Luhn Algorithm

find check digits

def number_with_check_digits(number)
digits = number.to_s.reverse.scan(/\d/).map { |x| x.to_i }
digits = digits.each_with_index.map { |d, i|
  d *= 2 if i.even?
  d > 9 ? d - 9 : d
}
sum = digits.inject(0) { |m, x| m + x }
mod = 10 - sum % 10
mod==10 ? 0 : mod
digits = number.to_s.reverse.scan(/\d/).map { |x| x.to_i }
digits = digits.each_with_index.map { |d, i|
  d *= 2 if i.even?
  d > 9 ? d - 9 : d
}
sum = digits.inject(0) { |m, x| m + x }
mod = 10 - sum % 10
mod==10 ? 0 : mod
number.coerce(mod).reverse.join.to_i
end

valid

def credit_card_valid?(account_number)
  digits = account_number.to_s.scan(/./).map(&:to_i) # account_number could be a integer as well
  check = digits.pop

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