reverse_bfs.rb

  ##  
  # Given a key/value list of nodes and their children, this returns the reversed,
  # breadth-first search, the driving need of which is to find the build order of 
  # applying seed tables and their dependencies.  
  #
  # - jodell 20080530 
  #
  def reverse_bfs(list, nodes = []) 
    return nodes unless nodes.any? { |v| not list[v].nil? }
    return reverse_bfs(
      list.reject { |k, v| not (list.invert.keys.flatten - [nil]).include?(k) },
      list.invert.keys.flatten - [nil]
    ) | nodes
  end