lucene/src/main/java/com/twitter/elephantbird/mapreduce/output/LuceneIndexOutputFormat.java
public static PathFilter newIndexDirFilter(Configuration conf) {
    return new PathFilters.CompositePathFilter(
      PathFilters.newExcludeFilesFilter(conf),
      PathFilters.EXCLUDE_HIDDEN_PATHS_FILTER,
      new PathFilter() {
        @Override
        public boolean accept(Path path) {
          return path.getName().startsWith("index-");
        }
      }
    );
  }
public static PathFilter newIndexDirFilter(Configuration conf) {
    return new PathFilters.CompositePathFilter(
      PathFilters.newExcludeFilesFilter(conf),
      PathFilters.EXCLUDE_HIDDEN_PATHS_FILTER,
      new PathFilter() {
        @Override
        public boolean accept(Path path) {
          return path.getName().startsWith("index-");
        }
      }
    );
  }
Creates a path filter that accepts non-hidden directories that start with "index-"
This is what the indexes created by this output format look like,
so this is useful for finding them when traversing the file system