/**
    * Copyright The Apache Software Foundation
    *
    * 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.hadoop.hbase.regionserver;
  
  import java.io.IOException;
  import java.util.List;
  import java.util.NavigableSet;
  
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  import org.apache.hadoop.hbase.Cell;
  import org.apache.hadoop.hbase.CellUtil;
  import org.apache.hadoop.hbase.HConstants;
  import org.apache.hadoop.hbase.KeyValue;
  import org.apache.hadoop.hbase.KeyValue.KVComparator;
  import org.apache.hadoop.hbase.client.Scan;
  
  /**
   * ReversedStoreScanner extends from StoreScanner, and is used to support
   * reversed scanning.
   */
  @InterfaceAudience.Private
  class ReversedStoreScanner extends StoreScanner implements KeyValueScanner {
  
    /**
     * Opens a scanner across memstore, snapshot, and all StoreFiles. Assumes we
     * are not in a compaction.
     * 
     * @param store who we scan
     * @param scanInfo
     * @param scan the spec
     * @param columns which columns we are scanning
     * @throws IOException
     */
    ReversedStoreScanner(Store store, ScanInfo scanInfo, Scan scan,
        NavigableSet<byte[]> columns, long readPt)
        throws IOException {
      super(store, scanInfo, scan, columns, readPt);
    }
  
    /** Constructor for testing. */
    ReversedStoreScanner(final Scan scan, ScanInfo scanInfo, ScanType scanType,
        final NavigableSet<byte[]> columns, final List<KeyValueScanner> scanners)
        throws IOException {
      super(scan, scanInfo, scanType, columns, scanners,
          HConstants.LATEST_TIMESTAMP);
    }
  
    @Override
    protected void resetKVHeap(List<? extends KeyValueScanner> scanners,
        KVComparator comparator) throws IOException {
      // Combine all seeked scanners with a heap
      heap = new ReversedKeyValueHeap(scanners, comparator);
    }
  
    @Override
    protected void seekScanners(List<? extends KeyValueScanner> scanners,
        Cell seekKey, boolean isLazy, boolean isParallelSeek)
        throws IOException {
      // Seek all scanners to the start of the Row (or if the exact matching row
      // key does not exist, then to the start of the previous matching Row).
      if (CellUtil.matchingRow(seekKey, HConstants.EMPTY_START_ROW)) {
        for (KeyValueScanner scanner : scanners) {
          scanner.seekToLastRow();
        }
      } else {
        for (KeyValueScanner scanner : scanners) {
          scanner.backwardSeek(seekKey);
        }
      }
    }
  
    @Override
    protected boolean seekToNextRow(Cell kv) throws IOException {
      return seekToPreviousRow(kv);
    }
  
    /**
     * Do a backwardSeek in a reversed StoreScanner(scan backward)
     */
    @Override
    protected boolean seekAsDirection(Cell kv) throws IOException {
      return backwardSeek(kv);
    }
 
   @Override
   protected void checkScanOrder(Cell prevKV, Cell kv,
       KeyValue.KVComparator comparator) throws IOException {
     // Check that the heap gives us KVs in an increasing order for same row and
     // decreasing order for different rows.
     assert prevKV == null || comparator == null || comparator.compareRows(kv, prevKV) < 0
         || (comparator.matchingRows(kv, prevKV) && comparator.compare(kv,
             prevKV) >= 0) : "Key " + prevKV
         + " followed by a " + "error order key " + kv + " in cf " + store
         + " in reversed scan";
   }
 
   @Override
   public boolean reseek(Cell kv) throws IOException {
     throw new IllegalStateException(
         "reseek cannot be called on ReversedStoreScanner");
   }
 
   @Override
   public boolean seek(Cell key) throws IOException {
     throw new IllegalStateException(
         "seek cannot be called on ReversedStoreScanner");
   }
 
   @Override
   public boolean seekToPreviousRow(Cell key) throws IOException {
     lock.lock();
     try {
       checkReseek();
       return this.heap.seekToPreviousRow(key);
     } finally {
       lock.unlock();
     }
 
   }
   
   @Override
   public boolean backwardSeek(Cell key) throws IOException {
     lock.lock();
     try {
       checkReseek();
       return this.heap.backwardSeek(key);
     } finally {
       lock.unlock();
     }
   }
 }