Package org.apache.lucene.util.fst

Finite state transducers

See:
Description

Class Summary

Builder<T>	Builds a minimal FST (maps an IntsRef term to an arbitrary output) from pre-sorted terms with outputs.
Builder.Arc<T>	Expert: holds a pending (seen but not yet serialized) arc.
Builder.FreezeTail<T>	Expert: this is invoked by Builder whenever a suffix is serialized.
Builder.UnCompiledNode<T>	Expert: holds a pending (seen but not yet serialized) Node.
ByteSequenceOutputs	An FST Outputs implementation where each output is a sequence of bytes.
BytesRefFSTEnum<T>	Enumerates all input (BytesRef) + output pairs in an FST.
BytesRefFSTEnum.InputOutput<T>	Holds a single input (BytesRef) + output pair.
CharSequenceOutputs	An FST Outputs implementation where each output is a sequence of characters.
FST<T>	Represents an finite state machine (FST), using a compact byte[] format.
FST.Arc<T>	Represents a single arc.
FST.BytesReader	Reads bytes stored in an FST.
IntSequenceOutputs	An FST Outputs implementation where each output is a sequence of ints.
IntsRefFSTEnum<T>	Enumerates all input (IntsRef) + output pairs in an FST.
IntsRefFSTEnum.InputOutput<T>	Holds a single input (IntsRef) + output pair.
NoOutputs	A null FST Outputs implementation; use this if you just want to build an FSA.
Outputs<T>	Represents the outputs for an FST, providing the basic algebra required for building and traversing the FST.
PairOutputs<A,B>	An FST Outputs implementation, holding two other outputs.
PairOutputs.Pair<A,B>	Holds a single pair of two outputs.
PositiveIntOutputs	An FST Outputs implementation where each output is a non-negative long value.
Util	Static helper methods.
Util.FSTPath<T>	Represents a path in TopNSearcher.
Util.MinResult<T>	Holds a single input (IntsRef) + output, returned by shortestPaths().
Util.TopNSearcher<T>	Utility class to find top N shortest paths from start point(s).

Enum Summary

FST.INPUT_TYPE

Specifies allowed range of each int input label for this FST.

Package org.apache.lucene.util.fst Description

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++) {
      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