Lua大数减法：先比大小再逐位借位

local function cmpBig(a, b)
    if #a ~= #b then
        return #a > #b and 1 or -1
    end
    return a < b and -1 or (a > b and 1 or 0)
end

local function subBig(a, b)
    local neg = false
    if cmpBig(a, b) < 0 then
        a, b, neg = b, a, true
    end
    local borrow, out = 0, {}
    for i = 1, #a do
        local da = tonumber(a:sub(#a - i + 1, #a - i + 1)) or 0
        local db = tonumber(b:sub(#b - i + 1, #b - i + 1)) or 0
        local d = da - db - borrow
        if d < 0 then d = d + 10; borrow = 1 else borrow = 0 end
        out[#out + 1] = d
    end
    local s = table.concat(out):reverse():gsub("^0+", "")
    if s == "" then s = "0" end
    return neg and "-" .. s or s
end
local p = getplayerbyname("sub01")
sendmsg(p, 1, subBig("301", "1000"))