FuzzyQuery.java

package org.apache.lucene.search;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.Term;
import org.apache.lucene.util.ToStringUtils;

import java.io.IOException;
import java.util.PriorityQueue;

/** Implements the fuzzy search query. The similarity measurement
 * is based on the Levenshtein (edit distance) algorithm.
 * 
 * Warning: this query is not very scalable with its default prefix
 * length of 0 - in this case, *every* term will be enumerated and
 * cause an edit score calculation.
 * 
 */
public class FuzzyQuery extends MultiTermQuery {
  
  public final static float defaultMinSimilarity = 0.5f;
  public final static int defaultPrefixLength = 0;
  
  private float minimumSimilarity;
  private int prefixLength;
  private boolean termLongEnough = false;
  
  protected Term term;
  
  /**
   * Create a new FuzzyQuery that will match terms with a similarity 
   * of at least <code>minimumSimilarity</code> to <code>term</code>.
   * If a <code>prefixLength</code> > 0 is specified, a common prefix
   * of that length is also required.
   * 
   * @param term the term to search for
   * @param minimumSimilarity a value between 0 and 1 to set the required similarity
   *  between the query term and the matching terms. For example, for a
   *  <code>minimumSimilarity</code> of <code>0.5</code> a term of the same length
   *  as the query term is considered similar to the query term if the edit distance
   *  between both terms is less than <code>length(term)*0.5</code>
   * @param prefixLength length of common (non-fuzzy) prefix
   * @throws IllegalArgumentException if minimumSimilarity is >= 1 or < 0
   * or if prefixLength < 0
   */
  public FuzzyQuery(Term term, float minimumSimilarity, int prefixLength) throws IllegalArgumentException {
    this.term = term;
    
    if (minimumSimilarity >= 1.0f)
      throw new IllegalArgumentException("minimumSimilarity >= 1");
    else if (minimumSimilarity < 0.0f)
      throw new IllegalArgumentException("minimumSimilarity < 0");
    if (prefixLength < 0)
      throw new IllegalArgumentException("prefixLength < 0");
    
    if (term.text().length() > 1.0f / (1.0f - minimumSimilarity)) {
      this.termLongEnough = true;
    }
    
    this.minimumSimilarity = minimumSimilarity;
    this.prefixLength = prefixLength;
    rewriteMethod = SCORING_BOOLEAN_QUERY_REWRITE;
  }
  
  /**
   * Calls {@link #FuzzyQuery(Term, float) FuzzyQuery(term, minimumSimilarity, 0)}.
   */
  public FuzzyQuery(Term term, float minimumSimilarity) throws IllegalArgumentException {
      this(term, minimumSimilarity, defaultPrefixLength);
  }

  /**
   * Calls {@link #FuzzyQuery(Term, float) FuzzyQuery(term, 0.5f, 0)}.
   */
  public FuzzyQuery(Term term) {
    this(term, defaultMinSimilarity, defaultPrefixLength);
  }
  
  /**
   * Returns the minimum similarity that is required for this query to match.
   * @return float value between 0.0 and 1.0
   */
  public float getMinSimilarity() {
    return minimumSimilarity;
  }
    
  /**
   * Returns the non-fuzzy prefix length. This is the number of characters at the start
   * of a term that must be identical (not fuzzy) to the query term if the query
   * is to match that term. 
   */
  public int getPrefixLength() {
    return prefixLength;
  }

  @Override
  protected FilteredTermEnum getEnum(IndexReader reader) throws IOException {
    return new FuzzyTermEnum(reader, getTerm(), minimumSimilarity, prefixLength);
  }
  
  /**
   * Returns the pattern term.
   */
  public Term getTerm() {
    return term;
  }

  @Override
  public void setRewriteMethod(RewriteMethod method) {
    throw new UnsupportedOperationException("FuzzyQuery cannot change rewrite method");
  }
  
  @Override
  public Query rewrite(IndexReader reader) throws IOException {
    if(!termLongEnough) {  // can only match if it's exact
      return new TermQuery(term);
    }

    int maxSize = BooleanQuery.getMaxClauseCount();
    PriorityQueue<ScoreTerm> stQueue = new PriorityQueue<ScoreTerm>();
    FilteredTermEnum enumerator = getEnum(reader);
    try {
      ScoreTerm st = new ScoreTerm();
      do {
        final Term t = enumerator.term();
        if (t == null) break;
        final float score = enumerator.difference();
        // ignore uncompetetive hits
        if (stQueue.size() >= maxSize && score <= stQueue.peek().score)
          continue;
        // add new entry in PQ
        st.term = t;
        st.score = score;
        stQueue.offer(st);
        // possibly drop entries from queue
        st = (stQueue.size() > maxSize) ? stQueue.poll() : new ScoreTerm();
      } while (enumerator.next());
    } finally {
      enumerator.close();
    }
    
    BooleanQuery query = new BooleanQuery(true);
    for (final ScoreTerm st : stQueue) {
      TermQuery tq = new TermQuery(st.term);      // found a match
      tq.setBoost(getBoost() * st.score); // set the boost
      query.add(tq, BooleanClause.Occur.SHOULD);          // add to query
    }

    return query;
  }
  
  private static final class ScoreTerm implements Comparable<ScoreTerm> {
    public Term term;
    public float score;
    
    public int compareTo(ScoreTerm other) {
      if (this.score == other.score)
        return other.term.compareTo(this.term);
      else
        return Float.compare(this.score, other.score);
    }
  }
    
  @Override
  public String toString(String field) {
    final StringBuilder buffer = new StringBuilder();
    if (!term.field().equals(field)) {
        buffer.append(term.field());
        buffer.append(":");
    }
    buffer.append(term.text());
    buffer.append('~');
    buffer.append(Float.toString(minimumSimilarity));
    buffer.append(ToStringUtils.boost(getBoost()));
    return buffer.toString();
  }
  
  @Override
  public int hashCode() {
    final int prime = 31;
    int result = super.hashCode();
    result = prime * result + Float.floatToIntBits(minimumSimilarity);
    result = prime * result + prefixLength;
    result = prime * result + ((term == null) ? 0 : term.hashCode());
    return result;
  }

  @Override
  public boolean equals(Object obj) {
    if (this == obj)
      return true;
    if (!super.equals(obj))
      return false;
    if (getClass() != obj.getClass())
      return false;
    FuzzyQuery other = (FuzzyQuery) obj;
    if (Float.floatToIntBits(minimumSimilarity) != Float
        .floatToIntBits(other.minimumSimilarity))
      return false;
    if (prefixLength != other.prefixLength)
      return false;
    if (term == null) {
      if (other.term != null)
        return false;
    } else if (!term.equals(other.term))
      return false;
    return true;
  }


}