Package org.apache.lucene.search.suggest.fst

Class FSTCompletionLookup

  • public class FSTCompletionLookup
extends Lookup
    An adapter from Lookup API to FSTCompletion.

    This adapter differs from FSTCompletion in that it attempts to discretize any "weights" as passed from in TermFreqIterator.weight() to match the number of buckets. For the rationale for bucketing, see FSTCompletion.

    Note:Discretization requires an additional sorting pass.

    The range of weights for bucketing/ discretization is determined by sorting the input by weight and then dividing into equal ranges. Then, scores within each range are assigned to that bucket.

    Note that this means that even large differences in weights may be lost during automaton construction, but the overall distinction between "classes" of weights will be preserved regardless of the distribution of weights.

    For fine-grained control over which weights are assigned to which buckets, use FSTCompletion directly or TSTLookup, for example.

    See Also:
    FSTCompletion
    WARNING: This API is experimental and might change in incompatible ways in the next release.

    • Constructor Detail

      • FSTCompletionLookup

        public FSTCompletionLookup()
        This constructor prepares for creating a suggested FST using the build(TermFreqIterator) method. The number of weight discretization buckets is set to FSTCompletion.DEFAULT_BUCKETS and exact matches are promoted to the top of the suggestions list.

      • FSTCompletionLookup

        public FSTCompletionLookup​(int buckets,
                           boolean exactMatchFirst)
        This constructor prepares for creating a suggested FST using the build(TermFreqIterator) method.
        Parameters:
        buckets - The number of weight discretization buckets (see FSTCompletion for details).
        exactMatchFirst - If true exact matches are promoted to the top of the suggestions list. Otherwise they appear in the order of discretized weight and alphabetical within the bucket.

      • FSTCompletionLookup

        public FSTCompletionLookup​(FSTCompletion completion,
                           boolean exactMatchFirst)
        This constructor takes a pre-built automaton.
        Parameters:
        completion - An instance of FSTCompletion.
        exactMatchFirst - If true exact matches are promoted to the top of the suggestions list. Otherwise they appear in the order of discretized weight and alphabetical within the bucket.

    • Method Detail

      • lookup

        public List<Lookup.LookupResult> lookup​(CharSequence key,
                                        boolean higherWeightsFirst,
                                        int num)
        Description copied from class: Lookup
        Look up a key and return possible completion for this key.
        Specified by:
        lookup in class Lookup
        Parameters:
        key - lookup key. Depending on the implementation this may be a prefix, misspelling, or even infix.
        higherWeightsFirst - return only more popular results
        num - maximum number of results to return
        Returns:
        a list of possible completions, with their relative weight (e.g. popularity)

      • store

        public boolean store​(OutputStream output)
              throws IOException
        Description copied from class: Lookup
        Persist the constructed lookup data to a directory. Optional operation.
        Specified by:
        store in class Lookup
        Parameters:
        output - OutputStream to write the data to.
        Returns:
        true if successful, false if unsuccessful or not supported.
        Throws:
        IOException - when fatal IO error occurs.

      • load

        public boolean load​(InputStream input)
             throws IOException
        Description copied from class: Lookup
        Discard current lookup data and load it from a previously saved copy. Optional operation.
        Specified by:
        load in class Lookup
        Parameters:
        input - the InputStream to load the lookup data.
        Returns:
        true if completed successfully, false if unsuccessful or not supported.
        Throws:
        IOException - when fatal IO error occurs.