【高级技巧】校验和：存档完整性的算术校验封装

local function checksum(data)
    local a, b = 1, 0
    for i = 1, #data do
        a = (a + string.byte(data, i)) % 65521
        b = (b + a) % 65521
    end
    return b * 65536 + a
end

local function saveWithSum(slot, blob)
    setsysvarex("Blob_" .. slot, blob, 1)
    setsysvarex("Sum_" .. slot, checksum(blob), 1)
end
local function loadWithCheck(slot)
    local blob = getsysvar("Blob_" .. slot) or ""
    local sum = tonumber(getsysvar("Sum_" .. slot)) or 0
    if checksum(blob) ~= sum then
        return nil
    end
    return blob
end