/*
 * 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.
 */
package org.apache.lucene.search;

import java.io.IOException;

/**
 * Re-scores the topN results ({@link TopDocs}) from an original query. See {@link QueryRescorer}
 * for an actual implementation. Typically, you run a low-cost first-pass query across the entire
 * index, collecting the top few hundred hits perhaps, and then use this class to mix in a more
 * costly second pass scoring.
 *
 * <p>See {@link QueryRescorer#rescore(IndexSearcher,TopDocs,Query,double,int)} for a simple static
 * method to call to rescore using a 2nd pass {@link Query}.
 *
 * @lucene.experimental
 */
public abstract class Rescorer {

  /**
   * Rescore an initial first-pass {@link TopDocs}.
   *
   * @param searcher {@link IndexSearcher} used to produce the first pass topDocs
   * @param firstPassTopDocs Hits from the first pass search. It's very important that these hits
   *     were produced by the provided searcher; otherwise the doc IDs will not match!
   * @param topN How many re-scored hits to return
   */
  public abstract TopDocs rescore(IndexSearcher searcher, TopDocs firstPassTopDocs, int topN)
      throws IOException;

  /** Explains how the score for the specified document was computed. */
  public abstract Explanation explain(
      IndexSearcher searcher, Explanation firstPassExplanation, int docID) throws IOException;
}
