def luhn_valid?(value)
  return false unless value.is_a?(String) && !value.strip.empty?

  value.gsub!(/\D/, '')

  sum = 0

  value.split(//).each_with_index do |number, index|

    if index.even?
      sum += (number * 2).to_s.split(//).map(&:to_i).sum
    else
      sum += number
    end

  end

  sum % 10 == 0
end