache[key] = result
    return result
end