Ruby implementation of the Luhn checksum algorithm

luhn_checksum.rb

# Luhn Checksum
#  
# http://en.wikipedia.org/wiki/Luhn_algorithm
#  
# Implement a Ruby program that outputs the validity of a number by applying the Luhn Checksum.
#  
# Input:
# The program should accept a single number to validate.
#  
# Output:
# The program should output the input along with the validity of the number.
#  
# Examples:
# With an input of 79927398713 the output would be:
# 79927398713 is valid.
#  
# With an input of 79927398710 the output would be:
# 79927398710 is not valid.

class LuhnChecksum
  attr_reader :value

  def initialize(value)
    @value = value
  end

  def valid?
    (checksum % 10).zero?
  end

  def invalid?
    !valid?
  end

  def checksum
    modified_digits.map { |i| sum_digits(i) }.reduce(0, :+)
  end

  private
  def sum_digits(number)
    digits(number).reduce(0, :+)
  end

  def digits(number=value)
    number.to_s.split('').map(&:to_i)
  end

  def modified_digits
    digits.reverse.map.with_index { |i, index| (index % 2).zero? ? i : i * 2 }.reverse
  end
end

# Run specs when run through the RSpec CLI
if defined? RSpec
  describe LuhnChecksum do
    it 'is valid for 79927398713' do
      LuhnChecksum.new(79927398713).should be_valid
    end

    it 'is invalid for 79927398710' do
      LuhnChecksum.new(79927398710).should be_invalid
    end

    # Common check value for credit cards
    it 'is valid for 4111111111111111' do
      LuhnChecksum.new(4111111111111111).should be_valid
    end
  end
end

# Run standalone if directly running this file.
if $0 == __FILE__
  checker = LuhnChecksum.new(ARGV[0].to_i)
  puts "#{checker.value} is #{'not ' if checker.invalid?}valid."
end