class FDB::DirectoryLayer

def node_containing_key(tr, key)
  return @root_node if key.start_with?(@node_subspace.key)
  tr.get_range(@node_subspace.range[0], 
               @node_subspace.pack([key]) + "\x00", 
               { :reverse => true, :limit => 1})
  .map { |kv|
    prev_prefix = @node_subspace.unpack(kv.key)[0]
    node_with_prefix(prev_prefix) if key.start_with?(prev_prefix)
  }[0]
end