content: package org.apache.lucene.search.grouping;
  
  /**
   * 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 java.io.IOException;
  
  import org.apache.lucene.index.IndexReader;
  import org.apache.lucene.search.FieldCache;
  import org.apache.lucene.search.Sort;
  
  /**
   * Concrete implementation of {@link AbstractFirstPassGroupingCollector} that groups based on
   * field values and more specifically uses {@link org.apache.lucene.search.FieldCache.StringIndex}
   * to collect groups.
   *
   * @lucene.experimental
   */
  public class TermFirstPassGroupingCollector extends AbstractFirstPassGroupingCollector<String> {
  
    private FieldCache.StringIndex index;
  
    private String groupField;
  
    /**
     * Create the first pass collector.
     *
     *  @param groupField The field used to group
     *    documents. This field must be single-valued and
     *    indexed (FieldCache is used to access its value
     *    per-document).
     *  @param groupSort The {@link Sort} used to sort the
     *    groups.  The top sorted document within each group
     *    according to groupSort, determines how that group
     *    sorts against other groups.  This must be non-null,
     *    ie, if you want to groupSort by relevance use
     *    Sort.RELEVANCE.
     *  @param topNGroups How many top groups to keep.
     *  @throws IOException When I/O related errors occur
     */
    public TermFirstPassGroupingCollector(String groupField, Sort groupSort, int topNGroups) throws IOException {
      super(groupSort, topNGroups);
      this.groupField = groupField;
    }
  
    @Override
    protected String getDocGroupValue(int doc) {
      final int ord = index.order[doc];
      return ord == 0 ? null : index.lookup[ord];
    }
  
    @Override
    protected String copyDocGroupValue(String groupValue, String reuse) {
      return groupValue;
    }
  
    @Override
    public void setNextReader(IndexReader reader, int docBase) throws IOException {
      super.setNextReader(reader, docBase);
      index = FieldCache.DEFAULT.getStringIndex(reader, groupField);
    }
  }
  