Hangman game with class, yield and self

hangman.rb

require_relative 'hangman_class_3'

hangman = Hangman.new

hangman.start do |h|
  puts h.answer
  print "#{h.limit} Enter your best guest: "
  h.guess(gets)
end

hangman.say_good_bye

hangman_class_3.rb

String.class_eval do
  def match_replace(word, char)
    index = 0
    word.each_char do |w|
      if w == char
        self[index] = char
      end
      index += 1
    end
  end
end

class Hangman
  attr_reader :finish, :answer, :limit

  def initialize
    @words = %w{spiderman superman powerpuf hangman}
    @finish = false
    @limit = 8
    @win = false
  end

  def random_word
    return @words[rand(@words.length-1)]
  end  

  def start(&block)
    @word = random_word
    @answer = "_" * @word.length
    until @finish
      yield self
    end
  end

  def guess(answer)
    @limit -=  1
    answer = answer[0..-2]
    if answer == 'exit' || limit == 0
      @finish = true
      return 
    end 
    if answer.length == 1
      @answer.match_replace(@word, answer)
    end
    if @answer == @word || answer == @word
      @finish = true
      @win = true
    end
  end

  def say_good_bye
    if @win
      puts "You are the winner"
    else
      puts "Bye loser, the answer is #{@word}" 
    end
  end

end

test_hangman_class_3.rb

require_relative 'hangman_class_3'
require 'test/unit'

class TestHangman < Test::Unit::TestCase

  def setup
    Hangman.class_eval do 
      attr_accessor :words, :finish, :limit, :win, :answer

      def random_word
        return "powerpuf"
      end      
    end
    @hangman = Hangman.new
  end

  def test_startup_value
    assert(@hangman.words.length > 0, "should have more than one in dictionary")
    assert_equal(@hangman.finish, false)
    assert_equal(@hangman.limit, 8)
    assert_equal(@hangman.win, false)
  end

  def test_exit
    @hangman.guess("exit")
    assert(@hangman.finish, "should exit when user type exit")
  end

  def test_find_one_missing_char
    @hangman.start do |h|
      h.guess("x\n")
      assert_equal("________", h.answer)   
      break
    end
  end

  def test_find_one_right_char
    @hangman.start do |h|
      h.guess("o\n")
      assert_equal("_o______", h.answer)
      break
    end
  end

  def test_find_double_right_char
    @hangman.start do |h|  
      h.guess("p\n")
      assert_equal("p____p__", h.answer)
      break
    end
  end

  def test_quick_answer
    @hangman.start do |h|
      h.guess("powerpuf\n")
      assert_equal(true, h.finish)
    end
  end
end