HashOfArray keeps track of the combined order in which elements are appended to its arrays.

hash_of_array.rb

# HashOfArray keeps track of the combined order in which elements are appended to its
# arrays.
# HashOfArray.items returns all items appended to the arrays, in the order they were
# appended.
#
# Example:
#
# h = HashOfArray.new
#
# h[:a] << 1
# h[:b] << 2
# h[:a] << 3
# h[:b] << 4
#
# h.items => [1, 2, 3, 4]
class HashOfArray < Hash
  attr_accessor :items

  def initialize
    self.items = []

    super { |h, k| h[k] = Arr.new(self) }
  end

  class Arr < Array
    attr_accessor :parent

    def initialize(parent)
      self.parent = parent
    end

    def <<(value)
      parent.items << value
      super
    end
  end
end