method_missing_iteration.rb

module Iteration
  def map
    each[0]
  end

  def to_a
    each.split("")
  end
end

module ForwardIteration
  include Iteration

  def each
    'Forward'
  end
end

module BackwardIteration
  include Iteration

  def method_missing(method, *args, &blk)
    if method.to_s =~ /^(.*)_reverse/
      define_each
      result = send($1.to_sym)
      undef_each

      result
    end
  end

  def define_each
    class << self
      define_method :each do
        e = BackwardIteration.instance_method(:each)
        e.bind(self).call
      end
    end
  end

  def undef_each
    class << self
      define_method :each do
        super()
      end
    end
  end

  def each
    'Backward'
  end
end

class SingleLinkedList
  include ForwardIteration
end

class DoubleLinkedList
  include BackwardIteration
  include ForwardIteration
end

spec.rb

describe 'LinkedList' do
  context SingleLinkedList do
    subject { described_class.new }

    it 'map' do
      subject.map.should == 'F'
    end
  end

  context DoubleLinkedList do
    subject { described_class.new }
    
    it 'map' do
      subject.map.should == 'F'
    end

    it 'map_reverse' do
      subject.map_reverse.should == 'B'
    end

    it 'returns array from head to tail' do
      subject.to_a.should == %w(F o r w a r d)
    end

    it 'returns array from tail to head' do
      subject.to_a_reverse.should == %w(B a c k w a r d)
    end

    it 'does not override #each method' do
      subject.to_a_reverse.should == %w(B a c k w a r d)
      subject.to_a.should == %w(F o r w a r d)
    end
  end
end