adapter.rb

# Convert the interface of a class into another interface clients expect.
# Adapter lets classes work together that couldn't otherwise because of
# incompatible interfaces
class Quest
  attr_accessor :difficulty, :hero
  
  def initialize(difficulty)
    @difficulty = difficulty
    @hero = nil 
  end

  def finish
    @hero.exp += calculate_experience
  end

  def calculate_experience
    @difficulty * 50 / @hero.level
  end
end

class Hero
  attr_accessor :level, :exp, :quests

  def initialize
    @level = 1
    @exp = 0
    @quests = []
  end

  def take_quest(quest)
    @quests << (quest.hero = self)
  end

  def finish_quest(quest)
    quest.finish
    @quests.delete quest
  end
end

class OldQuest
  attr_accessor :hero, :difficulty, :experience

  def initialize
    @difficulty = 3
    @experience = 10
  end

  def done
    difficulty * experience
  end
end

class QuestAdapter
  attr_accessor :hero

  def initialize(old_quest, difficulty)
    @old_quest = old_quest
    @old_quest.difficulty = difficulty
    @hero = nil
  end

  def finish
    @hero.exp += @old_quest.done
  end
end

# Usage
hero = Hero.new
quest = Quest.new 5
hero.take_quest quest
hero.finish_quest quest
puts hero.exp
# => 250
some_old_quest = OldQuest.new
old_quest_adapted = QuestAdapter.new(some_old_quest, 5)
hero.take_quest old_quest_adapted
hero.finish_quest old_quest_adapted
puts hero.exp
# => 300