Seq.rb

class Seq
  def initialize(string)
    @string = string.to_s
  end
   
  def next
    @string = generate(@string)
    self
  end
  
  def to_s
    @string
  end  

  private

  def generate(string)
    old_string = string
    counter = 1
    new_string = ""
    for index_of_char in 0..old_string.size-1
      char = old_string[index_of_char]
      if old_string[index_of_char] == old_string[index_of_char+1]
        counter += 1
        next
      else
        new_string << counter.to_s
        new_string << char.to_s
        counter = 1
      end
    end
    new_string
  end
end

seq_spec.rb

require File.expand_path('Seq.rb')

describe Seq do
  describe "#next" do
    it "should return self (same instance of the Seq class)" do
      s = Seq.new(1)
      s.next.should eq(s)
    end

    it "should assign @string with next calculated string" do
      s = Seq.new(1)
      s.next
      s.instance_variable_get(:@string).should == "11"
    end

    it "when 3 times chained should calculate new string 3 times" do
      s = Seq.new(1)
      s.next.next.next
      s.instance_variable_get(:@string).should == "1211"
    end
  end

  describe "#to_s" do
    it "should return last calculated string" do
      s = Seq.new(1)
      s.next.next.next.next
      s.to_s.should == "111221"
    end  
  end
end