class Cart
  def initialize
    @items_by_id = {}
  end
  
  def [](string_id)
    @items_by_id[string_id]
  end

  def add(item)
    if @items_by_id[item.string_id]
      @items_by_id[item.string_id].increment_quantity
    else
      @items_by_id[item.string_id] = CartItem.new item
    end
  end
  
  def empty?
    size.zero?
  end
  
  def items
    @items_by_id.values
  end
  
  def remove(string_id)
    @items_by_id.delete string_id
  end
  
  def size
    items.size
  end
  
  def total
    items.inject(0) { |sum,item| sum + item.total }
  end
  
  def update_quantities(quantity_by_string_id)
    quantity_by_string_id.each do |string_id, quantity|
      @items_by_id[string_id.to_s].quantity = quantity.to_i
    end
  end
end