story.rb

class Story
  attr_reader :forked_stories, :id
  
  class << self
    def get_id
      last_id = instance_variable_get("@last_id")
      next_id = last_id.nil? ? :a : last_id.to_s.succ.to_sym
      instance_variable_set("@last_id", next_id)
      # puts "XXX Returning #{next_id}"
      next_id
    end
  end
  
  def initialize
    @forked_stories = []
    @id = self.class.get_id
  end

  def fork!    
    forked_story = Story.new
    @forked_stories << forked_story
    forked_story
  end
  
  def fork_tree
    forked = forked_stories
    forked_from_children = forked.inject([]) do |fstories, fstory|
      fstories + fstory.fork_tree
    end
    forked + forked_from_children
  end
  
  def inspect
    "<#{object_id}:#{@id}>"
  end
  
  private
  def get_next_id
    last_story = forked_stories.empty? ? self : forked_stories.last
    last_story.id.to_s.succ
  end
end

story_spec.rb

require "story"

describe Story do
  before do
    @story = Story.new
  end

  describe "when assigning ids" do
    it "the first node should receive 'a'" do
      @story.id.should == :a
    end
  end
  
  describe "when not forked" do
    it "should not have any forked stories" do
      @story.forked_stories.should == []
    end
    it "its fork tree should be empty" do
      @story.fork_tree.should == []
    end
  end
  
  describe "when forked" do
    before do
      @forked_story = @story.fork!
    end
    it "should create a new story" do
      @forked_story.should_not be_nil
    end
    it "should have the new story as the only forked story" do
      @story.forked_stories.should == [@forked_story]
    end
  end
  
  describe "when forked multiple times" do
    before do
      @story_1 = @story.fork!
      @story_2 = @story.fork!
      @story_3 = @story.fork!
    end
    it "should contain all the forked stories in the order of their forking" do
      @story.forked_stories.should == [@story_1, @story_2, @story_3]
    end
  end
    
  describe "when asked about its fork tree" do
    before(:all) do
      # see my fork tree sketch on the paper
      @a = Story.new
      @b = @a.fork!
      @c = @b.fork!
      @d = @b.fork!
      @e = @b.fork!
      @f = @d.fork!
      @g = @d.fork!
      @h = @e.fork!
      @i = @h.fork!
      @j = @h.fork!
      @k = @h.fork!
      @l = @h.fork!
      # and http://gist.github.com/179186
    end
    it "the leaves should return an empty collection" do
      %w[c f g i j k l].each do |leaf|
        instance_variable_get("@#{leaf}").send(:fork_tree).should == []
      end
    end
    it "first level nodes should return their forked stories" do
      @d.fork_tree.should == [@f, @g]
      @h.fork_tree.should == [@i, @j, @k, @l]
    end
    it "higher level nodes should return the ensemble of the forked stories of their children" do
      @e.fork_tree.should == [@h, @i, @j, @k, @l]
      @b.fork_tree.should == [@c, @d, @e, @f, @g, @h, @i, @j, @k, @l]
      @a.fork_tree.should == [@b, @c, @d, @e, @f, @g, @h, @i, @j, @k, @l]      
    end
  end
end