package org.apache.lucene.index;

/*
 * 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.FileNotFoundException;
import java.io.IOException;
import java.nio.file.NoSuchFileException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

import org.apache.lucene.search.SearcherManager; // javadocs
import org.apache.lucene.store.Directory;
import org.apache.lucene.store.NoSuchDirectoryException;

/** 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 name="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<AtomicReader> {

  /** Default termInfosIndexDivisor. */
  public static final int DEFAULT_TERMS_INDEX_DIVISOR = 1;

  /** 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, DEFAULT_TERMS_INDEX_DIVISOR);
  }
  
  /** Expert: Returns a IndexReader reading the index in the given
   *  Directory with the given termInfosIndexDivisor.
   * @param directory the index directory
   * @param termInfosIndexDivisor Subsamples which indexed
   *  terms are loaded into RAM. This has the same effect as {@link
   *  IndexWriterConfig#setTermIndexInterval} except that setting
   *  must be done at indexing time while this setting can be
   *  set per reader.  When set to N, then one in every
   *  N*termIndexInterval terms in the index is loaded into
   *  memory.  By setting this to a value > 1 you can reduce
   *  memory usage, at the expense of higher latency when
   *  loading a TermInfo.  The default value is 1.  Set this
   *  to -1 to skip loading the terms index entirely.
   *  <b>NOTE:</b> divisor settings &gt; 1 do not apply to all PostingsFormat
   *  implementations, including the default one in this release. It only makes
   *  sense for terms indexes that can efficiently re-sample terms at load time.
   * @throws IOException if there is a low-level IO error
   */
  public static DirectoryReader open(final Directory directory, int termInfosIndexDivisor) throws IOException {
    return StandardDirectoryReader.open(directory, null, termInfosIndexDivisor);
  }
  
  /**
   * Open a near real time IndexReader from the {@link org.apache.lucene.index.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.
   * @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, boolean applyAllDeletes) throws IOException {
    return writer.getReader(applyAllDeletes);
  }

  /** 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, DEFAULT_TERMS_INDEX_DIVISOR);
  }

  /** Expert: returns an IndexReader reading the index in the given
   *  {@link IndexCommit} and termInfosIndexDivisor.
   * @param commit the commit point to open
   * @param termInfosIndexDivisor Subsamples which indexed
   *  terms are loaded into RAM. This has the same effect as {@link
   *  IndexWriterConfig#setTermIndexInterval} except that setting
   *  must be done at indexing time while this setting can be
   *  set per reader.  When set to N, then one in every
   *  N*termIndexInterval terms in the index is loaded into
   *  memory.  By setting this to a value > 1 you can reduce
   *  memory usage, at the expense of higher latency when
   *  loading a TermInfo.  The default value is 1.  Set this
   *  to -1 to skip loading the terms index entirely.
   *  <b>NOTE:</b> divisor settings &gt; 1 do not apply to all PostingsFormat
   *  implementations, including the default one in this release. It only makes
   *  sense for terms indexes that can efficiently re-sample terms at load time.
   * @throws IOException if there is a low-level IO error
   */
  public static DirectoryReader open(final IndexCommit commit, int termInfosIndexDivisor) throws IOException {
    return StandardDirectoryReader.open(commit.getDirectory(), commit, termInfosIndexDivisor);
  }

  /**
   * 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 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;
  }

  /**
   * 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>
   *
   * <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>
   *
   * <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>
   *
   * <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>
   *
   * <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}.</p>
   *
   * @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 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;
  }

  /** 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 = new SegmentInfos();
    latest.read(dir);
    final long currentGen = latest.getGeneration();

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

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

      final String fileName = files[i];

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

        SegmentInfos sis = new SegmentInfos();
        try {
          // IOException allowed to throw there, in case
          // segments_N is corrupt
          sis.read(dir, fileName);
        } catch (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
          sis = null;
        }

        if (sis != null)
          commits.add(new StandardDirectoryReader.ReaderCommit(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;
    try {
      files = directory.listAll();
    } catch (NoSuchDirectoryException nsde) {
      // Directory does not exist --> no index exists
      return false;
    }

    // Defensive: maybe a Directory impl returns null
    // instead of throwing NoSuchDirectoryException:
    if (files != null) {
      String prefix = IndexFileNames.SEGMENTS + "_";
      for(String file : files) {
        if (file.startsWith(prefix) || file.equals(IndexFileNames.SEGMENTS_GEN)) {
          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()}.
   */
  protected DirectoryReader(Directory directory, AtomicReader[] segmentReaders) {
    super(segmentReaders);
    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,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,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;

  /**
   * 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}.</p>
   */
  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>
   *
   * <p>If instead this reader is a near real-time reader
   * (ie, obtained by a call to {@link
   * DirectoryReader#open(IndexWriter,boolean)}, 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>
   *
   * <p>In any event, if this returns false, you should call
   * {@link #openIfChanged} to get a new reader that sees the
   * changes.</p>
   *
   * @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.
   * <p/>
   * @lucene.experimental
   */
  public abstract IndexCommit getIndexCommit() throws IOException;

}