module ActiveSupport::DescendantsTracker

def accumulate_descendants(klass, acc)
  if direct_descendants = @@direct_descendants[klass]
    acc.concat(direct_descendants)
    direct_descendants.each { |direct_descendant| accumulate_descendants(direct_descendant, acc) }
  end
end

def clear
  if defined? ActiveSupport::Dependencies
    @@direct_descendants.each do |klass, descendants|
      if ActiveSupport::Dependencies.autoloaded?(klass)
        @@direct_descendants.delete(klass)
      else
        descendants.reject! { |v| ActiveSupport::Dependencies.autoloaded?(v) }
      end
    end
  else
    @@direct_descendants.clear
  end
end

def descendants(klass)
  arr = []
  accumulate_descendants(klass, arr)
  arr
end

def descendants
  DescendantsTracker.descendants(self)
end

def direct_descendants(klass)
  @@direct_descendants[klass] || []
end

def direct_descendants
  DescendantsTracker.direct_descendants(self)
end

def inherited(base)
  DescendantsTracker.store_inherited(self, base)
  super
end

def store_inherited(klass, descendant)
  (@@direct_descendants[klass] ||= []) << descendant
end