Functional version of gilded rose kata. See https://github.com/jimweirich/gilded_rose_kata

gilded_rose.rb

module ItemAging
  AGING_FUNCTIONS = {
    'NORMAL ITEM'                               => ->(s,q) { normal(s,q)    },
    'Aged Brie'                                 => ->(s,q) { brie(s,q)      },
    'Backstage passes to a TAFKAL80ETC concert' => ->(s,q) { backstage(s,q) },
    'Conjured Mana Cake'                        => ->(s,q) { conjured(s,q)  },
  }

  # A convenience function. The *only* function in the module that
  # mutates non-local state.
  def self.age_item item
    item.quality, item.sell_in =
      aging_func(item).call(item.sell_in, item.quality)
  end

  def self.aging_func item, funcs=AGING_FUNCTIONS
    funcs[item.name] || ->(s,q) { [ q, s ] }
  end

  # -------------------------------------------------------------------------------------------

  def self.backstage sell_in, quality
    if quality < 50
      if sell_in < 1
        quality = 0
      else
        quality += 1
        quality += 1 if sell_in < 11
        quality += 1 if sell_in < 6
      end
    end

    [ quality, sell_in - 1]
  end

  def self.brie sell_in, quality
    if quality < 50
      quality += 1
      quality += 1 if sell_in <= 1 && quality < 50
    end

    [ quality, sell_in - 1 ]
  end

  def self.conjured sell_in, quality
    if quality != 0
      quality -= 2
      quality -= 2 if sell_in <= 1
    end

    [ quality, sell_in - 1]
  end

  def self.normal sell_in, quality
    if quality != 0
      quality -= 1
      quality -= 1 if sell_in <= 1
    end

    [ quality, sell_in - 1 ]
  end
end

def update_quality(items)
  items.each {|item| ItemAging.age_item(item) }
end

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