class ViewModel::GarbageCollection

def collect_references(tree, &block)
  return enum_for(__method__, tree) unless block_given?
  case tree
  when Hash
    if tree.size == 1 && (ref = tree[ViewModel::REFERENCE_ATTRIBUTE])
      block.(ref)
    else
      tree.each_value { |t| collect_references(t, &block) }
    end
  when Array
    tree.each { |t| collect_references(t, &block) }
  end
end

def garbage_collect_references!(serialization)
  return unless serialization.has_key?('references')
  roots      = serialization.except('references')
  references = serialization['references']
  worklist = Set.new(collect_references(roots))
  visited  = Set.new
  while (live = worklist.first)
    worklist.delete(live)
    visited << live
    collect_references(references[live]) do |ref|
      worklist << ref unless visited.include?(ref)
    end
  end
  references.keep_if { |ref, _val| visited.include?(ref) }
end