docs: public Set<String> getGroupIds( String parentGroupId )
      {
          File parentDir =
              StringUtils.isEmpty( parentGroupId ) ? root : new File( root, parentGroupId.replace( '.', '/' ) );
          if ( !parentDir.isDirectory() )
          {
              return Collections.emptySet();
          }
          File[] groupDirs = parentDir.listFiles();
          Set<String> result = new HashSet<String>();
          for ( int i = 0; i < groupDirs.length; i++ )
          {
              if ( groupDirs[i].isDirectory() )
              {
                  result.add( groupDirs[i].getName() );
              }
          }
          return result;
      }