gilded_rose.rb

require 'forwardable'

Item = Struct.new(:name, :sell_in, :quality)

class NormalAging
  def self.age(item)
    item.quality -= 1 if item.sell_in >  0
    item.quality -= 2 if item.sell_in <= 0
    item.quality =  0 if item.quality <  0
    item.quality = 50 if item.quality > 50
    item.sell_in -= 1
  end
end

class BrieAging
  def self.age(item)
    item.quality += 1 if item.sell_in >  0
    item.quality += 2 if item.sell_in <= 0
    item.quality =  0 if item.quality <  0
    item.quality = 50 if item.quality > 50
    item.sell_in -= 1
  end
end

class TicketAging
  def self.age(item)
    item.quality += 1 if item.sell_in > 10
    item.quality += 2 if item.sell_in > 5 && item.sell_in <= 10
    item.quality += 3 if item.sell_in > 0 && item.sell_in <= 5

    item.quality =  0 if item.sell_in <= 0 || item.quality < 0
    item.quality = 50 if item.quality > 50
    item.sell_in -= 1
  end
end

class ManaCakeAging
  def self.age(item)
    item.quality -= 2 if item.sell_in <= 5 && item.sell_in > 0
    item.quality -= 4 if item.sell_in <= 0
    item.quality =  0 if item.quality <  0
    item.quality = 50 if item.quality > 50
    item.sell_in -= 1
  end
end

class NoAging
  def self.age(item)
  end
end

class AgingItem
  attr_reader :item, :aging_strategy
  extend Forwardable
  def_delegators :@item, :name, :name=, :sell_in, :sell_in=, :quality, :quality=

  def initialize(item, aging_strategy)
    @item = item
    @aging_strategy = aging_strategy
  end

  def age
    aging_strategy.age(item)
  end

end

class GildedRose

  AGING_STRATEGIES = {
    'NORMAL ITEM'                               => NormalAging,
    'Aged Brie'                                 => BrieAging,
    'Backstage passes to a TAFKAL80ETC concert' => TicketAging,
    'Sulfuras, Hand of Ragnaros'                => NoAging,
    'Conjured Mana Cake'                        => ManaCakeAging
  }

  def self.update_quality(items)
    items.each do |item|
      aging_strategy = AGING_STRATEGIES[item.name]
      AgingItem.new(item, aging_strategy).age
    end
  end

end