---
uid: Lucene.Net.Util.Fst
summary: *content
---

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Finite state transducers

This package implements [
Finite State Transducers](http://en.wikipedia.org/wiki/Finite_state_transducer) 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](xref:Lucene.Net.Util.Fst.FST#methods)

*   [Lookup-by-output](xref:Lucene.Net.Util.Fst.Util#methods) when the 
       outputs are in sorted order (e.g., ordinals or file pointers)

*   Pluggable [Outputs](xref:Lucene.Net.Util.Fst.Outputs) representation

*   [N-shortest-paths](xref:Lucene.Net.Util.Fst.Util#methods) search by
       weight

*   Enumerators ([IntsRef](xref:Lucene.Net.Util.Fst.IntsRefFSTEnum) and [BytesRef](xref:Lucene.Net.Util.Fst.BytesRefFSTEnum)) that behave like {@link java.util.SortedMap 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++) {
          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
        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:

        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