Package org.apache.lucene.util.fst

Finite state transducers

This package implements Finite State Transducers with the following characteristics:

  • Fast and low memory overhead construction of the minimal FST (but inputs must be provided in sorted order)
  • Low object overhead and quick deserialization (byte[] representation)
  • Optional two-pass compression: FST.pack()
  • Lookup-by-output when the outputs are in sorted order (e.g., ordinals or file pointers)
  • Pluggable Outputs representation
  • N-shortest-paths search by weight
  • Enumerators (IntsRef and BytesRef) that behave like SortedMap iterators

FST Construction example:

    // Input values (keys). These must be provided to Builder in Unicode sorted order!
    String inputValues[] = {"cat", "dog", "dogs"};
    long outputValues[] = {5, 7, 12};
    PositiveIntOutputs outputs = PositiveIntOutputs.getSingleton();
    Builder<Long> builder = new Builder<Long>(INPUT_TYPE.BYTE1, outputs);
    BytesRef scratchBytes = new BytesRef();
    IntsRef scratchInts = new IntsRef();
    for (int i = 0; i < inputValues.length; i++) {
      builder.add(Util.toIntsRef(scratchBytes, scratchInts), outputValues[i]);
    FST<Long> fst = builder.finish();
Retrieval by key:
    Long value = Util.get(fst, new BytesRef("dog"));
    System.out.println(value); // 7
Retrieval by value:
    // Only works because outputs are also in sorted order
    IntsRef key = Util.getByOutput(fst, 12);
    System.out.println(Util.toBytesRef(key, scratchBytes).utf8ToString()); // dogs
Iterate over key-value pairs in sorted order:
    // Like TermsEnum, this also supports seeking (advance)
    BytesRefFSTEnum<Long> iterator = new BytesRefFSTEnum<Long>(fst);
    while ( != null) {
      InputOutput<Long> mapEntry = iterator.current();
N-shortest paths by weight:
    Comparator<Long> comparator = new Comparator<Long>() {
      public int compare(Long left, Long right) {
        return left.compareTo(right);
    Arc<Long> firstArc = fst.getFirstArc(new Arc<Long>());
    MinResult<Long> paths[] = Util.shortestPaths(fst, firstArc, comparator, 2);
    System.out.println(Util.toBytesRef(paths[0].input, scratchBytes).utf8ToString()); // cat
    System.out.println(paths[0].output); // 5
    System.out.println(Util.toBytesRef(paths[1].input, scratchBytes).utf8ToString()); // dog
    System.out.println(paths[1].output); // 7