luhn.rb

def luhn_valid?(number)
  return unless number.is_a?(String)
  # Convert to a reversed list of digits
  digits = number.scan(/\d/).map(&:to_i).reverse
  # No numbers in the string
  return false if digits.empty?
  check_digit = digits.shift
  # For each of the normal integers, compute the correct sum
  final_digits = []
  digits.each_with_index do |value, index|
    # Second from the left is double.
    if index.even?
      final_digits << (value * 2).to_s.split('').map(&:to_i)
    else
      final_digits << value
    end
  end
  # Actually compute the product, including check digit.
  (final_digits.flatten.inject(:+) + check_digit) % 10 == 0
end

puts "#{luhn_valid?('4111 1111 1111 1111')} should be true"
puts "#{luhn_valid?('5500 0000 0000 0004')} should be true"
puts "#{luhn_valid?('3400 0000 0000 009')} should be true"
puts "#{luhn_valid?('3000 0000 0000 04')} should be true"
puts "#{luhn_valid?('3000 0000 0000 04')} should be true"
puts "#{luhn_valid?('6011 0000 0000 0004')} should be true"
puts "#{luhn_valid?('2014 0000 0000 009')} should be true"
puts "#{luhn_valid?('3088 0000 0000 0009')} should be true"
puts "#{luhn_valid?('3088 0000 0000 0008')} should be false"