gilded_rose.rb

class GildedRose
  NAME_SULFURAS = 'Sulfuras, Hand of Ragnaros'.freeze
  NAME_BRIE = 'Aged Brie'.freeze
  NAME_PASS = 'Backstage passes to a TAFKAL80ETC concert'.freeze
  MAX_QUALITY = 50

  def initialize(items)
    @items = items
  end

  def update_quality
    @items.each do |item|
      next if item.name == NAME_SULFURAS

      item.quality = calc_quality(item)
      item.sell_in -= 1
      item.quality = calc_quality_of_over_sell_in(item) if item.sell_in.negative?
    end
  end

  def calc_quality_of_over_sell_in(item)
    return 0 if item.name == NAME_PASS
    return plus_quality item, 1 if item.name == NAME_BRIE

    minus_quality(item, 1)
  end

  def calc_quality(item)
    case item.name
    when NAME_BRIE
      plus_quality item, 1
    when NAME_PASS
      pass_quality item
    else
      return item.quality unless item.quality.positive?

      minus_quality item, 1
    end
  end

  def plus_quality(item, quality)
    [item.quality + quality, MAX_QUALITY].min
  end

  def minus_quality(item, quality)
    [item.quality - quality, 0].max
  end

  def pass_quality(pass)
    if pass.sell_in < 6
      plus_quality(pass, 3)
    elsif pass.sell_in < 11
      plus_quality(pass, 2)
    else
      plus_quality(pass, 1)
    end
  end
end

class Item
  attr_accessor :name, :sell_in, :quality

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

  def to_s
    "#{@name}, #{@sell_in}, #{@quality}"
  end
end