+ dir_to_tree = lambda do |dirname|
+ # First list subdirectories, with their files inside.
+ subnodes = tree.keys.select { |bd, td| (bd == dirname) and (td != '.') }
+ .sort.flat_map do |parts|
+ [parts + [nil]] + dir_to_tree.call(File.join(parts))
+ end
+ # Then extend that list with files in this directory.
+ subnodes + tree[File.split(dirname)]
+ end
+ dir_to_tree.call('.')