神秘商店：随机刷新的限时折扣货架

local POOL = {2101, 2102, 2103, 2104,
              2105, 2106, 2107, 2108}
local function refresh()
    local today = os.date("%Y%m%d")
    if getsysvar("shopDate") == today then return end
    setsysvarex("shopDate", today)
    local used = {}
    for i = 1, 6 do
        local idx = math.random(1, #POOL)
        while used[idx] do
            idx = math.random(1, #POOL)
        end
        used[idx] = true
        setsysvarex("shop" .. i, POOL[idx])
        setsysvarex("shopOff" .. i, math.random(50, 80))
    end
end

local function price(slot, base)
    local off = (getsysvar("shopOff" .. slot) or 100) / 100
    return math.floor(base * off)
end