Package org.apache.lucene.util.fst

Finite state transducers

See: Description

Package org.apache.lucene.util.fst Description

Finite state transducers

This package implements Finite State Transducers with the following characteristics:

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(true);
    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++) {
      scratchBytes.copyChars(inputValues[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, and
    // we passed 'true' for sharing to PositiveIntOutputs.getSingleton
    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 (iterator.next() != null) {
      InputOutput<Long> mapEntry = iterator.current();
      System.out.println(mapEntry.input.utf8ToString());
      System.out.println(mapEntry.output);
    }
N-shortest paths by weight:
    // Only works because we passed 'true' for sharing to PositiveIntOutputs.getSingleton
    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

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