/**
    *
    * 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.io.InterruptedIOException;
  import java.util.ArrayList;
  import java.util.List;
  import java.util.NavigableSet;
  import java.util.concurrent.CountDownLatch;
  import java.util.concurrent.locks.ReentrantLock;
  
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.hadoop.hbase.Cell;
  import org.apache.hadoop.hbase.CellUtil;
  import org.apache.hadoop.hbase.DoNotRetryIOException;
  import org.apache.hadoop.hbase.HConstants;
  import org.apache.hadoop.hbase.KeyValue;
  import org.apache.hadoop.hbase.KeyValue.KVComparator;
  import org.apache.hadoop.hbase.KeyValueUtil;
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  import org.apache.hadoop.hbase.client.IsolationLevel;
  import org.apache.hadoop.hbase.client.Scan;
  import org.apache.hadoop.hbase.executor.ExecutorService;
  import org.apache.hadoop.hbase.filter.Filter;
  import org.apache.hadoop.hbase.regionserver.ScanQueryMatcher.MatchCode;
  import org.apache.hadoop.hbase.regionserver.ScannerContext.LimitScope;
  import org.apache.hadoop.hbase.regionserver.ScannerContext.NextState;
  import org.apache.hadoop.hbase.regionserver.handler.ParallelSeekHandler;
  import org.apache.hadoop.hbase.util.Bytes;
  import org.apache.hadoop.hbase.util.EnvironmentEdgeManager;
  
  import com.google.common.annotations.VisibleForTesting;
  
  /**
   * Scanner scans both the memstore and the Store. Coalesce KeyValue stream
   * into List<KeyValue> for a single row.
   */
  @InterfaceAudience.Private
  public class StoreScanner extends NonReversedNonLazyKeyValueScanner
      implements KeyValueScanner, InternalScanner, ChangedReadersObserver {
    private static final Log LOG = LogFactory.getLog(StoreScanner.class);
    // In unit tests, the store could be null
    protected final Store store;
    protected ScanQueryMatcher matcher;
    protected KeyValueHeap heap;
    protected boolean cacheBlocks;
  
    protected long countPerRow = 0;
    protected int storeLimit = -1;
    protected int storeOffset = 0;
  
    // Used to indicate that the scanner has closed (see HBASE-1107)
    // Doesnt need to be volatile because it's always accessed via synchronized methods
    protected boolean closing = false;
    protected final boolean get;
    protected final boolean explicitColumnQuery;
    protected final boolean useRowColBloom;
    /**
     * A flag that enables StoreFileScanner parallel-seeking
     */
    protected boolean parallelSeekEnabled = false;
    protected ExecutorService executor;
    protected final Scan scan;
    protected final NavigableSet<byte[]> columns;
    protected final long oldestUnexpiredTS;
    protected final long now;
    protected final int minVersions;
    protected final long maxRowSize;
    protected final long cellsPerHeartbeatCheck;
  
    /**
     * The number of KVs seen by the scanner. Includes explicitly skipped KVs, but not
     * KVs skipped via seeking to next row/column. TODO: estimate them?
     */
    private long kvsScanned = 0;
    private Cell prevCell = null;
  
    /** We don't ever expect to change this, the constant is just for clarity. */
    static final boolean LAZY_SEEK_ENABLED_BY_DEFAULT = true;
    public static final String STORESCANNER_PARALLEL_SEEK_ENABLE =
       "hbase.storescanner.parallel.seek.enable";
 
   /** Used during unit testing to ensure that lazy seek does save seek ops */
   protected static boolean lazySeekEnabledGlobally =
       LAZY_SEEK_ENABLED_BY_DEFAULT;
 
   /**
    * The number of cells scanned in between timeout checks. Specifying a larger value means that
    * timeout checks will occur less frequently. Specifying a small value will lead to more frequent
    * timeout checks.
    */
   public static final String HBASE_CELLS_SCANNED_PER_HEARTBEAT_CHECK =
       "hbase.cells.scanned.per.heartbeat.check";
 
   /**
    * Default value of {@link #HBASE_CELLS_SCANNED_PER_HEARTBEAT_CHECK}.
    */
   public static final long DEFAULT_HBASE_CELLS_SCANNED_PER_HEARTBEAT_CHECK = 10000;
 
   // if heap == null and lastTop != null, you need to reseek given the key below
   protected Cell lastTop = null;
 
   // A flag whether use pread for scan
   private boolean scanUsePread = false;
   // Indicates whether there was flush during the course of the scan
   private volatile boolean flushed = false;
   // generally we get one file from a flush
   private List<StoreFile> flushedStoreFiles = new ArrayList<StoreFile>(1);
   // The current list of scanners
   private List<KeyValueScanner> currentScanners = new ArrayList<KeyValueScanner>();
   // flush update lock
   private ReentrantLock flushLock = new ReentrantLock();
 
   private final long readPt;
 
   // used by the injection framework to test race between StoreScanner construction and compaction
   enum StoreScannerCompactionRace {
     BEFORE_SEEK,
     AFTER_SEEK,
     COMPACT_COMPLETE
   }
 
   /** An internal constructor. */
   protected StoreScanner(Store store, Scan scan, final ScanInfo scanInfo,
       final NavigableSet<byte[]> columns, long readPt, boolean cacheBlocks) {
     this.readPt = readPt;
     this.store = store;
     this.cacheBlocks = cacheBlocks;
     get = scan.isGetScan();
     int numCol = columns == null ? 0 : columns.size();
     explicitColumnQuery = numCol > 0;
     this.scan = scan;
     this.columns = columns;
     this.now = EnvironmentEdgeManager.currentTime();
     this.oldestUnexpiredTS = now - scanInfo.getTtl();
     this.minVersions = scanInfo.getMinVersions();
 
      // We look up row-column Bloom filters for multi-column queries as part of
      // the seek operation. However, we also look the row-column Bloom filter
      // for multi-row (non-"get") scans because this is not done in
      // StoreFile.passesBloomFilter(Scan, SortedSet<byte[]>).
      this.useRowColBloom = numCol > 1 || (!get && numCol == 1);
 
      this.maxRowSize = scanInfo.getTableMaxRowSize();
      this.scanUsePread = scan.isSmall()? true: scanInfo.isUsePread();
      this.cellsPerHeartbeatCheck = scanInfo.getCellsPerTimeoutCheck();
      // Parallel seeking is on if the config allows and more there is more than one store file.
      if (this.store != null && this.store.getStorefilesCount() > 1) {
        RegionServerServices rsService = ((HStore)store).getHRegion().getRegionServerServices();
        if (rsService != null && scanInfo.isParallelSeekEnabled()) {
          this.parallelSeekEnabled = true;
          this.executor = rsService.getExecutorService();
        }
      }
   }
 
   protected void addCurrentScanners(List<? extends KeyValueScanner> scanners) {
     this.currentScanners.addAll(scanners);
   }
   /**
    * Opens a scanner across memstore, snapshot, and all StoreFiles. Assumes we
    * are not in a compaction.
    *
    * @param store who we scan
    * @param scan the spec
    * @param columns which columns we are scanning
    * @throws IOException
    */
   public StoreScanner(Store store, ScanInfo scanInfo, Scan scan, final NavigableSet<byte[]> columns,
       long readPt)
   throws IOException {
     this(store, scan, scanInfo, columns, readPt, scan.getCacheBlocks());
     if (columns != null && scan.isRaw()) {
       throw new DoNotRetryIOException("Cannot specify any column for a raw scan");
     }
     matcher = new ScanQueryMatcher(scan, scanInfo, columns,
         ScanType.USER_SCAN, Long.MAX_VALUE, HConstants.LATEST_TIMESTAMP,
         oldestUnexpiredTS, now, store.getCoprocessorHost());
 
     this.store.addChangedReaderObserver(this);
 
     try {
       // Pass columns to try to filter out unnecessary StoreFiles.
       List<KeyValueScanner> scanners = getScannersNoCompaction();
 
       // 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 next matching Row).
       // Always check bloom filter to optimize the top row seek for delete
       // family marker.
       seekScanners(scanners, matcher.getStartKey(), explicitColumnQuery && lazySeekEnabledGlobally,
         parallelSeekEnabled);
 
       // set storeLimit
       this.storeLimit = scan.getMaxResultsPerColumnFamily();
 
       // set rowOffset
       this.storeOffset = scan.getRowOffsetPerColumnFamily();
       addCurrentScanners(scanners);
       // Combine all seeked scanners with a heap
       resetKVHeap(scanners, store.getComparator());
     } catch (IOException e) {
       // remove us from the HStore#changedReaderObservers here or we'll have no chance to
       // and might cause memory leak
       this.store.deleteChangedReaderObserver(this);
       throw e;
     }
   }
 
   /**
    * Used for compactions.<p>
    *
    * Opens a scanner across specified StoreFiles.
    * @param store who we scan
    * @param scan the spec
    * @param scanners ancillary scanners
    * @param smallestReadPoint the readPoint that we should use for tracking
    *          versions
    */
   public StoreScanner(Store store, ScanInfo scanInfo, Scan scan,
       List<? extends KeyValueScanner> scanners, ScanType scanType,
       long smallestReadPoint, long earliestPutTs) throws IOException {
     this(store, scanInfo, scan, scanners, scanType, smallestReadPoint, earliestPutTs, null, null);
   }
 
   /**
    * Used for compactions that drop deletes from a limited range of rows.<p>
    *
    * Opens a scanner across specified StoreFiles.
    * @param store who we scan
    * @param scan the spec
    * @param scanners ancillary scanners
    * @param smallestReadPoint the readPoint that we should use for tracking versions
    * @param dropDeletesFromRow The inclusive left bound of the range; can be EMPTY_START_ROW.
    * @param dropDeletesToRow The exclusive right bound of the range; can be EMPTY_END_ROW.
    */
   public StoreScanner(Store store, ScanInfo scanInfo, Scan scan,
       List<? extends KeyValueScanner> scanners, long smallestReadPoint, long earliestPutTs,
       byte[] dropDeletesFromRow, byte[] dropDeletesToRow) throws IOException {
     this(store, scanInfo, scan, scanners, ScanType.COMPACT_RETAIN_DELETES, smallestReadPoint,
         earliestPutTs, dropDeletesFromRow, dropDeletesToRow);
   }
 
   private StoreScanner(Store store, ScanInfo scanInfo, Scan scan,
       List<? extends KeyValueScanner> scanners, ScanType scanType, long smallestReadPoint,
       long earliestPutTs, byte[] dropDeletesFromRow, byte[] dropDeletesToRow) throws IOException {
     this(store, scan, scanInfo, null,
       ((HStore)store).getHRegion().getReadpoint(IsolationLevel.READ_COMMITTED), false);
     if (dropDeletesFromRow == null) {
       matcher = new ScanQueryMatcher(scan, scanInfo, null, scanType, smallestReadPoint,
           earliestPutTs, oldestUnexpiredTS, now, store.getCoprocessorHost());
     } else {
       matcher = new ScanQueryMatcher(scan, scanInfo, null, smallestReadPoint, earliestPutTs,
           oldestUnexpiredTS, now, dropDeletesFromRow, dropDeletesToRow, store.getCoprocessorHost());
     }
 
     // Filter the list of scanners using Bloom filters, time range, TTL, etc.
     scanners = selectScannersFrom(scanners);
 
     // Seek all scanners to the initial key
     seekScanners(scanners, matcher.getStartKey(), false, parallelSeekEnabled);
     addCurrentScanners(scanners);
     // Combine all seeked scanners with a heap
     resetKVHeap(scanners, store.getComparator());
   }
 
   @VisibleForTesting
   StoreScanner(final Scan scan, ScanInfo scanInfo,
       ScanType scanType, final NavigableSet<byte[]> columns,
       final List<KeyValueScanner> scanners) throws IOException {
     this(scan, scanInfo, scanType, columns, scanners,
         HConstants.LATEST_TIMESTAMP,
         // 0 is passed as readpoint because the test bypasses Store
         0);
   }
 
   @VisibleForTesting
   StoreScanner(final Scan scan, ScanInfo scanInfo,
     ScanType scanType, final NavigableSet<byte[]> columns,
     final List<KeyValueScanner> scanners, long earliestPutTs)
         throws IOException {
     this(scan, scanInfo, scanType, columns, scanners, earliestPutTs,
       // 0 is passed as readpoint because the test bypasses Store
       0);
   }
 
   private StoreScanner(final Scan scan, ScanInfo scanInfo,
       ScanType scanType, final NavigableSet<byte[]> columns,
       final List<KeyValueScanner> scanners, long earliestPutTs, long readPt)
   throws IOException {
     this(null, scan, scanInfo, columns, readPt, scan.getCacheBlocks());
     this.matcher = new ScanQueryMatcher(scan, scanInfo, columns, scanType,
         Long.MAX_VALUE, earliestPutTs, oldestUnexpiredTS, now, null);
 
     // In unit tests, the store could be null
     if (this.store != null) {
       this.store.addChangedReaderObserver(this);
     }
     // Seek all scanners to the initial key
     seekScanners(scanners, matcher.getStartKey(), false, parallelSeekEnabled);
     addCurrentScanners(scanners);
     resetKVHeap(scanners, scanInfo.getComparator());
   }
 
   /**
    * Get a filtered list of scanners. Assumes we are not in a compaction.
    * @return list of scanners to seek
    */
   protected List<KeyValueScanner> getScannersNoCompaction() throws IOException {
     final boolean isCompaction = false;
     boolean usePread = get || scanUsePread;
     return selectScannersFrom(store.getScanners(cacheBlocks, get, usePread,
         isCompaction, matcher, scan.getStartRow(), scan.getStopRow(), this.readPt));
   }
 
   /**
    * Seek the specified scanners with the given key
    * @param scanners
    * @param seekKey
    * @param isLazy true if using lazy seek
    * @param isParallelSeek true if using parallel seek
    * @throws IOException
    */
   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 next matching Row).
     // Always check bloom filter to optimize the top row seek for delete
     // family marker.
     if (isLazy) {
       for (KeyValueScanner scanner : scanners) {
         scanner.requestSeek(seekKey, false, true);
       }
     } else {
       if (!isParallelSeek) {
         long totalScannersSoughtBytes = 0;
         for (KeyValueScanner scanner : scanners) {
           if (totalScannersSoughtBytes >= maxRowSize) {
             throw new RowTooBigException("Max row size allowed: " + maxRowSize
               + ", but row is bigger than that");
           }
           scanner.seek(seekKey);
           Cell c = scanner.peek();
           if (c != null) {
             totalScannersSoughtBytes += CellUtil.estimatedSerializedSizeOf(c);
           }
         }
       } else {
         parallelSeek(scanners, seekKey);
       }
     }
   }
 
   protected void resetKVHeap(List<? extends KeyValueScanner> scanners,
       KVComparator comparator) throws IOException {
     // Combine all seeked scanners with a heap
     heap = new KeyValueHeap(scanners, comparator);
   }
 
   /**
    * Filters the given list of scanners using Bloom filter, time range, and
    * TTL.
    */
   protected List<KeyValueScanner> selectScannersFrom(
       final List<? extends KeyValueScanner> allScanners) {
     boolean memOnly;
     boolean filesOnly;
     if (scan instanceof InternalScan) {
       InternalScan iscan = (InternalScan)scan;
       memOnly = iscan.isCheckOnlyMemStore();
       filesOnly = iscan.isCheckOnlyStoreFiles();
     } else {
       memOnly = false;
       filesOnly = false;
     }
 
     List<KeyValueScanner> scanners =
         new ArrayList<KeyValueScanner>(allScanners.size());
 
     // We can only exclude store files based on TTL if minVersions is set to 0.
     // Otherwise, we might have to return KVs that have technically expired.
     long expiredTimestampCutoff = minVersions == 0 ? oldestUnexpiredTS: Long.MIN_VALUE;
 
     // include only those scan files which pass all filters
     for (KeyValueScanner kvs : allScanners) {
       boolean isFile = kvs.isFileScanner();
       if ((!isFile && filesOnly) || (isFile && memOnly)) {
         continue;
       }
 
       if (kvs.shouldUseScanner(scan, store, expiredTimestampCutoff)) {
         scanners.add(kvs);
       } else {
         kvs.close();
       }
     }
     return scanners;
   }
 
   @Override
   public Cell peek() {
     if (this.heap == null) {
       return this.lastTop;
     }
     return this.heap.peek();
   }
 
   @Override
   public KeyValue next() {
     // throw runtime exception perhaps?
     throw new RuntimeException("Never call StoreScanner.next()");
   }
 
   @Override
   public void close() {
     if (this.closing) return;
     this.closing = true;
     // Under test, we dont have a this.store
     if (this.store != null)
       this.store.deleteChangedReaderObserver(this);
     if (this.heap != null)
       this.heap.close();
     this.heap = null; // CLOSED!
     this.lastTop = null; // If both are null, we are closed.
   }
 
   @Override
   public boolean seek(Cell key) throws IOException {
     boolean flushed = checkFlushed();
     // reset matcher state, in case that underlying store changed
     checkReseek(flushed);
     return this.heap.seek(key);
   }
 
   @Override
   public boolean next(List<Cell> outResult) throws IOException {
     return next(outResult, NoLimitScannerContext.getInstance());
   }
 
   /**
    * Get the next row of values from this Store.
    * @param outResult
    * @param scannerContext
    * @return true if there are more rows, false if scanner is done
    */
   @Override
   public boolean next(List<Cell> outResult, ScannerContext scannerContext) throws IOException {
     if (scannerContext == null) {
       throw new IllegalArgumentException("Scanner context cannot be null");
     }
     boolean flushed = checkFlushed();
     if (checkReseek(flushed)) {
       return scannerContext.setScannerState(NextState.MORE_VALUES).hasMoreValues();
     }
 
     // if the heap was left null, then the scanners had previously run out anyways, close and
     // return.
     if (this.heap == null) {
       close();
       return scannerContext.setScannerState(NextState.NO_MORE_VALUES).hasMoreValues();
     }
 
     Cell cell = this.heap.peek();
     if (cell == null) {
       close();
       return scannerContext.setScannerState(NextState.NO_MORE_VALUES).hasMoreValues();
     }
 
     // only call setRow if the row changes; avoids confusing the query matcher
     // if scanning intra-row
     byte[] row = cell.getRowArray();
     int offset = cell.getRowOffset();
     short length = cell.getRowLength();
 
     // If no limits exists in the scope LimitScope.Between_Cells then we are sure we are changing
     // rows. Else it is possible we are still traversing the same row so we must perform the row
     // comparison.
     if (!scannerContext.hasAnyLimit(LimitScope.BETWEEN_CELLS) || matcher.row == null) {
       this.countPerRow = 0;
       matcher.setRow(row, offset, length);
     }
 
     // Clear progress away unless invoker has indicated it should be kept.
     if (!scannerContext.getKeepProgress()) scannerContext.clearProgress();
 
     // Only do a sanity-check if store and comparator are available.
     KeyValue.KVComparator comparator =
         store != null ? store.getComparator() : null;
 
     int count = 0;
     long totalBytesRead = 0;
 
     LOOP: do {
       // Update and check the time limit based on the configured value of cellsPerTimeoutCheck
       if ((kvsScanned % cellsPerHeartbeatCheck == 0)) {
         scannerContext.updateTimeProgress();
         if (scannerContext.checkTimeLimit(LimitScope.BETWEEN_CELLS)) {
           return scannerContext.setScannerState(NextState.TIME_LIMIT_REACHED).hasMoreValues();
         }
       }
 
       if (prevCell != cell) ++kvsScanned; // Do object compare - we set prevKV from the same heap.
       checkScanOrder(prevCell, cell, comparator);
       prevCell = cell;
 
       ScanQueryMatcher.MatchCode qcode = matcher.match(cell);
       qcode = optimize(qcode, cell);
       switch(qcode) {
         case INCLUDE:
         case INCLUDE_AND_SEEK_NEXT_ROW:
         case INCLUDE_AND_SEEK_NEXT_COL:
 
           Filter f = matcher.getFilter();
           if (f != null) {
             // TODO convert Scan Query Matcher to be Cell instead of KV based ?
             cell = f.transformCell(cell);
           }
 
           this.countPerRow++;
           if (storeLimit > -1 &&
               this.countPerRow > (storeLimit + storeOffset)) {
             // do what SEEK_NEXT_ROW does.
             if (!matcher.moreRowsMayExistAfter(cell)) {
               return scannerContext.setScannerState(NextState.NO_MORE_VALUES).hasMoreValues();
             }
             // Setting the matcher.row = null, will mean that after the subsequent seekToNextRow()
             // the heap.peek() will any way be in the next row. So the SQM.match(cell) need do
             // another compareRow to say the current row is DONE
             matcher.row = null;
             seekToNextRow(cell);
             break LOOP;
           }
 
           // add to results only if we have skipped #storeOffset kvs
           // also update metric accordingly
           if (this.countPerRow > storeOffset) {
             outResult.add(cell);
 
             // Update local tracking information
             count++;
             totalBytesRead += CellUtil.estimatedSerializedSizeOf(cell);
 
             // Update the progress of the scanner context
             scannerContext.incrementSizeProgress(CellUtil.estimatedHeapSizeOfWithoutTags(cell));
             scannerContext.incrementBatchProgress(1);
 
             if (totalBytesRead > maxRowSize) {
               throw new RowTooBigException("Max row size allowed: " + maxRowSize
                   + ", but the row is bigger than that.");
             }
           }
 
           if (qcode == ScanQueryMatcher.MatchCode.INCLUDE_AND_SEEK_NEXT_ROW) {
             if (!matcher.moreRowsMayExistAfter(cell)) {
               return scannerContext.setScannerState(NextState.NO_MORE_VALUES).hasMoreValues();
             }
             // Setting the matcher.row = null, will mean that after the subsequent seekToNextRow()
             // the heap.peek() will any way be in the next row. So the SQM.match(cell) need do
             // another compareRow to say the current row is DONE
             matcher.row = null;
             seekToNextRow(cell);
           } else if (qcode == ScanQueryMatcher.MatchCode.INCLUDE_AND_SEEK_NEXT_COL) {
             seekAsDirection(matcher.getKeyForNextColumn(cell));
           } else {
             this.heap.next();
           }
 
           if (scannerContext.checkBatchLimit(LimitScope.BETWEEN_CELLS)) {
             break LOOP;
           }
           if (scannerContext.checkSizeLimit(LimitScope.BETWEEN_CELLS)) {
             break LOOP;
           }
           continue;
 
         case DONE:
           // Optimization for Gets! If DONE, no more to get on this row, early exit!
           if (this.scan.isGetScan()) {
             // Then no more to this row... exit.
             close();// Do all cleanup except heap.close()
             return scannerContext.setScannerState(NextState.NO_MORE_VALUES).hasMoreValues();
           }
           // We are sure that this row is done and we are in the next row.
           // So subsequent StoresScanner.next() call need not do another compare
           // and set the matcher.row
           matcher.row = null;
           return scannerContext.setScannerState(NextState.MORE_VALUES).hasMoreValues();
 
         case DONE_SCAN:
           close();
           return scannerContext.setScannerState(NextState.NO_MORE_VALUES).hasMoreValues();
 
         case SEEK_NEXT_ROW:
           // This is just a relatively simple end of scan fix, to short-cut end
           // us if there is an endKey in the scan.
           if (!matcher.moreRowsMayExistAfter(cell)) {
             return scannerContext.setScannerState(NextState.NO_MORE_VALUES).hasMoreValues();
           }
           // Setting the matcher.row = null, will mean that after the subsequent seekToNextRow()
           // the heap.peek() will any way be in the next row. So the SQM.match(cell) need do
           // another compareRow to say the current row is DONE
           matcher.row = null;
           seekToNextRow(cell);
           break;
 
         case SEEK_NEXT_COL:
           seekAsDirection(matcher.getKeyForNextColumn(cell));
           break;
 
         case SKIP:
           this.heap.next();
           break;
 
         case SEEK_NEXT_USING_HINT:
           // TODO convert resee to Cell?
           Cell nextKV = matcher.getNextKeyHint(cell);
           if (nextKV != null) {
             seekAsDirection(nextKV);
           } else {
             heap.next();
           }
           break;
 
         default:
           throw new RuntimeException("UNEXPECTED");
       }
     } while((cell = this.heap.peek()) != null);
 
     if (count > 0) {
       return scannerContext.setScannerState(NextState.MORE_VALUES).hasMoreValues();
     }
 
     // No more keys
     close();
     return scannerContext.setScannerState(NextState.NO_MORE_VALUES).hasMoreValues();
   }
 
   /**
    * See if we should actually SEEK or rather just SKIP to the next Cell (see HBASE-13109).
    * This method works together with ColumnTrackers and Filters. ColumnTrackers may issue SEEK
    * hints, such as seek to next column, next row, or seek to an arbitrary seek key.
    * This method intercepts these qcodes and decides whether a seek is the most efficient _actual_
    * way to get us to the requested cell (SEEKs are more expensive than SKIP, SKIP, SKIP inside the
    * current, loaded block).
    * It does this by looking at the next indexed key of the current HFile. This key
    * is then compared with the _SEEK_ key, where a SEEK key is an artificial 'last possible key
    * on the row' (only in here, we avoid actually creating a SEEK key; in the compare we work with
    * the current Cell but compare as though it were a seek key; see down in
    * matcher.compareKeyForNextRow, etc). If the compare gets us onto the
    * next block we *_SEEK, otherwise we just INCLUDE or SKIP, and let the ColumnTrackers or Filters
    * go through the next Cell, and so on)
    *
    * <p>The ColumnTrackers and Filters must behave correctly in all cases, i.e. if they are past the
    * Cells they care about they must issues a SKIP or SEEK.
    *
    * <p>Other notes:
    * <ul>
    * <li>Rows can straddle block boundaries</li>
    * <li>Versions of columns can straddle block boundaries (i.e. column C1 at T1 might be in a
    * different block than column C1 at T2)</li>
    * <li>We want to SKIP and INCLUDE if the chance is high that we'll find the desired Cell after a
    * few SKIPs...</li>
    * <li>We want to INCLUDE_AND_SEEK and SEEK when the chance is high that we'll be able to seek
    * past many Cells, especially if we know we need to go to the next block.</li>
    * </ul>
    * <p>A good proxy (best effort) to determine whether INCLUDE/SKIP is better than SEEK is whether
    * we'll likely end up seeking to the next block (or past the next block) to get our next column.
    * Example:
    * <pre>
    * |    BLOCK 1              |     BLOCK 2                   |
    * |  r1/c1, r1/c2, r1/c3    |    r1/c4, r1/c5, r2/c1        |
    *                                   ^         ^
    *                                   |         |
    *                           Next Index Key   SEEK_NEXT_ROW (before r2/c1)
    *
    *
    * |    BLOCK 1                       |     BLOCK 2                      |
    * |  r1/c1/t5, r1/c1/t4, r1/c1/t3    |    r1/c1/t2, r1/c1/T1, r1/c2/T3  |
    *                                            ^              ^
    *                                            |              |
    *                                    Next Index Key        SEEK_NEXT_COL
    * </pre>
    * Now imagine we want columns c1 and c3 (see first diagram above), the 'Next Index Key' of r1/c4
    * is > r1/c3 so we should seek to get to the c1 on the next row, r2. In second case, say we only
    * want one version of c1, after we have it, a SEEK_COL will be issued to get to c2. Looking at
    * the 'Next Index Key', it would land us in the next block, so we should SEEK. In other scenarios
    * where the SEEK will not land us in the next block, it is very likely better to issues a series
    * of SKIPs.
    */
   @VisibleForTesting
   protected ScanQueryMatcher.MatchCode optimize(ScanQueryMatcher.MatchCode qcode, Cell cell) {
     switch(qcode) {
     case INCLUDE_AND_SEEK_NEXT_COL:
     case SEEK_NEXT_COL:
     {
       Cell nextIndexedKey = getNextIndexedKey();
       if (nextIndexedKey != null && nextIndexedKey != KeyValueScanner.NO_NEXT_INDEXED_KEY
           && matcher.compareKeyForNextColumn(nextIndexedKey, cell) >= 0) {
         return qcode == MatchCode.SEEK_NEXT_COL ? MatchCode.SKIP : MatchCode.INCLUDE;
       }
       break;
     }
     case INCLUDE_AND_SEEK_NEXT_ROW:
     case SEEK_NEXT_ROW:
     {
       // If it is a Get Scan, then we know that we are done with this row; there are no more
       // rows beyond the current one: don't try to optimize. We are DONE. Return the *_NEXT_ROW
       // qcode as is. When the caller gets these flags on a Get Scan, it knows it can shut down the
       // Scan.
       if (!this.scan.isGetScan()) {
         Cell nextIndexedKey = getNextIndexedKey();
         if (nextIndexedKey != null && nextIndexedKey != KeyValueScanner.NO_NEXT_INDEXED_KEY
             && matcher.compareKeyForNextRow(nextIndexedKey, cell) > 0) {
           return qcode == MatchCode.SEEK_NEXT_ROW ? MatchCode.SKIP : MatchCode.INCLUDE;
         }
       }
       break;
     }
     default:
       break;
     }
     return qcode;
   }
 
   // Implementation of ChangedReadersObserver
   @Override
   public void updateReaders(List<StoreFile> sfs) throws IOException {
     flushed = true;
     flushLock.lock();
     try {
       flushedStoreFiles.addAll(sfs);
     } finally {
       flushLock.unlock();
     }
   }
 
   // Implementation of ChangedReadersObserver
   protected void nullifyCurrentHeap() throws IOException {
     if (this.closing) return;
 
     // All public synchronized API calls will call 'checkReseek' which will cause
     // the scanner stack to reseek if this.heap==null && this.lastTop != null.
     // But if two calls to updateReaders() happen without a 'next' or 'peek' then we
     // will end up calling this.peek() which would cause a reseek in the middle of a updateReaders
     // which is NOT what we want, not to mention could cause an NPE. So we early out here.
     if (this.heap == null) return;
 
     // this could be null.
     this.lastTop = this.heap.peek();
 
     //DebugPrint.println("SS updateReaders, topKey = " + lastTop);
 
     // close scanners to old obsolete Store files
     this.heap.close(); // bubble thru and close all scanners.
     this.heap = null; // the re-seeks could be slow (access HDFS) free up memory ASAP
 
     // Let the next() call handle re-creating and seeking
   }
 
   /**
    * @param flushed indicates if there was a flush
    * @return true if top of heap has changed (and KeyValueHeap has to try the
    *         next KV)
    * @throws IOException
    */
   protected boolean checkReseek(boolean flushed) throws IOException {
     if (flushed && this.lastTop != null) {
       resetScannerStack(this.lastTop);
       if (this.heap.peek() == null
           || store.getComparator().compareRows(this.lastTop, this.heap.peek()) != 0) {
         LOG.debug("Storescanner.peek() is changed where before = "
             + this.lastTop.toString() + ",and after = " + this.heap.peek());
         this.lastTop = null;
         return true;
       }
       this.lastTop = null; // gone!
     }
     // else dont need to reseek
     return false;
   }
 
   protected void resetScannerStack(Cell lastTopKey) throws IOException {
     /* When we have the scan object, should we not pass it to getScanners()
      * to get a limited set of scanners? We did so in the constructor and we
      * could have done it now by storing the scan object from the constructor
      */
 
     final boolean isCompaction = false;
     boolean usePread = get || scanUsePread;
     List<KeyValueScanner> scanners = null;
     try {
       flushLock.lock();
       scanners = selectScannersFrom(store.getScanners(flushedStoreFiles, cacheBlocks, get, usePread,
         isCompaction, matcher, scan.getStartRow(), scan.getStopRow(), this.readPt, true));
       // Clear the current set of flushed store files so that they don't get added again
       flushedStoreFiles.clear();
     } finally {
       flushLock.unlock();
     }
 
     // Seek the new scanners to the last key
     seekScanners(scanners, lastTopKey, false, parallelSeekEnabled);
     // remove the older memstore scanner
     for (int i = 0; i < currentScanners.size(); i++) {
       if (!currentScanners.get(i).isFileScanner()) {
         currentScanners.remove(i);
         break;
       }
     }
     // add the newly created scanners on the flushed files and the current active memstore scanner
     addCurrentScanners(scanners);
     // Combine all seeked scanners with a heap
     resetKVHeap(this.currentScanners, store.getComparator());
     // Reset the state of the Query Matcher and set to top row.
     // Only reset and call setRow if the row changes; avoids confusing the
     // query matcher if scanning intra-row.
     Cell kv = heap.peek();
     if (kv == null) {
       kv = lastTopKey;
     }
     byte[] row = kv.getRowArray();
     int offset = kv.getRowOffset();
     short length = kv.getRowLength();
     if ((matcher.row == null) || !Bytes.equals(row, offset, length, matcher.row,
         matcher.rowOffset, matcher.rowLength)) {
       this.countPerRow = 0;
       matcher.reset();
       matcher.setRow(row, offset, length);
     }
   }
 
   /**
    * Check whether scan as expected order
    * @param prevKV
    * @param kv
    * @param comparator
    * @throws IOException
    */
   protected void checkScanOrder(Cell prevKV, Cell kv,
       KeyValue.KVComparator comparator) throws IOException {
     // Check that the heap gives us KVs in an increasing order.
     assert prevKV == null || comparator == null
         || comparator.compare(prevKV, kv) <= 0 : "Key " + prevKV
         + " followed by a " + "smaller key " + kv + " in cf " + store;
   }
 
   protected boolean seekToNextRow(Cell kv) throws IOException {
     return reseek(KeyValueUtil.createLastOnRow(kv));
   }
 
   /**
    * Do a reseek in a normal StoreScanner(scan forward)
    * @param kv
    * @return true if scanner has values left, false if end of scanner
    * @throws IOException
    */
   protected boolean seekAsDirection(Cell kv)
       throws IOException {
     return reseek(kv);
   }
 
   @Override
   public boolean reseek(Cell kv) throws IOException {
     boolean flushed = checkFlushed();
     // Heap will not be null, if this is called from next() which.
     // If called from RegionScanner.reseek(...) make sure the scanner
     // stack is reset if needed.
     checkReseek(flushed);
     if (explicitColumnQuery && lazySeekEnabledGlobally) {
       return heap.requestSeek(kv, true, useRowColBloom);
     }
     return heap.reseek(kv);
   }
 
   protected boolean checkFlushed() {
     // check the var without any lock. Suppose even if we see the old
     // value here still it is ok to continue because we will not be resetting
     // the heap but will continue with the referenced memstore's snapshot. For compactions
     // any way we don't need the updateReaders at all to happen as we still continue with
     // the older files
     if (flushed) {
       // If there is a flush and the current scan is notified on the flush ensure that the
       // scan's heap gets reset and we do a seek on the newly flushed file.
       if(!this.closing) {
         this.lastTop = this.peek();
       } else {
         return false;
       }
       // reset the flag
       flushed = false;
       return true;
     }
     return false;
   }
 
   @Override
   public long getSequenceID() {
     return 0;
   }
 
   /**
    * Seek storefiles in parallel to optimize IO latency as much as possible
    * @param scanners the list {@link KeyValueScanner}s to be read from
    * @param kv the KeyValue on which the operation is being requested
    * @throws IOException
    */
   private void parallelSeek(final List<? extends KeyValueScanner>
       scanners, final Cell kv) throws IOException {
     if (scanners.isEmpty()) return;
     int storeFileScannerCount = scanners.size();
     CountDownLatch latch = new CountDownLatch(storeFileScannerCount);
     List<ParallelSeekHandler> handlers =
         new ArrayList<ParallelSeekHandler>(storeFileScannerCount);
     for (KeyValueScanner scanner : scanners) {
       if (scanner instanceof StoreFileScanner) {
         ParallelSeekHandler seekHandler = new ParallelSeekHandler(scanner, kv,
           this.readPt, latch);
         executor.submit(seekHandler);
         handlers.add(seekHandler);
       } else {
         scanner.seek(kv);
         latch.countDown();
       }
     }
 
     try {
       latch.await();
     } catch (InterruptedException ie) {
       throw (InterruptedIOException)new InterruptedIOException().initCause(ie);
     }
 
     for (ParallelSeekHandler handler : handlers) {
       if (handler.getErr() != null) {
         throw new IOException(handler.getErr());
       }
     }
   }
 
   /**
    * Used in testing.
    * @return all scanners in no particular order
    */
   List<KeyValueScanner> getAllScannersForTesting() {
     List<KeyValueScanner> allScanners = new ArrayList<KeyValueScanner>();
     KeyValueScanner current = heap.getCurrentForTesting();
     if (current != null)
       allScanners.add(current);
     for (KeyValueScanner scanner : heap.getHeap())
       allScanners.add(scanner);
     return allScanners;
   }
 
   static void enableLazySeekGlobally(boolean enable) {
     lazySeekEnabledGlobally = enable;
   }
 
   /**
    * @return The estimated number of KVs seen by this scanner (includes some skipped KVs).
    */
   public long getEstimatedNumberOfKvsScanned() {
     return this.kvsScanned;
   }
 
   @Override
   public Cell getNextIndexedKey() {
     return this.heap.getNextIndexedKey();
   }
 }