Sum 2 big int | LUA

--[[
    ####--------------------------------####
    #--# Author:   by uriid1            #--#
    #--# License:  GNU GPLv3            #--#
    ####--------------------------------####
--]]

local ASCII_ZERO_CODE = 48
local ASCII_NINE_CODE = 57

io.stdout:write('First number: ')
local a_str = io.stdin:read()
io.stdout:write('Second number: ')
local b_str = io.stdin:read()

local function stringToNumArr(str)
  local result = {}
  for s in string.gmatch(str, '.') do
    local code = string.byte(s)
    if code < ASCII_ZERO_CODE or code > ASCII_NINE_CODE then
      return nil, 'Out of ascii range'
    end
    table.insert(result, code - ASCII_ZERO_CODE)
  end
  return result
end

local a, err = stringToNumArr(a_str)
if err then
  print(err)
  return
end

local b, err = stringToNumArr(b_str)
if err then
  print(err)
  return
end

local c = {}
local x = 0

for i = #a, 1, -1 do
  local sum = a[i] + x + (b[i-1] or 0)
  if sum > 9 then
    x = math.floor(sum / 10)
    table.insert(c, sum - 10)
  else
    table.insert(c, sum)
  end
end

local result = {}
for i = #c, 1, -1 do
  table.insert(result, c[i])
end
print('result:', table.concat(result))