The Gilded Rose Code Kata

gistfile1.rb

require 'guerrilla_patch'

SPECIAL_ITEMS = {
     legendary: [ 'Sulfuras, Hand of Ragnaros' ],
     one_time_event: [ 'Backstage passes to a TAFKAL80ETC concert'],
     better_with_age: ['Aged Brie'],
     conjured: ['Conjured Mana Cake']
}

QUALITY_AMOUNT_RULES = [
   [ "self.standard?", "-1"],
   [ "self.is? :conjured", "-2"],
   [ "(self.is?(:one_time_event) && (self.sell_in >= 6 && self.sell_in < 11) )", "2"],
   [ "(self.is?(:one_time_event) && (self.sell_in < 6 ))", "3"],
   [ "true", "1"]
]

EXPIRED_QUALITY_AMOUNT_RULES = [
   [ "self.is? :one_time_event", "-self.quality"],
   [ "self.is? :better_with_age", "1"],
   [ "self.is? :conjured", "-2"],
   [ "self.standard?", "-1" ],
   [ "true", "0"]
]

def update_quality(items)
  items.reject do |item|
    item.extend Gilded
    item.is? :legendary 
  end.each(&:tick_time)
end

module Gilded
  def tick_time
    adjust_quality
    adjust_age
  end

  def adjust_quality
    self.quality += find_amount(expired? ? EXPIRED_QUALITY_AMOUNT_RULES : QUALITY_AMOUNT_RULES)
    limit_quality
  end

  def limit_quality
    self.quality = 0 if self.quality < 0
    self.quality = 50 if self.quality > 50 
  end

  def adjust_age
    self.sell_in -= 1 
    adjust_quality if expired?
  end

  let(:expired?) { self.sell_in < 0 }
  let(:standard?) { (SPECIAL_ITEMS.each_value.reduce([]){|result, array| result += array }).include?(self.name) == false }
  let(:is?) {|name| SPECIAL_ITEMS[name].include?(self.name) }
  let(:find_amount) { |rules| eval(rules.select {|item| eval(item[0]) }.flatten[1]) }
end

# DO NOT CHANGE THINGS BELOW -----------------------------------------

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

I don't think that optimal solution for the problem is sub-classing since classes hardly carry their weight in this example. I liked the JEG2 refactoring with modules.

Still I think the underlying abstraction for this is that you have arbitrary rules for quality with two cases: before and after expiration. You also have one exception of legendary items not being dependent on time.

IMHO code above communicates this nicely and rules are easy to grasp and extend.