Luhn Algorithm

luhn.rb

class Luhn
  attr_reader :numbers

  def initialize(numbers)
    @numbers = numbers.to_s.chars.map(&:to_i)
  end

  def self.create(numbers)
    create_valid_number(new(numbers))
  end

  def addends
    new_numbers = []
    numbers.reverse.each_with_index do |number, index|
      if index.odd?
        new_numbers << double_then_subtract_9_if_over_ten(number)
      else
        new_numbers << number
      end
    end
    new_numbers.reverse
  end

  def checksum
    addends.inject(:+)
  end

  def valid?
    checksum % 10 == 0
  end

  private

  def double_then_subtract_9_if_over_ten(number)
    doubled_number = number * 2
    doubled_number -= 9 if doubled_number >= 10
    doubled_number
  end

  def self.create_valid_number(instance)
    number = 1
    until instance.valid?
      instance.numbers << return_0_if_equal_to_10(number)
      remove_last_number_from_invalid instance
      number += 1
    end
    instance.numbers.join.to_i
  end

  def self.return_0_if_equal_to_10(number)
    return 0 if number == 10
    number
  end

  def self.remove_last_number_from_invalid(instance)
    instance.valid? || instance.numbers.pop
  end
end