begin
  require 'psych'
rescue LoadError
end

require 'yaml'

YAML.add_builtin_type("omap") do |type, val|
  ActiveSupport::OrderedHash[val.map{ |v| v.to_a.first }]
end

module ActiveSupport
  # The order of iteration over hashes in Ruby 1.8 is undefined. For example, you do not know the
  # order in which +keys+ will return keys, or +each+ yield pairs. <tt>ActiveSupport::OrderedHash</tt>
  # implements a hash that preserves insertion order, as in Ruby 1.9:
  #
  #   oh = ActiveSupport::OrderedHash.new
  #   oh[:a] = 1
  #   oh[:b] = 2
  #   oh.keys # => [:a, :b], this order is guaranteed
  #
  # <tt>ActiveSupport::OrderedHash</tt> is namespaced to prevent conflicts with other implementations.
  class OrderedHash < ::Hash #:nodoc:
    def to_yaml_type
      "!tag:yaml.org,2002:omap"
    end

    def encode_with(coder)
      coder.represent_seq '!omap', map { |k,v| { k => v } }
    end

    def to_yaml(opts = {})
      if YAML.const_defined?(:ENGINE) && !YAML::ENGINE.syck?
        return super
      end

      YAML.quick_emit(self, opts) do |out|
        out.seq(taguri) do |seq|
          each do |k, v|
            seq.add(k => v)
          end
        end
      end
    end

    def nested_under_indifferent_access
      self
    end

    # Hash is ordered in Ruby 1.9!
    if RUBY_VERSION < '1.9'

      # 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

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

      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
  end
end
