src/java/org/apache/lucene/search/QueryFilter.java

package org.apache.lucene.search;

/**
 * Copyright 2004 The Apache Software Foundation
 *
 * Licensed 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 java.io.IOException;
import java.util.WeakHashMap;
import java.util.BitSet;
import org.apache.lucene.index.IndexReader;

/** Constrains search results to only match those which also match a provided
 * query.  Results are cached, so that searches after the first on the same
 * index using this filter are much faster.
 *
 * <p> This could be used, for example, with a {@link RangeQuery} on a suitably
 * formatted date field to implement date filtering.  One could re-use a single
 * QueryFilter that matches, e.g., only documents modified within the last
 * week.  The QueryFilter and RangeQuery would only need to be reconstructed
 * once per day.
 *
 * @version $Id$
 */
public class QueryFilter extends Filter {
  private Query query;
  private transient WeakHashMap cache = null;

  /** Constructs a filter which only matches documents matching
   * <code>query</code>.
   */
  public QueryFilter(Query query) {
    this.query = query;
  }

  public BitSet bits(IndexReader reader) throws IOException {

    if (cache == null) {
      cache = new WeakHashMap();
    }

    synchronized (cache) {  // check cache
      BitSet cached = (BitSet) cache.get(reader);
      if (cached != null) {
        return cached;
      }
    }

    final BitSet bits = new BitSet(reader.maxDoc());

    new IndexSearcher(reader).search(query, new HitCollector() {
      public final void collect(int doc, float score) {
        bits.set(doc);  // set bit for hit
      }
    });

    synchronized (cache) {  // update cache
      cache.put(reader, bits);
    }

    return bits;
  }

  public String toString() {
    return "QueryFilter("+query+")";
  }
}