Implementation of Gilded Rose

gilded_rose.rb

module GildedRose
  def self.new(name:, days_remaining:, quality:)
    item_class = case name
    when "Aged Brie"
      AgedItem
    when "Sulfuras, Hand of Ragnaros"
      LegendaryItem
    when "Backstage passes to a TAFKAL80ETC concert"
      BackstagePass
    when "Conjured Mana Cake"
      ConjuredItem
    else
      NormalItem
    end

    item_class.new(name: name, days_remaining: days_remaining, quality: quality)
  end

  class BaseItem
    MAX_QUALITY = 50
    attr_reader :name, :days_remaining, :quality

    def initialize(name:, days_remaining:, quality:)
      @name = name
      @days_remaining = days_remaining
      @quality = quality
    end

    def tick
      raise "not implemented"
    end

    private

    def decrease_days_remaining
      @days_remaining -= 1
    end

    def before_sell_date?
      @days_remaining > 0
    end

    def decrease_quality(n)
      @quality.tap do
        @quality = @quality - n
        @quality = 0 if @quality < 0
      end
    end

    def increase_quality(n)
      @quality.tap do
        @quality = @quality + n
        @quality = MAX_QUALITY if @quality > MAX_QUALITY
      end
    end
  end

  class NormalItem < BaseItem
    def tick
      if before_sell_date?
        decrease_quality(1)
      else
        decrease_quality(2)
      end

      decrease_days_remaining
    end
  end

  class AgedItem < BaseItem
    def tick
      if before_sell_date?
        increase_quality(1)
      else
        increase_quality(2)
      end

      decrease_days_remaining
    end
  end

  class BackstagePass < BaseItem
    def tick
      if days_remaining <= 0
        @quality = 0
      elsif days_remaining <= 5
        increase_quality(3)
      elsif days_remaining <= 10
        increase_quality(2)
      elsif days_remaining > 10
        increase_quality(1)
      end

      decrease_days_remaining
    end
  end

  class ConjuredItem < BaseItem
    def tick
      if before_sell_date?
        decrease_quality(2)
      else
        decrease_quality(4)
      end

      decrease_days_remaining
    end
  end

  class LegendaryItem < BaseItem
    QUALITY = 80

    def tick
      # no-op
    end

    def quality
      @quality = QUALITY
    end
  end
end