Luhn algorythm

luhn.rb

#https://en.wikipedia.org/wiki/Luhn_algorithm
# v1
def self.check_digit_for(number)
  digits = number.to_s.reverse.split(//)

  sum = 0
  digits.each_with_index do |digit, index|
    digit = digit.to_i
    digit *= 2 if index.even?
    digit -= 9 if digit > 9
    sum += digit
  end
  mod = 10 - sum % 10
  mod == 10 ? 0 : mod
end

# v2
def self.check_digit_for(number)
  digits = account_number.scan(/./).map(&:to_i)

  sum = digits.reverse.each_slice(2).map do |x, y|
    [(x * 2).divmod(10), y || 0]
  end.flatten.inject(:+)

  (10 - sum % 10)
end