test_issue_from_Robarin.rb

class Checkout
  attr_reader :items, :pricing_rules

  def initialize(pricing_rules)
    @pricing_rules = pricing_rules
    @items = []
  end

  def add(item)
    items.push item
  end

  def total
    process_rules
    items.sum { |item| item.cost_with_discount }
  end

  private 

  def process_rules
    pricing_rules.each do |type, rule|
      items_by_rule = items.select { |item| item.type.to_sym == type }
      process_discount(rule[:action]) if items_by_rule.count >= rule[:min_count]
    end
  end

  def process_discount(action)
    case action
    when :each_PC_minus_20percent
      items.select { |item| item.type == 'PC' }.each { |item| item.cost_with_discount = item.cost - item.cost * 20 / 100 }
    when :plus_1_CC
      cc_items = items.select { |item| item.type == 'CC' }
      if cc_items.count > 1
        free_cc_items = cc_items.count / 2
        cc_items.first(free_cc_items).each { |item| item.cost_with_discount = 0 }
      end
    end
  end
end

class Item
  attr_reader :cost, :type
  attr_accessor :cost_with_discount

  def initialize(data)
    @type = data[:type] 
    @name = data[:name]
    @cost = data[:cost]
    @cost_with_discount = data[:cost]
  end
end

pricing_rules = { PC: { min_count: 3, action: :each_PC_minus_20percent }, CC: { min_count: 1, action: :plus_1_CC } }
co = Checkout.new(pricing_rules)
items_data = [
  { type: 'CC', name: 'Кока-Кола', cost: 1.50 }, 
  { type: 'CC', name: 'Кока-Кола', cost: 1.50 }, 
  { type: 'PC', name: 'Пепси Кола', cost: 2.00 }, 
  { type: 'PC', name: 'Пепси Кола', cost: 2.00 }, 
  { type: 'PC', name: 'Пепси Кола', cost: 2.00 }, 
  { type: 'WA', name: 'Вода', cost: 0.85 }
]
items_data.each do |item_data|
  co.add(Item.new(item_data))
end

p co.total