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JaeHyoLee's solution

to Luhn in the Lua Track

Published at Jul 13 2018 · 2 comments
Test suite

Given a number determine whether or not it is valid per the Luhn formula.

The Luhn algorithm is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers and Canadian Social Insurance Numbers.

The task is to check if a given string is valid.

Validating a Number

Strings of length 1 or less are not valid. Spaces are allowed in the input, but they should be stripped before checking. All other non-digit characters are disallowed.

Example 1: valid credit card number

4539 1488 0343 6467

The first step of the Luhn algorithm is to double every second digit, starting from the right. We will be doubling

4_3_ 1_8_ 0_4_ 6_6_

If doubling the number results in a number greater than 9 then subtract 9 from the product. The results of our doubling:

8569 2478 0383 3437

Then sum all of the digits:

8+5+6+9+2+4+7+8+0+3+8+3+3+4+3+7 = 80

If the sum is evenly divisible by 10, then the number is valid. This number is valid!

Example 2: invalid credit card number

8273 1232 7352 0569

Double the second digits, starting from the right

7253 2262 5312 0539

Sum the digits

7+2+5+3+2+2+6+2+5+3+1+2+0+5+3+9 = 57

57 is not evenly divisible by 10, so this number is not valid.

Running the tests

To run the tests, run the command busted from within the exercise directory.

Further information

For more detailed information about the Lua track, including how to get help if you're having trouble, please visit the exercism.io Lua language page.


The Luhn Algorithm on Wikipedia http://en.wikipedia.org/wiki/Luhn_algorithm

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


local luhn = require('luhn')

describe('luhn', function()
  it('should indicate that single digits are invalid', function()
    assert.equal(false, luhn.valid('1'))
    assert.equal(false, luhn.valid('0'))

  it('should check a valid Canadian SIN', function()
    assert.equal(true, luhn.valid('046 454 286'))

  it('should check an invalid Canadian SIN', function()
    assert.equal(false, luhn.valid('046 454 287'))

  it('should check an invalid credit card', function()
    assert.equal(false, luhn.valid('8273 1232 7352 0569'))

  it('should not allow non-digits', function()
    assert.equal(false, luhn.valid('a46 454 286'))
local Luhn = {}
Luhn.__index = Luhn

local function new(input)
  local o = {_input = input }
  return setmetatable(o, Luhn)

local function create(base)
  local new_base = base..'0'
  local check_sum = new(new_base):checksum()
  local last_digit = (10 - check_sum % 10)
  return base..(last_digit % 10)

function Luhn:valid()
  local check_sum = self:checksum()
  return (check_sum % 10 == 0)

function Luhn:checksum()
  local addends = self:addends()
  local sum = 0
  for i = 1, #addends do
    sum = sum + addends[i]
  return sum

function Luhn:addends()
  local translated = {}
  local even = nil
  if #self._input % 2 == 0 then even = true end

  for i = 1, #self._input do
    local n = tonumber(self._input:sub(i,i))
    if even then n = n * 2 even = nil else even = true end
    table.insert(translated, n > 9 and n-9 or n)
  return translated

function Luhn:check_digit()
  return tonumber(self._input:sub(-1))

return {
  new = new,
  create = create

Community comments

Find this solution interesting? Ask the author a question to learn more.
Avatar of ryanplusplus

I'm surprised that this is valid syntax: then n = n * 2 even = nil

I would have thought that a semicolon would have been required before even.

Avatar of JaeHyoLee

@ryanplusplus That's what I thought. but it worked~! :)

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