lunh.rb

# 1. From the rightmost digit, which is the check digit, moving left, double the
# value of every second digit; if the product of this doubling operation is
# greater than 9 (e.g., 8 × 2 = 16), then sum the digits of the products
# (e.g., 16: 1 + 6 = 7, 18: 1 + 8 = 9).

# 2. Take the sum of all the digits.

# 3. If the total modulo 10 is equal to 0 (if the total ends in zero) then the
# number is valid according to the Luhn formula; else it is not valid.



def lunh_valid?(num)
  numbers = num.to_s.chars.map(&:to_i)

  i = 0
  sum = 0

  # 1. From the rightmost digit, which is the check digit, moving left,
  numbers.reverse.each do |n|

    # double the value of every second digit;
    n *= 2 if i.odd?

    # if the product of this doubling operation is greater than 9, then
    # sum the digits of the products
    n = 1 + (n - 10) if n >= 10

    # 2. Take the sum of all the digits.
    sum += n

    i += 1
  end

  # 3. If the total modulo 10 is equal to 0, then it's valid.
  sum % 10 == 0
end

lunh_cli.rb

require './lunh.rb'

def get_number
  gets.chomp!
end

puts "Please enter a number to check using the Luhn Algorithm"
num = get_number
puts("Usage: ruby lunh.rb {thenumber}") && exit if num.nil?
valid = lunh_valid?(num)
puts "\e[1A#{num} is #{"not " unless valid}valid"

lunh_test.rb

require 'minitest'
require 'minitest/autorun'
require './lunh.rb'


class TestAlgorithm < Minitest::Test
  describe "Lunh Algorithm" do
    good_test_credit_card_numbers = [
      378282246310005,
      371449635398431,
      378734493671000,
      5610591081018250,
      30569309025904,
      38520000023237,
      3530111333300000,
      5105105105105100,
      4222222222222
    ]
    bad_test_credit_card_numbers = [
      6011111111111110,
      3566002020360500,
      4111111111111110,
      76009244561,
      5019717010103740,
      6331101999990010,
      6011000990139420,
      5555555555554440,
      4012888888881880
    ]

    it "returns true for valid numbers" do
      good_test_credit_card_numbers.each do |n|
        assert lunh_valid?(n) == true
      end
    end

    it "returns false for invalid numbers" do
      bad_test_credit_card_numbers.each do |n|
        assert lunh_valid?(n) == false
      end
    end
  end
end