My Solution to Gilded Rose Kata-- SPOILER ALERT

Gemfile

source 'http://rubygems.org'

gem 'rspec'

gilded_rose.rb

require './item.rb'
# This is a fantastical store. Items are updated and
# generally lose or gain quality with the passage of time.
class GildedRose
  include ItemHandler
  attr_accessor :items

  def initialize
    @items = []
    @items << GenericItem.new("+5 Dexterity Vest", 10, 20)
    @items << CheeseItem.new("Aged Brie", 2, 0)
    @items << GenericItem.new("Elixir of the Mongoose", 5, 7)
    @items << LegendaryItem.new("Sulfuras, Hand of Ragnaros", 0, 80)
    @items << TicketItem.new("Backstage passes to a TAFKAL80ETC concert", 15, 20)
    @items << ConjuredItem.new("Conjured Mana Cake", 3, 6)
  end

  def update_quality
    @items.each { |item| item.update_item }
  end
end

gilded_rose_spec.rb

require 'rubygems'
require './gilded_rose.rb'
require 'rspec'

describe GildedRose do

  before(:each) do
     @gilded_rose = GildedRose.new
  end

  def update_should_decrement_sell_in
    expect {
      @gilded_rose.update_quality
    }.to change { @item.sell_in }.by(-1)
  end

  def update_should_change_quality_by(number)
    expect {
      @gilded_rose.update_quality
    }.to change { @item.quality }.by(number)
  end

  def update_should_not_change_quality
    expect {
      @gilded_rose.update_quality
    }.to_not change { @item.quality }
  end

  describe "generic item, when updated" do

    before(:each) do
      @item = @gilded_rose.items.find { |item| item.name =~ /elixir/i }
      @item.sell_in = 1
      @item.quality = 10
    end

    it "should reduce sell_in by 1" do
      update_should_decrement_sell_in
    end

    it "should reduce quality by 1 before it expires" do
      update_should_change_quality_by(-1)
    end

    it "should reduce quality by 2 after it expires" do
      @item.sell_in = 0
      update_should_change_quality_by(-2)
    end

    it "should not reduce quality below 0" do
      @item.quality = 0
      update_should_not_change_quality
      @item.sell_in = 0
      update_should_not_change_quality
    end
  end

  describe "cheese item, when updated" do
    before(:each) do
      @item = @gilded_rose.items.find { |item| item.name =~ /brie/i }
      @item.sell_in = 50
      @item.quality = 10
    end

    it "should reduce sell_in by 1" do
      update_should_decrement_sell_in
    end

    it "should increase quality by 1 before expiration" do
      update_should_change_quality_by(1)
    end

    it "should increase quality by 2 after expiration" do
      @item.sell_in = 0
      update_should_change_quality_by(2)
    end

    it "should not increase quality above 50" do
      @item.quality = 50
      update_should_not_change_quality
      @item.sell_in = 0
      update_should_not_change_quality
    end
  end

  describe "legendary item, when updated" do
    before(:each) do
      @item = @gilded_rose.items.find { |item| item.name =~ /hand/i }
      @item.sell_in = 50
      @item.quality = 80
    end

    it "should not decrease in sell_in" do
      expect {
        @gilded_rose.update_quality
      }.to_not change { @item.sell_in }
    end

    it "should not change quality" do
      update_should_not_change_quality
    end

    it "should not change quality if created in expired state" do
      @item.sell_in = -1
      update_should_not_change_quality
    end
  end

  describe "ticket item, when updated" do
    before(:each) do
      @item = @gilded_rose.items.find { |item| item.name =~ /backstage/i }
      @item.quality = 10
      @item.sell_in = 11
    end

    it "should reduce sell_in by 1" do
      update_should_decrement_sell_in
    end

    it "should increase quality by 1 more than 10 days before concert" do
      update_should_change_quality_by(1)
    end

    it "should increase quality by 2 6-10 days before concert" do
      [6, 10].each do |num|
        @item.sell_in = num
        update_should_change_quality_by(2)
      end
    end

    it "should increase quality by 3 1-5 days before concert" do
      [1, 5].each do |num|
        @item.sell_in = num
        update_should_change_quality_by(3)
      end
    end

    it "should drop to 0 after the concert" do
      @item.sell_in = 0
      expect {
        @gilded_rose.update_quality
      }.to change { @item.quality }.to(0)
    end

    it "should not go above 50 in any case" do
      [15, 10, 5].each do |num|
        @item.quality = 50
        @item.sell_in = num
        update_should_not_change_quality
      end
    end
  end

  describe "a conjured item, when updated" do
    before(:each) do
      @item = @gilded_rose.items.find { |item| item.name =~ /conjured/i }
      @item.sell_in = 1
      @item.quality = 10
    end

    it "should reduce sell_in by 1" do
      update_should_decrement_sell_in
    end

    it "should reduce quality by 2 when not expired" do
      update_should_change_quality_by(-2)
    end

    it "should reduce quality by 4 when expired" do
      @item.sell_in = 0
      update_should_change_quality_by(-4)
    end

    it "should not reduce quality below 0" do
      @item.quality = 0
      update_should_not_change_quality
      @item.sell_in = 0
      update_should_not_change_quality
    end
  end
end

item.rb

# Wrapper for item classes
module ItemHandler
  # Item would be our generic item class if not
  # for that pesky goblin. GenericItem adds the
  # necessary functionality instead.
  class Item
    attr_accessor :name, :sell_in, :quality

    def initialize (name, sell_in, quality)
      @name = name
      @sell_in = sell_in
      @quality = quality
    end
  end

  # All of the actual updating should be done in this class.
  # @quality is changed by amount of QLTY_CHANGE, or that number
  # multiplied by 2 if the @sell_in date is passed. Quality
  # may not be less than QLTY_MIN or greater than QLTY_MAX.
  class GenericItem < Item
    QLTY_MIN = 0
    QLTY_MAX = 50
    QLTY_CHANGE = -1

    def update_item
      @sell_in -= 1
      result = @quality + qlty_change
      @quality = [QLTY_MIN, result, QLTY_MAX].sort[1]
    end

    def qlty_change
      self.class::QLTY_CHANGE * (expired? ? 2 : 1)
    end

    def expired?
      @sell_in < 0
    end
  end

  # Cheese items gain in value over time, so
  # QLTY_CHANGE is positive.
  class CheeseItem < GenericItem
    QLTY_CHANGE = 1
  end

  # Conjured items decrease in value 2x
  # faster than generic items, so QLTY_CHANGE is -2.
  class ConjuredItem < GenericItem
    QLTY_CHANGE = -2
  end

  # Legendary items never change, so the update_item
  # method does nothing. They can have @quality > 50.
  class LegendaryItem < GenericItem
    def update_item
      false
    end
  end

  # Ticket items increase in value more quickly as
  # the concert approaches, then drop to zero.
  # * > 10 days to concert: +1 @quality
  # * 6-10 days to concert: +2 @quality
  # * 1-5 days to concert:  +3 @quality
  # * After concert:         @quality is zero
  class TicketItem < GenericItem
    def qlty_change
      return 3 if (0..4).include? @sell_in
      return 2 if (5..9).include? @sell_in
      expired? ? @quality * -1 : 1
    end
  end
end