Luhn Checksum

luhn_checksum.rb

class LuhnChecksum
  attr_accessor :number
  
  def initialize(number = "")
    @number = number
  end  
  
  def valid?
    calculate_checksum % 10 == 0
  end

  def calculate_checksum
    all_individual_digits.inject(:+)
  end
  
  private
  
  def digits
    number.scan(/\d/).collect{ |c| c.to_i }.to_a.reverse
  end
  
  def all_individual_digits
    odd + double_digits(even)
  end

  def even
    digits.select.with_index { |digit, index| digit if (index + 1).even? }
  end
  
  def odd
    digits.select.with_index { |digit, index| digit if (index + 1).odd? }
  end
  
  def double_digits(array = [])
    array.collect { |digit| individual_digits(digit * 2) }.flatten
  end
  
  def individual_digits(digit)   
    digit.to_s.scan(/\d/).collect { |i| i.to_i }
  end
end

luhn_checksum_spec.rb

require "rspec"
require "./luhn_checksum"

describe LuhnChecksum do
  it "initializes a new object with a string of numbers" do
    checksum = LuhnChecksum.new
    checksum.number.should eq ""
  
    checksum = LuhnChecksum.new("79927398719")
    checksum.number.should eq "79927398719"
  end

  it "calculates the checksum" do
    checksum = LuhnChecksum.new("79927398713")
    checksum.calculate_checksum.should eq 70
  end
  
  it "correctly validates the checksum" do
    LuhnChecksum.new("79927398710").valid?.should be_false
    LuhnChecksum.new("79927398711").valid?.should be_false
    LuhnChecksum.new("79927398712").valid?.should be_false
    LuhnChecksum.new("79927398713").valid?.should be_true
    LuhnChecksum.new("79927398714").valid?.should be_false
    LuhnChecksum.new("79927398715").valid?.should be_false
    LuhnChecksum.new("79927398716").valid?.should be_false
    LuhnChecksum.new("79927398717").valid?.should be_false
    LuhnChecksum.new("79927398718").valid?.should be_false
    LuhnChecksum.new("79927398719").valid?.should be_false
  end
end