if RUBY_VERSION.to_f <= 1.8

  # Simplified version copied from Activesupport::OrderedHash
  class OrderedHash < ::Hash #:nodoc:
    # In MRI the Hash class is core and written in C. In particular, methods are
    # programmed with explicit C function calls and polymorphism is not honored.
    #
    # For example, []= is crucial in this implementation to maintain the @keys
    # array but hash.c invokes rb_hash_aset() originally. This prevents method
    # reuse through inheritance and forces us to reimplement stuff.
    #
    # For instance, we cannot use the inherited #merge! because albeit the algorithm
    # itself would work, our []= is not being called at all by the C code.

    def initialize(*args, &block)
      super
      @keys = []
    end

    def self.[](*args)
      ordered_hash = new

      if (args.length == 1 && args.first.is_a?(Array))
        args.first.each do |key_value_pair|
          next unless (key_value_pair.is_a?(Array))
          ordered_hash[key_value_pair[0]] = key_value_pair[1]
        end

        return ordered_hash
      end

      unless (args.size % 2 == 0)
        raise ArgumentError.new("odd number of arguments for Hash")
      end

      args.each_with_index do |val, ind|
        next if (ind % 2 != 0)
        ordered_hash[val] = args[ind + 1]
      end

      ordered_hash
    end

    def initialize_copy(other)
      super
      # make a deep copy of keys
      @keys = other.keys
    end

    def []=(key, value)
      @keys << key unless has_key?(key)
      super
    end

    def delete(key)
      if has_key? key
        index = @keys.index(key)
        @keys.delete_at index
      end
      super
    end

    def delete_if
      super
      sync_keys!
      self
    end

    def reject!
      super
      sync_keys!
      self
    end

    def reject(&block)
      dup.reject!(&block)
    end

    def keys
      @keys.dup
    end

    def values
      @keys.collect { |key| self[key] }
    end

    def to_hash
      self
    end

    def to_a
      @keys.map { |key| [ key, self[key] ] }
    end

    def each_key
      return to_enum(:each_key) unless block_given?
      @keys.each { |key| yield key }
      self
    end

    def each_value
      return to_enum(:each_value) unless block_given?
      @keys.each { |key| yield self[key]}
      self
    end

    def each
      return to_enum(:each) unless block_given?
      @keys.each {|key| yield [key, self[key]]}
      self
    end

    alias_method :each_pair, :each

    alias_method :select, :find_all

    def clear
      super
      @keys.clear
      self
    end

    def shift
      k = @keys.first
      v = delete(k)
      [k, v]
    end

    def merge!(other_hash)
      if block_given?
        other_hash.each { |k, v| self[k] = key?(k) ? yield(k, self[k], v) : v }
      else
        other_hash.each { |k, v| self[k] = v }
      end
      self
    end

    alias_method :update, :merge!

    def merge(other_hash, &block)
      dup.merge!(other_hash, &block)
    end

    # When replacing with another hash, the initial order of our keys must come from the other hash -ordered or not.
    def replace(other)
      super
      @keys = other.keys
      self
    end

    def invert
      OrderedHash[self.to_a.map!{|key_value_pair| key_value_pair.reverse}]
    end

    def inspect
      "#<OrderedHash #{super}>"
    end

    private
      def sync_keys!
        @keys.delete_if {|k| !has_key?(k)}
      end
  end
  
else
  ::OrderedHash = ::Hash
end