gistfile1.rb

class OrderedHash
  def initialize
    @data  = {}
    @order = []
  end

  def set(key, value)
    @data[key] = value
    @order << key
  end

  def get(key)
    @data[key]
  end

  def each
    @order.each do |key|
      yield key
    end
  end
end

hash = OrderedHash.new
hash.set('foo', 'bar')
hash.set('bar', 'baz')
hash.set('baz', 'quux')
hash.each do |item|
  puts "#{item} -> #{hash.get(item)}"
end

# travis@travis-DX4860:/tmp$ ruby ordered_hash.rb 
# foo -> bar
# bar -> baz
# baz -> quux