Lua大数乘法：竖式逐位乘的错位累加

local function bigMul(a, b)
    local m, n = #a, #b
    local res = {}
    for i = 1, m + n do res[i] = 0 end
    for i = m, 1, -1 do
        for j = n, 1, -1 do
            res[i + j] = res[i + j]
                + tonumber(a:sub(i, i)) * tonumber(b:sub(j, j))
        end
    end
    local carry = 0
    for k = m + n, 1, -1 do
        local v = res[k] + carry
        res[k] = v % 10
        carry = math.floor(v / 10)
    end
    return (table.concat(res):gsub("^0+", ""))
end

local p = getplayerbyname("bignum")
sendmsg(p, 1, bigMul("123456789", "987654321"))
-- 输出 121932631112635269，18 位精确到个位