class Hash
  # Returns a deep copy of hash.
  #
  #   hash = { :a => { :b => 'b' } }
  #   dup  = hash.deep_dup
  #   dup[:a][:c] = 'c'
  #
  #   hash[:a][:c] #=> nil
  #   dup[:a][:c]  #=> "c"
  def deep_dup
    duplicate = self.dup
    duplicate.each_pair do |k,v|
      tv = duplicate[k]
      duplicate[k] = tv.is_a?(Hash) && v.is_a?(Hash) ? tv.deep_dup : v
    end
    duplicate
  end
end