add_to_cart.rb

require_relative "inet2"
puts ARGV
cart = Cart.new(ARGV.delete_at(0))
ARGV.each do |argument|
  @items.each { |item| cart.add_expensive_item(item) if argument == item.name }
end

cart.save_to_file

cart.rb

class Cart

  attr_reader :items

  include ItemContainer

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

  def save_to_file
    File.open("#{@owner}_cart.txt" , "w") do |f|
      @items.each{ |i| f.puts i}
    end
  end

  def read_from_file
    @items = []
    File.readlines("#{@owner}_cart.txt").each {|item| @items << item.to_real_item}
    @items.uniq!
  rescue Errno::ENOENT
    File.open("#{@owner}_cart.txt" , "w") {}
    puts "file #{@owner}_cart.txt created"
  end
end

inet2.rb

require_relative "string"
require_relative "item_container"
require_relative "cart"
require_relative "item"
#require_relative "order" - забыл файл добавить?
require_relative "virtual_item"
require_relative "real_item"

@items = []
@items<< RealItem.new( { :price=>109, :weight=>100, :name=> "car"} )
@items<< RealItem.new( { :price=>101, :weight=>100, :name=> "kettle"} )
@items<< RealItem.new( { :weight=>100, :name=> "dishwasher", :price=>100} )

item.rb

class Item

  @@discount = 0.05

  def self.discount
    if Time.now.month == 3
      @@discount + 0.1
    else
      @@discount
    end
  end

  def initialize(options={})
    @real_price = options[:price]
    @name = options[:name]
  end

  def info
    yield(price)
    yield(name)
  end

  attr_reader :real_price, :name
  attr_writer :price

  def price
    (@real_price - @real_price * self.class.discount) + tax if @real_price
  end

  def to_s
    "#{self.name}. Price: #{self.price}, Weight: #{self.weight}"
  end

  def valid?
    !self.price.nil?
  end

  private

  def tax
    cost_tax + type_tax
  end

  def type_tax
    if self.class == VirtualItem
      1
    else
      2
    end
  end

  def cost_tax
    if @real_price > 5
      @real_price * 0.2
    else
      @real_price * 0.1
    end
  end
end

item_container.rb

module ItemContainer
  module ClassMethods
    def min_price
      50
    end
  end
  
  module InstantMethods
    def add_expensive_item(item)
      unless item.price < self.class.min_price
        @items.push item
      end
    end
  
    def remove_item_from_tail
      @items.pop
    end
  
    def validate
      @items.each{ |item| puts "Item has no price" if item.valid? }
    end
  
    def delete_invalid_items
      @items.delete_if { |item| !item.valid? }
    end
  
  
    def count_valid_items
      @items.select{ |item| item.valid? }.count
    end
  end

  def self.included(base)
    base.extend ClassMethods

    base.class_eval do
      include InstantMethods
    end
  end
end

real_item.rb

class RealItem < Item

  attr_accessor :weight

  def initialize(options)
    @weight = options[:weight]
    super
  end

  def info
    yield (@weight)
    super
  end
end

string.rb

class String
  def to_real_item
    fields = self.chomp
    fields = fields.split(":")
    RealItem.new(name: fields[0], price: fields[1].to_i, weight: fields[2].to_i)
  end
end

virtual_item.rb

class VirtualItem < Item

end