Java – How to do query auto-completion/suggestions in Lucene

autocompletejavalucene

I'm looking for a way to do query auto-completion/suggestions in Lucene. I've Googled around a bit and played around a bit, but all of the examples I've seen seem to be setting up filters in Solr. We don't use Solr and aren't planning to move to using Solr in the near future, and Solr is obviously just wrapping around Lucene anyway, so I imagine there must be a way to do it!

I've looked into using EdgeNGramFilter, and I realise that I'd have to run the filter on the index fields and get the tokens out and then compare them against the inputted Query… I'm just struggling to make the connection between the two into a bit of code, so help is much appreciated!

To be clear on what I'm looking for (I realised I wasn't being overly clear, sorry) – I'm looking for a solution where when searching for a term, it'd return a list of suggested queries. When typing 'inter' into the search field, it'll come back with a list of suggested queries, such as 'internet', 'international', etc.

Best Answer

Based on @Alexandre Victoor's answer, I wrote a little class based on the Lucene Spellchecker in the contrib package (and using the LuceneDictionary included in it) that does exactly what I want.

This allows re-indexing from a single source index with a single field, and provides suggestions for terms. Results are sorted by the number of matching documents with that term in the original index, so more popular terms appear first. Seems to work pretty well :)

import java.io.IOException;
import java.io.Reader;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;

import org.apache.lucene.analysis.Analyzer;
import org.apache.lucene.analysis.ISOLatin1AccentFilter;
import org.apache.lucene.analysis.LowerCaseFilter;
import org.apache.lucene.analysis.StopFilter;
import org.apache.lucene.analysis.TokenStream;
import org.apache.lucene.analysis.ngram.EdgeNGramTokenFilter;
import org.apache.lucene.analysis.ngram.EdgeNGramTokenFilter.Side;
import org.apache.lucene.analysis.standard.StandardFilter;
import org.apache.lucene.analysis.standard.StandardTokenizer;
import org.apache.lucene.document.Document;
import org.apache.lucene.document.Field;
import org.apache.lucene.index.CorruptIndexException;
import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.IndexWriter;
import org.apache.lucene.index.Term;
import org.apache.lucene.search.IndexSearcher;
import org.apache.lucene.search.Query;
import org.apache.lucene.search.ScoreDoc;
import org.apache.lucene.search.Sort;
import org.apache.lucene.search.TermQuery;
import org.apache.lucene.search.TopDocs;
import org.apache.lucene.search.spell.LuceneDictionary;
import org.apache.lucene.store.Directory;
import org.apache.lucene.store.FSDirectory;

/**
 * Search term auto-completer, works for single terms (so use on the last term
 * of the query).
 * <p>
 * Returns more popular terms first.
 * 
 * @author Mat Mannion, M.Mannion@warwick.ac.uk
 */
public final class Autocompleter {

    private static final String GRAMMED_WORDS_FIELD = "words";

    private static final String SOURCE_WORD_FIELD = "sourceWord";

    private static final String COUNT_FIELD = "count";

    private static final String[] ENGLISH_STOP_WORDS = {
    "a", "an", "and", "are", "as", "at", "be", "but", "by",
    "for", "i", "if", "in", "into", "is",
    "no", "not", "of", "on", "or", "s", "such",
    "t", "that", "the", "their", "then", "there", "these",
    "they", "this", "to", "was", "will", "with"
    };

    private final Directory autoCompleteDirectory;

    private IndexReader autoCompleteReader;

    private IndexSearcher autoCompleteSearcher;

    public Autocompleter(String autoCompleteDir) throws IOException {
        this.autoCompleteDirectory = FSDirectory.getDirectory(autoCompleteDir,
                null);

        reOpenReader();
    }

    public List<String> suggestTermsFor(String term) throws IOException {
        // get the top 5 terms for query
        Query query = new TermQuery(new Term(GRAMMED_WORDS_FIELD, term));
        Sort sort = new Sort(COUNT_FIELD, true);

        TopDocs docs = autoCompleteSearcher.search(query, null, 5, sort);
        List<String> suggestions = new ArrayList<String>();
        for (ScoreDoc doc : docs.scoreDocs) {
            suggestions.add(autoCompleteReader.document(doc.doc).get(
                    SOURCE_WORD_FIELD));
        }

        return suggestions;
    }

    @SuppressWarnings("unchecked")
    public void reIndex(Directory sourceDirectory, String fieldToAutocomplete)
            throws CorruptIndexException, IOException {
        // build a dictionary (from the spell package)
        IndexReader sourceReader = IndexReader.open(sourceDirectory);

        LuceneDictionary dict = new LuceneDictionary(sourceReader,
                fieldToAutocomplete);

        // code from
        // org.apache.lucene.search.spell.SpellChecker.indexDictionary(
        // Dictionary)
        IndexReader.unlock(autoCompleteDirectory);

        // use a custom analyzer so we can do EdgeNGramFiltering
        IndexWriter writer = new IndexWriter(autoCompleteDirectory,
        new Analyzer() {
            public TokenStream tokenStream(String fieldName,
                    Reader reader) {
                TokenStream result = new StandardTokenizer(reader);

                result = new StandardFilter(result);
                result = new LowerCaseFilter(result);
                result = new ISOLatin1AccentFilter(result);
                result = new StopFilter(result,
                    ENGLISH_STOP_WORDS);
                result = new EdgeNGramTokenFilter(
                    result, Side.FRONT,1, 20);

                return result;
            }
        }, true);

        writer.setMergeFactor(300);
        writer.setMaxBufferedDocs(150);

        // go through every word, storing the original word (incl. n-grams) 
        // and the number of times it occurs
        Map<String, Integer> wordsMap = new HashMap<String, Integer>();

        Iterator<String> iter = (Iterator<String>) dict.getWordsIterator();
        while (iter.hasNext()) {
            String word = iter.next();

            int len = word.length();
            if (len < 3) {
                continue; // too short we bail but "too long" is fine...
            }

            if (wordsMap.containsKey(word)) {
                throw new IllegalStateException(
                        "This should never happen in Lucene 2.3.2");
                // wordsMap.put(word, wordsMap.get(word) + 1);
            } else {
                // use the number of documents this word appears in
                wordsMap.put(word, sourceReader.docFreq(new Term(
                        fieldToAutocomplete, word)));
            }
        }

        for (String word : wordsMap.keySet()) {
            // ok index the word
            Document doc = new Document();
            doc.add(new Field(SOURCE_WORD_FIELD, word, Field.Store.YES,
                    Field.Index.UN_TOKENIZED)); // orig term
            doc.add(new Field(GRAMMED_WORDS_FIELD, word, Field.Store.YES,
                    Field.Index.TOKENIZED)); // grammed
            doc.add(new Field(COUNT_FIELD,
                    Integer.toString(wordsMap.get(word)), Field.Store.NO,
                    Field.Index.UN_TOKENIZED)); // count

            writer.addDocument(doc);
        }

        sourceReader.close();

        // close writer
        writer.optimize();
        writer.close();

        // re-open our reader
        reOpenReader();
    }

    private void reOpenReader() throws CorruptIndexException, IOException {
        if (autoCompleteReader == null) {
            autoCompleteReader = IndexReader.open(autoCompleteDirectory);
        } else {
            autoCompleteReader.reopen();
        }

        autoCompleteSearcher = new IndexSearcher(autoCompleteReader);
    }

    public static void main(String[] args) throws Exception {
        Autocompleter autocomplete = new Autocompleter("/index/autocomplete");

        // run this to re-index from the current index, shouldn't need to do
        // this very often
        // autocomplete.reIndex(FSDirectory.getDirectory("/index/live", null),
        // "content");

        String term = "steve";

        System.out.println(autocomplete.suggestTermsFor(term));
        // prints [steve, steven, stevens, stevenson, stevenage]
    }

}
Related Topic