org.apache.lucene.search.suggest.fst
Class FSTCompletionLookup

java.lang.Object
  extended by org.apache.lucene.search.suggest.Lookup
      extended by org.apache.lucene.search.suggest.fst.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.

Nested Class Summary
 
Nested classes/interfaces inherited from class org.apache.lucene.search.suggest.Lookup
Lookup.LookupPriorityQueue, Lookup.LookupResult
 
Field Summary
 
Fields inherited from class org.apache.lucene.search.suggest.Lookup
CHARSEQUENCE_COMPARATOR
 
Constructor Summary
FSTCompletionLookup()
          This constructor prepares for creating a suggested FST using the build(TermFreqIterator) method.
FSTCompletionLookup(FSTCompletion completion, boolean exactMatchFirst)
          This constructor takes a pre-built automaton.
FSTCompletionLookup(int buckets, boolean exactMatchFirst)
          This constructor prepares for creating a suggested FST using the build(TermFreqIterator) method.
 
Method Summary
 void build(TermFreqIterator tfit)
          Builds up a new internal Lookup representation based on the given TermFreqIterator.
 Object get(CharSequence key)
          Returns the bucket (weight) as a Long for the provided key if it exists, otherwise null if it does not.
 boolean load(InputStream input)
          Discard current lookup data and load it from a previously saved copy.
 List<Lookup.LookupResult> lookup(CharSequence key, boolean higherWeightsFirst, int num)
          Look up a key and return possible completion for this key.
 boolean store(OutputStream output)
          Persist the constructed lookup data to a directory.
 
Methods inherited from class org.apache.lucene.search.suggest.Lookup
build
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

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

build

public void build(TermFreqIterator tfit)
           throws IOException
Description copied from class: Lookup
Builds up a new internal Lookup representation based on the given TermFreqIterator. The implementation might re-sort the data internally.

Specified by:
build in class Lookup
Throws:
IOException

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)

get

public Object get(CharSequence key)
Returns the bucket (weight) as a Long for the provided key if it exists, otherwise null if it does not.


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.


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