content: package org.apache.lucene.util;
  
  /**
   * 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 java.util.ArrayList;
  import java.util.List;
  
  import org.apache.lucene.index.IndexReader;
  
  /**
   * Common util methods for dealing with {@link IndexReader}s.
   *
   * @lucene.internal
   */
  public final class ReaderUtil {
  
    private ReaderUtil() {} // no instance
  
    /**
     * Gathers sub-readers from reader into a List.
     * 
     * @param allSubReaders
     * @param reader
     */
    public static void gatherSubReaders(List<IndexReader> allSubReaders, IndexReader reader) {
      IndexReader[] subReaders = reader.getSequentialSubReaders();
      if (subReaders == null) {
        // Add the reader itself, and do not recurse
        allSubReaders.add(reader);
      } else {
        for (int i = 0; i < subReaders.length; i++) {
          gatherSubReaders(allSubReaders, subReaders[i]);
        }
      }
    }
  
    /** Recursively visits all sub-readers of a reader.  You
     *  should subclass this and override the add method to
     *  gather what you need.
     *
     * @lucene.experimental */
    public static abstract class Gather {
      private final IndexReader topReader;
  
      public Gather(IndexReader r) {
        topReader = r;
      }
  
      public int run() throws IOException {
        return run(0, topReader);
      }
  
      public int run(int docBase) throws IOException {
        return run(docBase, topReader);
      }
  
      private int run(int base, IndexReader reader) throws IOException {
        IndexReader[] subReaders = reader.getSequentialSubReaders();
        if (subReaders == null) {
          // atomic reader
          add(base, reader);
          base += reader.maxDoc();
        } else {
          // composite reader
          for (int i = 0; i < subReaders.length; i++) {
            base = run(base, subReaders[i]);
          }
        }
  
        return base;
      }
  
      protected abstract void add(int base, IndexReader r) throws IOException;
    }
  
    /**
     * Returns sub IndexReader that contains the given document id.
     *    
     * @param doc id of document
     * @param reader parent reader
     * @return sub reader of parent which contains the specified doc id
     */
    public static IndexReader subReader(int doc, IndexReader reader) {
      List<IndexReader> subReadersList = new ArrayList<IndexReader>();
      ReaderUtil.gatherSubReaders(subReadersList, reader);
      IndexReader[] subReaders = subReadersList
          .toArray(new IndexReader[subReadersList.size()]);
      int[] docStarts = new int[subReaders.length];
      int maxDoc = 0;
      for (int i = 0; i < subReaders.length; i++) {
        docStarts[i] = maxDoc;
        maxDoc += subReaders[i].maxDoc();
      }
      return subReaders[ReaderUtil.subIndex(doc, docStarts)];
    }
    
    /**
     * Returns sub-reader subIndex from reader.
     * 
     * @param reader parent reader
     * @param subIndex index of desired sub reader
     * @return the subreader at subIndex
     */
    public static IndexReader subReader(IndexReader reader, int subIndex) {
      List<IndexReader> subReadersList = new ArrayList<IndexReader>();
      ReaderUtil.gatherSubReaders(subReadersList, reader);
      IndexReader[] subReaders = subReadersList
          .toArray(new IndexReader[subReadersList.size()]);
      return subReaders[subIndex];
    }
  
  
    /**
     * Returns index of the searcher/reader for document <code>n</code> in the
     * array used to construct this searcher/reader.
     */
    public static int subIndex(int n, int[] docStarts) { // find
      // searcher/reader for doc n:
      int size = docStarts.length;
      int lo = 0; // search starts array
      int hi = size - 1; // for first element less than n, return its index
      while (hi >= lo) {
        int mid = (lo + hi) >>> 1;
        int midValue = docStarts[mid];
        if (n < midValue)
          hi = mid - 1;
        else if (n > midValue)
          lo = mid + 1;
        else { // found a match
          while (mid + 1 < size && docStarts[mid + 1] == midValue) {
            mid++; // scan to last match
          }
          return mid;
        }
      }
      return hi;
    }
  }
  