gilded_rose_with_config

config.yml

Normal:
  :change: -1
  :decay: [0]
Sulfuras, Hand of Ragnaros:
  :eternal: true
Aged Brie:
  :change: +1
  :decay: [0]
Backstage passes to a TAFKAL80ETC concert:
  :change: +1
  :decay: [10, 5]
  :expires: true
Conjured Mana Cake:
  :change: -2
  :decay: [1]

gilded_rose.rb

require 'yaml'

class GildedRose < Struct.new(:name, :days_remaining, :quality)
  def tick(item = config[name] || config['Normal'])
    return quality if item[:eternal]

    self.days_remaining -= 1
    self.quality = new_quality(item[:decay], item[:change], item[:expires])
  end

  protected

  def new_quality(decay, change, expires)
    return 0 if expires and days_remaining < 0
    [[0, quality + decays(decay) * change].max, 50].min
  end

  def decays(decay)
    1 + decay.count { |days| days_remaining < days }
  end

  def config
    @config ||= YAML.load_file("#{__dir__}/config.yml")
  end
end