/*
 * 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.index;

import java.io.FileNotFoundException;
import java.io.IOException;
import java.nio.file.NoSuchFileException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.concurrent.ExecutorService;
import org.apache.lucene.search.SearcherManager; // javadocs
import org.apache.lucene.store.Directory;

/**
 * DirectoryReader is an implementation of {@link CompositeReader} that can read indexes in a {@link
 * Directory}.
 *
 * <p>DirectoryReader instances are usually constructed with a call to one of the static <code>
 * open()</code> methods, e.g. {@link #open(Directory)}.
 *
 * <p>For efficiency, in this API documents are often referred to via <i>document numbers</i>,
 * non-negative integers which each name a unique document in the index. These document numbers are
 * ephemeral -- they may change as documents are added to and deleted from an index. Clients should
 * thus not rely on a given document having the same number between sessions.
 *
 * <p><a id="thread-safety"></a>
 *
 * <p><b>NOTE</b>: {@link IndexReader} instances are completely thread safe, meaning multiple
 * threads can call any of its methods, concurrently. If your application requires external
 * synchronization, you should <b>not</b> synchronize on the <code>IndexReader</code> instance; use
 * your own (non-Lucene) objects instead.
 */
public abstract class DirectoryReader extends BaseCompositeReader<LeafReader> {

  /** The index directory. */
  protected final Directory directory;

  /**
   * Returns a IndexReader reading the index in the given Directory
   *
   * @param directory the index directory
   * @throws IOException if there is a low-level IO error
   */
  public static DirectoryReader open(final Directory directory) throws IOException {
    return StandardDirectoryReader.open(directory, null, null);
  }

  /**
   * Returns a IndexReader reading the index in the given Directory
   *
   * @param directory the index directory
   * @param executorService used to open segment readers in parallel
   * @throws IOException if there is a low-level IO error
   */
  public static DirectoryReader open(final Directory directory, ExecutorService executorService)
      throws IOException {
    return StandardDirectoryReader.open(directory, null, null, executorService);
  }

  /**
   * Returns a IndexReader for the index in the given Directory
   *
   * @param directory the index directory
   * @param leafSorter a comparator for sorting leaf readers. Providing leafSorter is useful for
   *     indices on which it is expected to run many queries with particular sort criteria (e.g. for
   *     time-based indices this is usually a descending sort on timestamp). In this case {@code
   *     leafSorter} should sort leaves according to this sort criteria. Providing leafSorter allows
   *     to speed up this particular type of sort queries by early terminating while iterating
   *     through segments and segments' documents.
   * @throws IOException if there is a low-level IO error
   */
  public static DirectoryReader open(final Directory directory, Comparator<LeafReader> leafSorter)
      throws IOException {
    return StandardDirectoryReader.open(directory, null, leafSorter, null);
  }

  /**
   * Returns a IndexReader for the index in the given Directory
   *
   * @param directory the index directory
   * @param leafSorter a comparator for sorting leaf readers. Providing leafSorter is useful for
   *     indices on which it is expected to run many queries with particular sort criteria (e.g. for
   *     time-based indices this is usually a descending sort on timestamp). In this case {@code
   *     leafSorter} should sort leaves according to this sort criteria. Providing leafSorter allows
   *     to speed up this particular type of sort queries by early terminating while iterating
   *     through segments and segments' documents.
   * @param executorService used to open segment readers in parallel
   * @throws IOException if there is a low-level IO error
   */
  public static DirectoryReader open(
      final Directory directory, Comparator<LeafReader> leafSorter, ExecutorService executorService)
      throws IOException {
    return StandardDirectoryReader.open(directory, null, leafSorter, executorService);
  }

  /**
   * Open a near real time IndexReader from the {@link org.apache.lucene.index.IndexWriter}.
   *
   * @param writer The IndexWriter to open from
   * @return The new IndexReader
   * @throws CorruptIndexException if the index is corrupt
   * @throws IOException if there is a low-level IO error
   * @see #openIfChanged(DirectoryReader,IndexWriter,boolean)
   * @lucene.experimental
   */
  public static DirectoryReader open(final IndexWriter writer) throws IOException {
    return open(writer, true, false);
  }

  /**
   * Expert: open a near real time IndexReader from the {@link org.apache.lucene.index.IndexWriter},
   * controlling whether past deletions should be applied.
   *
   * @param writer The IndexWriter to open from
   * @param applyAllDeletes If true, all buffered deletes will be applied (made visible) in the
   *     returned reader. If false, the deletes are not applied but remain buffered (in IndexWriter)
   *     so that they will be applied in the future. Applying deletes can be costly, so if your app
   *     can tolerate deleted documents being returned you might gain some performance by passing
   *     false.
   * @param writeAllDeletes If true, new deletes will be written down to index files instead of
   *     carried over from writer to reader directly in heap
   * @see #open(IndexWriter)
   * @lucene.experimental
   */
  public static DirectoryReader open(
      final IndexWriter writer, boolean applyAllDeletes, boolean writeAllDeletes)
      throws IOException {
    return writer.getReader(applyAllDeletes, writeAllDeletes);
  }

  /**
   * Expert: returns an IndexReader reading the index in the given {@link IndexCommit}.
   *
   * @param commit the commit point to open
   * @throws IOException if there is a low-level IO error
   */
  public static DirectoryReader open(final IndexCommit commit) throws IOException {
    return StandardDirectoryReader.open(commit.getDirectory(), commit, null, null);
  }

  /**
   * Expert: returns an IndexReader reading the index in the given {@link IndexCommit}.
   *
   * @param commit the commit point to open
   * @param executorService used to open segment readers in parallel
   * @throws IOException if there is a low-level IO error
   */
  public static DirectoryReader open(final IndexCommit commit, ExecutorService executorService)
      throws IOException {
    return StandardDirectoryReader.open(commit.getDirectory(), commit, null, executorService);
  }

  /**
   * Expert: returns an IndexReader reading the index on the given {@link IndexCommit}. This method
   * allows to open indices that were created wih a Lucene version older than N-1 provided that all
   * codecs for this index are available in the classpath and the segment file format used was
   * created with Lucene 7 or newer. Users of this API must be aware that Lucene doesn't guarantee
   * semantic compatibility for indices created with versions older than N-1. All backwards
   * compatibility aside from the file format is optional and applied on a best effort basis.
   *
   * @param commit the commit point to open
   * @param minSupportedMajorVersion the minimum supported major index version
   * @param leafSorter a comparator for sorting leaf readers. Providing leafSorter is useful for
   *     indices on which it is expected to run many queries with particular sort criteria (e.g. for
   *     time-based indices, this is usually a descending sort on timestamp). In this case {@code
   *     leafSorter} should sort leaves according to this sort criteria. Providing leafSorter allows
   *     to speed up this particular type of sort queries by early terminating while iterating
   *     through segments and segments' documents
   * @throws IOException if there is a low-level IO error
   */
  public static DirectoryReader open(
      final IndexCommit commit, int minSupportedMajorVersion, Comparator<LeafReader> leafSorter)
      throws IOException {
    return StandardDirectoryReader.open(
        commit.getDirectory(), minSupportedMajorVersion, commit, leafSorter, null);
  }

  /**
   * Expert: returns an IndexReader reading the index on the given {@link IndexCommit}. This method
   * allows to open indices that were created with a Lucene version older than N-1 provided that all
   * codecs for this index are available in the classpath and the segment file format used was
   * created with Lucene 7 or newer. Users of this API must be aware that Lucene doesn't guarantee
   * semantic compatibility for indices created with versions older than N-1. All backwards
   * compatibility aside from the file format is optional and applied on a best effort basis.
   *
   * @param commit the commit point to open
   * @param minSupportedMajorVersion the minimum supported major index version
   * @param leafSorter a comparator for sorting leaf readers. Providing leafSorter is useful for
   *     indices on which it is expected to run many queries with particular sort criteria (e.g. for
   *     time-based indices, this is usually a descending sort on timestamp). In this case {@code
   *     leafSorter} should sort leaves according to this sort criteria. Providing leafSorter allows
   *     to speed up this particular type of sort queries by early terminating while iterating
   *     through segments and segments' documents
   * @param executorService used to open segment readers in parallel
   * @throws IOException if there is a low-level IO error
   */
  public static DirectoryReader open(
      final IndexCommit commit,
      int minSupportedMajorVersion,
      Comparator<LeafReader> leafSorter,
      ExecutorService executorService)
      throws IOException {
    return StandardDirectoryReader.open(
        commit.getDirectory(), minSupportedMajorVersion, commit, leafSorter, executorService);
  }

  /**
   * If the index has changed since the provided reader was opened, open and return a new reader;
   * else, return null. The new reader, if not null, will be the same type of reader as the previous
   * one, ie an NRT reader will open a new NRT reader, a MultiReader will open a new MultiReader,
   * etc.
   *
   * <p>This method is typically far less costly than opening a fully new <code>DirectoryReader
   * </code> as it shares resources (for example sub-readers) with the provided <code>
   * DirectoryReader</code>, when possible.
   *
   * <p>The provided reader is not closed (you are responsible for doing so); if a new reader is
   * returned you also must eventually close it. Be sure to never close a reader while other threads
   * are still using it; see {@link SearcherManager} to simplify managing this.
   *
   * @throws CorruptIndexException if the index is corrupt
   * @throws IOException if there is a low-level IO error
   * @return null if there are no changes; else, a new DirectoryReader instance which you must
   *     eventually close
   */
  public static DirectoryReader openIfChanged(DirectoryReader oldReader) throws IOException {
    final DirectoryReader newReader = oldReader.doOpenIfChanged();
    assert newReader != oldReader;
    return newReader;
  }

  /**
   * If the index has changed since the provided reader was opened, open and return a new reader;
   * else, return null. The new reader, if not null, will be the same type of reader as the previous
   * one, ie an NRT reader will open a new NRT reader etc.
   *
   * <p>This method is typically far less costly than opening a fully new <code>DirectoryReader
   * </code> as it shares resources (for example sub-readers) with the provided <code>
   * DirectoryReader</code>, when possible.
   *
   * <p>The provided reader is not closed (you are responsible for doing so); if a new reader is
   * returned you also must eventually close it. Be sure to never close a reader while other threads
   * are still using it; see {@link SearcherManager} to simplify managing this.
   *
   * @throws CorruptIndexException if the index is corrupt
   * @throws IOException if there is a low-level IO error
   * @return null if there are no changes; else, a new DirectoryReader instance which you must
   *     eventually close
   */
  public static DirectoryReader openIfChanged(
      DirectoryReader oldReader, ExecutorService executorService) throws IOException {
    final DirectoryReader newReader = oldReader.doOpenIfChanged(executorService);
    assert newReader != oldReader;
    return newReader;
  }

  /**
   * If the IndexCommit differs from what the provided reader is searching, open and return a new
   * reader; else, return null.
   *
   * @see #openIfChanged(DirectoryReader)
   */
  public static DirectoryReader openIfChanged(DirectoryReader oldReader, IndexCommit commit)
      throws IOException {
    final DirectoryReader newReader = oldReader.doOpenIfChanged(commit);
    assert newReader != oldReader;
    return newReader;
  }

  /**
   * If the IndexCommit differs from what the provided reader is searching, open and return a new
   * reader; else, return null. Executor service used to open segment readers in parallel.
   *
   * @see #openIfChanged(DirectoryReader)
   */
  public static DirectoryReader openIfChanged(
      DirectoryReader oldReader, IndexCommit commit, ExecutorService executorService)
      throws IOException {
    final DirectoryReader newReader = oldReader.doOpenIfChanged(commit, executorService);
    assert newReader != oldReader;
    return newReader;
  }

  /**
   * Expert: If there changes (committed or not) in the {@link IndexWriter} versus what the provided
   * reader is searching, then open and return a new IndexReader searching both committed and
   * uncommitted changes from the writer; else, return null (though, the current implementation
   * never returns null).
   *
   * <p>This provides "near real-time" searching, in that changes made during an {@link IndexWriter}
   * session can be quickly made available for searching without closing the writer nor calling
   * {@link IndexWriter#commit}.
   *
   * <p>It's <i>near</i> real-time because there is no hard guarantee on how quickly you can get a
   * new reader after making changes with IndexWriter. You'll have to experiment in your situation
   * to determine if it's fast enough. As this is a new and experimental feature, please report back
   * on your findings so we can learn, improve and iterate.
   *
   * <p>The very first time this method is called, this writer instance will make every effort to
   * pool the readers that it opens for doing merges, applying deletes, etc. This means additional
   * resources (RAM, file descriptors, CPU time) will be consumed.
   *
   * <p>For lower latency on reopening a reader, you should call {@link
   * IndexWriterConfig#setMergedSegmentWarmer} to pre-warm a newly merged segment before it's
   * committed to the index. This is important for minimizing index-to-search delay after a large
   * merge.
   *
   * <p>If an addIndexes* call is running in another thread, then this reader will only search those
   * segments from the foreign index that have been successfully copied over, so far.
   *
   * <p><b>NOTE</b>: Once the writer is closed, any outstanding readers may continue to be used.
   * However, if you attempt to reopen any of those readers, you'll hit an {@link
   * org.apache.lucene.store.AlreadyClosedException}.
   *
   * @return DirectoryReader that covers entire index plus all changes made so far by this
   *     IndexWriter instance, or null if there are no new changes
   * @param writer The IndexWriter to open from
   * @throws IOException if there is a low-level IO error
   * @lucene.experimental
   */
  public static DirectoryReader openIfChanged(DirectoryReader oldReader, IndexWriter writer)
      throws IOException {
    return openIfChanged(oldReader, writer, true);
  }

  /**
   * Expert: If there changes (committed or not) in the {@link IndexWriter} versus what the provided
   * reader is searching, then open and return a new IndexReader searching both committed and
   * uncommitted changes from the writer; else, return null (though, the current implementation
   * never returns null).
   *
   * <p>This provides "near real-time" searching, in that changes made during an {@link IndexWriter}
   * session can be quickly made available for searching without closing the writer or calling
   * {@link IndexWriter#commit}.
   *
   * <p>It's <i>near</i> real-time because there is no hard guarantee on how quickly you can get a
   * new reader after making changes with IndexWriter. You'll have to experiment in your situation
   * to determine if it's fast enough. As this is a new and experimental feature, please report back
   * on your findings so we can learn, improve and iterate.
   *
   * <p>The very first time this method is called, this writer instance will make every effort to
   * pool the readers that it opens for doing merges, applying deletes, etc. This means additional
   * resources (RAM, file descriptors, CPU time) will be consumed.
   *
   * <p>For lower latency on reopening a reader, you should call {@link
   * IndexWriterConfig#setMergedSegmentWarmer} to pre-warm a newly merged segment before it's
   * committed to the index. This is important for minimizing index-to-search delay after a large
   * merge.
   *
   * <p>If an addIndexes* call is running in another thread, then this reader will only search those
   * segments from the foreign index that have been successfully copied over, so far.
   *
   * <p><b>NOTE</b>: Once the writer is closed, any outstanding readers may continue to be used.
   * However, if you attempt to reopen any of those readers, you'll hit an {@link
   * org.apache.lucene.store.AlreadyClosedException}.
   *
   * @return DirectoryReader that covers entire index plus all changes made so far by this
   *     IndexWriter instance, or null if there are no new changes
   * @param writer The IndexWriter to open from
   * @param executorService used to open segment readers in parallel
   * @throws IOException if there is a low-level IO error
   * @lucene.experimental
   */
  public static DirectoryReader openIfChanged(
      DirectoryReader oldReader, IndexWriter writer, ExecutorService executorService)
      throws IOException {
    return openIfChanged(oldReader, writer, true, executorService);
  }

  /**
   * Expert: Opens a new reader, if there are any changes, controlling whether past deletions should
   * be applied.
   *
   * @see #openIfChanged(DirectoryReader,IndexWriter)
   * @param writer The IndexWriter to open from
   * @param applyAllDeletes If true, all buffered deletes will be applied (made visible) in the
   *     returned reader. If false, the deletes are not applied but remain buffered (in IndexWriter)
   *     so that they will be applied in the future. Applying deletes can be costly, so if your app
   *     can tolerate deleted documents being returned you might gain some performance by passing
   *     false.
   * @throws IOException if there is a low-level IO error
   * @lucene.experimental
   */
  public static DirectoryReader openIfChanged(
      DirectoryReader oldReader, IndexWriter writer, boolean applyAllDeletes) throws IOException {
    final DirectoryReader newReader = oldReader.doOpenIfChanged(writer, applyAllDeletes);
    assert newReader != oldReader;
    return newReader;
  }

  /**
   * Expert: Opens a new reader, if there are any changes, controlling whether past deletions should
   * be applied.
   *
   * @see #openIfChanged(DirectoryReader,IndexWriter)
   * @param writer The IndexWriter to open from
   * @param applyAllDeletes If true, all buffered deletes will be applied (made visible) in the
   *     returned reader. If false, the deletes are not applied but remain buffered (in IndexWriter)
   *     so that they will be applied in the future. Applying deletes can be costly, so if your app
   *     can tolerate deleted documents being returned you might gain some performance by passing
   *     false.
   * @param executorService used to open segment readers in parallel
   * @throws IOException if there is a low-level IO error
   * @lucene.experimental
   */
  public static DirectoryReader openIfChanged(
      DirectoryReader oldReader,
      IndexWriter writer,
      boolean applyAllDeletes,
      ExecutorService executorService)
      throws IOException {
    final DirectoryReader newReader =
        oldReader.doOpenIfChanged(writer, applyAllDeletes, executorService);
    assert newReader != oldReader;
    return newReader;
  }

  /**
   * Returns all commit points that exist in the Directory. Normally, because the default is {@link
   * KeepOnlyLastCommitDeletionPolicy}, there would be only one commit point. But if you're using a
   * custom {@link IndexDeletionPolicy} then there could be many commits. Once you have a given
   * commit, you can open a reader on it by calling {@link DirectoryReader#open(IndexCommit)} There
   * must be at least one commit in the Directory, else this method throws {@link
   * IndexNotFoundException}. Note that if a commit is in progress while this method is running,
   * that commit may or may not be returned.
   *
   * @return a sorted list of {@link IndexCommit}s, from oldest to latest.
   */
  public static List<IndexCommit> listCommits(Directory dir) throws IOException {
    final String[] files = dir.listAll();

    List<IndexCommit> commits = new ArrayList<>();

    SegmentInfos latest = SegmentInfos.readLatestCommit(dir, 0);
    final long currentGen = latest.getGeneration();

    commits.add(new StandardDirectoryReader.ReaderCommit(null, latest, dir));

    for (int i = 0; i < files.length; i++) {

      final String fileName = files[i];

      if (fileName.startsWith(IndexFileNames.SEGMENTS)
          && SegmentInfos.generationFromSegmentsFileName(fileName) < currentGen) {

        SegmentInfos sis = null;
        try {
          // IOException allowed to throw there, in case
          // segments_N is corrupt
          sis = SegmentInfos.readCommit(dir, fileName, 0);
        } catch (@SuppressWarnings("unused") FileNotFoundException | NoSuchFileException fnfe) {
          // LUCENE-948: on NFS (and maybe others), if
          // you have writers switching back and forth
          // between machines, it's very likely that the
          // dir listing will be stale and will claim a
          // file segments_X exists when in fact it
          // doesn't.  So, we catch this and handle it
          // as if the file does not exist
        }

        if (sis != null) {
          commits.add(new StandardDirectoryReader.ReaderCommit(null, sis, dir));
        }
      }
    }

    // Ensure that the commit points are sorted in ascending order.
    Collections.sort(commits);

    return commits;
  }

  /**
   * Returns <code>true</code> if an index likely exists at the specified directory. Note that if a
   * corrupt index exists, or if an index in the process of committing
   *
   * @param directory the directory to check for an index
   * @return <code>true</code> if an index exists; <code>false</code> otherwise
   */
  public static boolean indexExists(Directory directory) throws IOException {
    // LUCENE-2812, LUCENE-2727, LUCENE-4738: this logic will
    // return true in cases that should arguably be false,
    // such as only IW.prepareCommit has been called, or a
    // corrupt first commit, but it's too deadly to make
    // this logic "smarter" and risk accidentally returning
    // false due to various cases like file description
    // exhaustion, access denied, etc., because in that
    // case IndexWriter may delete the entire index.  It's
    // safer to err towards "index exists" than try to be
    // smart about detecting not-yet-fully-committed or
    // corrupt indices.  This means that IndexWriter will
    // throw an exception on such indices and the app must
    // resolve the situation manually:
    String[] files = directory.listAll();

    String prefix = IndexFileNames.SEGMENTS + "_";
    for (String file : files) {
      if (file.startsWith(prefix)) {
        return true;
      }
    }
    return false;
  }

  /**
   * Expert: Constructs a {@code DirectoryReader} on the given subReaders.
   *
   * @param segmentReaders the wrapped atomic index segment readers. This array is returned by
   *     {@link #getSequentialSubReaders} and used to resolve the correct subreader for docID-based
   *     methods. <b>Please note:</b> This array is <b>not</b> cloned and not protected for
   *     modification outside of this reader. Subclasses of {@code DirectoryReader} should take care
   *     to not allow modification of this internal array, e.g. {@link #doOpenIfChanged()}.
   * @param leafSorter – a comparator for sorting leaf readers. If not {@code null}, this comparator
   *     is used for sorting leaf readers.
   */
  protected DirectoryReader(
      Directory directory, LeafReader[] segmentReaders, Comparator<LeafReader> leafSorter)
      throws IOException {
    super(segmentReaders, leafSorter);
    this.directory = directory;
  }

  /** Returns the directory this index resides in. */
  public final Directory directory() {
    // Don't ensureOpen here -- in certain cases, when a
    // cloned/reopened reader needs to commit, it may call
    // this method on the closed original reader
    return directory;
  }

  /**
   * Implement this method to support {@link #openIfChanged(DirectoryReader)}. If this reader does
   * not support reopen, return {@code null}, so client code is happy. This should be consistent
   * with {@link #isCurrent} (should always return {@code true}) if reopen is not supported.
   *
   * @throws IOException if there is a low-level IO error
   * @return null if there are no changes; else, a new DirectoryReader instance.
   */
  protected abstract DirectoryReader doOpenIfChanged() throws IOException;

  /**
   * Implement this method to support {@link #openIfChanged(DirectoryReader)}. If this reader does
   * not support reopen, return {@code null}, so client code is happy. This should be consistent
   * with {@link #isCurrent} (should always return {@code true}) if reopen is not supported.
   * ExecutorService provides intra-opening concurrency.
   *
   * @throws IOException if there is a low-level IO error
   * @return null if there are no changes; else, a new DirectoryReader instance.
   */
  protected abstract DirectoryReader doOpenIfChanged(ExecutorService executorService)
      throws IOException;

  /**
   * Implement this method to support {@link #openIfChanged(DirectoryReader,IndexCommit)}. If this
   * reader does not support reopen from a specific {@link IndexCommit}, throw {@link
   * UnsupportedOperationException}.
   *
   * @throws IOException if there is a low-level IO error
   * @return null if there are no changes; else, a new DirectoryReader instance.
   */
  protected abstract DirectoryReader doOpenIfChanged(final IndexCommit commit) throws IOException;

  /**
   * Implement this method to support {@link #openIfChanged(DirectoryReader,IndexCommit)}. If this
   * reader does not support reopen from a specific {@link IndexCommit}, throw {@link
   * UnsupportedOperationException}. ExecutorService provides intra-opening concurrency.
   *
   * @throws IOException if there is a low-level IO error
   * @return null if there are no changes; else, a new DirectoryReader instance.
   */
  protected abstract DirectoryReader doOpenIfChanged(
      final IndexCommit commit, ExecutorService executorService) throws IOException;

  /**
   * Implement this method to support {@link #openIfChanged(DirectoryReader,IndexWriter,boolean)}.
   * If this reader does not support reopen from {@link IndexWriter}, throw {@link
   * UnsupportedOperationException}.
   *
   * @throws IOException if there is a low-level IO error
   * @return null if there are no changes; else, a new DirectoryReader instance.
   */
  protected abstract DirectoryReader doOpenIfChanged(IndexWriter writer, boolean applyAllDeletes)
      throws IOException;

  /**
   * Implement this method to support {@link #openIfChanged(DirectoryReader,IndexWriter,boolean)}.
   * If this reader does not support reopen from {@link IndexWriter}, throw {@link
   * UnsupportedOperationException}. ExecutorService used to open segment readers in parallel.
   *
   * @throws IOException if there is a low-level IO error
   * @return null if there are no changes; else, a new DirectoryReader instance.
   */
  protected abstract DirectoryReader doOpenIfChanged(
      IndexWriter writer, boolean applyAllDeletes, ExecutorService executorService)
      throws IOException;

  /**
   * Version number when this IndexReader was opened.
   *
   * <p>This method returns the version recorded in the commit that the reader opened. This version
   * is advanced every time a change is made with {@link IndexWriter}.
   */
  public abstract long getVersion();

  /**
   * Check whether any new changes have occurred to the index since this reader was opened.
   *
   * <p>If this reader was created by calling {@link #open}, then this method checks if any further
   * commits (see {@link IndexWriter#commit}) have occurred in the directory.
   *
   * <p>If instead this reader is a near real-time reader (ie, obtained by a call to {@link
   * DirectoryReader#open(IndexWriter)}, or by calling {@link #openIfChanged} on a near real-time
   * reader), then this method checks if either a new commit has occurred, or any new uncommitted
   * changes have taken place via the writer. Note that even if the writer has only performed
   * merging, this method will still return false.
   *
   * <p>In any event, if this returns false, you should call {@link #openIfChanged} to get a new
   * reader that sees the changes.
   *
   * @throws IOException if there is a low-level IO error
   */
  public abstract boolean isCurrent() throws IOException;

  /**
   * Expert: return the IndexCommit that this reader has opened.
   *
   * @lucene.experimental
   */
  public abstract IndexCommit getIndexCommit() throws IOException;
}
