/**
    *
    * 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.Comparator;
  import java.util.List;
  import java.util.PriorityQueue;
  
  import org.apache.hadoop.hbase.Cell;
  import org.apache.hadoop.hbase.KeyValue.KVComparator;
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  import org.apache.hadoop.hbase.regionserver.ScannerContext.NextState;
  
  /**
   * Implements a heap merge across any number of KeyValueScanners.
   * <p>
   * Implements KeyValueScanner itself.
   * <p>
   * This class is used at the Region level to merge across Stores
   * and at the Store level to merge across the memstore and StoreFiles.
   * <p>
   * In the Region case, we also need InternalScanner.next(List), so this class
   * also implements InternalScanner.  WARNING: As is, if you try to use this
   * as an InternalScanner at the Store level, you will get runtime exceptions.
   */
  @InterfaceAudience.Private
  public class KeyValueHeap extends NonReversedNonLazyKeyValueScanner
      implements KeyValueScanner, InternalScanner {
    protected PriorityQueue<KeyValueScanner> heap = null;
  
    /**
     * The current sub-scanner, i.e. the one that contains the next key/value
     * to return to the client. This scanner is NOT included in {@link #heap}
     * (but we frequently add it back to the heap and pull the new winner out).
     * We maintain an invariant that the current sub-scanner has already done
     * a real seek, and that current.peek() is always a real key/value (or null)
     * except for the fake last-key-on-row-column supplied by the multi-column
     * Bloom filter optimization, which is OK to propagate to StoreScanner. In
     * order to ensure that, always use {@link #pollRealKV()} to update current.
     */
    protected KeyValueScanner current = null;
  
    protected KVScannerComparator comparator;
    
    /**
     * Constructor.  This KeyValueHeap will handle closing of passed in
     * KeyValueScanners.
     * @param scanners
     * @param comparator
     */
    public KeyValueHeap(List<? extends KeyValueScanner> scanners,
        KVComparator comparator) throws IOException {
      this(scanners, new KVScannerComparator(comparator));
    }
  
    /**
     * Constructor.
     * @param scanners
     * @param comparator
     * @throws IOException
     */
    KeyValueHeap(List<? extends KeyValueScanner> scanners,
        KVScannerComparator comparator) throws IOException {
      this.comparator = comparator;
      if (!scanners.isEmpty()) {
        this.heap = new PriorityQueue<KeyValueScanner>(scanners.size(),
            this.comparator);
        for (KeyValueScanner scanner : scanners) {
          if (scanner.peek() != null) {
            this.heap.add(scanner);
          } else {
            scanner.close();
          }
        }
        this.current = pollRealKV();
      }
    }
  
    public Cell peek() {
      if (this.current == null) {
        return null;
     }
     return this.current.peek();
   }
 
   public Cell next()  throws IOException {
     if(this.current == null) {
       return null;
     }
     Cell kvReturn = this.current.next();
     Cell kvNext = this.current.peek();
     if (kvNext == null) {
       this.current.close();
       this.current = null;
       this.current = pollRealKV();
     } else {
       KeyValueScanner topScanner = this.heap.peek();
       // no need to add current back to the heap if it is the only scanner left
       if (topScanner != null && this.comparator.compare(kvNext, topScanner.peek()) >= 0) {
         this.heap.add(this.current);
         this.current = null;
         this.current = pollRealKV();
       }
     }
     return kvReturn;
   }
 
   /**
    * Gets the next row of keys from the top-most scanner.
    * <p>
    * This method takes care of updating the heap.
    * <p>
    * This can ONLY be called when you are using Scanners that implement InternalScanner as well as
    * KeyValueScanner (a {@link StoreScanner}).
    * @param result
    * @return true if more rows exist after this one, false if scanner is done
    */
   @Override
   public boolean next(List<Cell> result) throws IOException {
     return next(result, NoLimitScannerContext.getInstance());
   }
 
   @Override
   public boolean next(List<Cell> result, ScannerContext scannerContext) throws IOException {
     if (this.current == null) {
       return scannerContext.setScannerState(NextState.NO_MORE_VALUES).hasMoreValues();
     }
     InternalScanner currentAsInternal = (InternalScanner)this.current;
     boolean moreCells = currentAsInternal.next(result, scannerContext);
     Cell pee = this.current.peek();
 
     /*
      * By definition, any InternalScanner must return false only when it has no
      * further rows to be fetched. So, we can close a scanner if it returns
      * false. All existing implementations seem to be fine with this. It is much
      * more efficient to close scanners which are not needed than keep them in
      * the heap. This is also required for certain optimizations.
      */
 
     if (pee == null || !moreCells) {
       this.current.close();
     } else {
       this.heap.add(this.current);
     }
     this.current = null;
     this.current = pollRealKV();
     if (this.current == null) {
       moreCells = scannerContext.setScannerState(NextState.NO_MORE_VALUES).hasMoreValues();
     }
     return moreCells;
   }
 
   protected static class KVScannerComparator implements Comparator<KeyValueScanner> {
     protected KVComparator kvComparator;
     /**
      * Constructor
      * @param kvComparator
      */
     public KVScannerComparator(KVComparator kvComparator) {
       this.kvComparator = kvComparator;
     }
     public int compare(KeyValueScanner left, KeyValueScanner right) {
       int comparison = compare(left.peek(), right.peek());
       if (comparison != 0) {
         return comparison;
       } else {
         // Since both the keys are exactly the same, we break the tie in favor
         // of the key which came latest.
         long leftSequenceID = left.getSequenceID();
         long rightSequenceID = right.getSequenceID();
         if (leftSequenceID > rightSequenceID) {
           return -1;
         } else if (leftSequenceID < rightSequenceID) {
           return 1;
         } else {
           return 0;
         }
       }
     }
     /**
      * Compares two KeyValue
      * @param left
      * @param right
      * @return less than 0 if left is smaller, 0 if equal etc..
      */
     public int compare(Cell left, Cell right) {
       return this.kvComparator.compare(left, right);
     }
     /**
      * @return KVComparator
      */
     public KVComparator getComparator() {
       return this.kvComparator;
     }
   }
 
   public void close() {
     if (this.current != null) {
       this.current.close();
     }
     if (this.heap != null) {
       KeyValueScanner scanner;
       while ((scanner = this.heap.poll()) != null) {
         scanner.close();
       }
     }
   }
 
   /**
    * Seeks all scanners at or below the specified seek key.  If we earlied-out
    * of a row, we may end up skipping values that were never reached yet.
    * Rather than iterating down, we want to give the opportunity to re-seek.
    * <p>
    * As individual scanners may run past their ends, those scanners are
    * automatically closed and removed from the heap.
    * <p>
    * This function (and {@link #reseek(Cell)}) does not do multi-column
    * Bloom filter and lazy-seek optimizations. To enable those, call
    * {@link #requestSeek(Cell, boolean, boolean)}.
    * @param seekKey KeyValue to seek at or after
    * @return true if KeyValues exist at or after specified key, false if not
    * @throws IOException
    */
   @Override
   public boolean seek(Cell seekKey) throws IOException {
     return generalizedSeek(false,    // This is not a lazy seek
                            seekKey,
                            false,    // forward (false: this is not a reseek)
                            false);   // Not using Bloom filters
   }
 
   /**
    * This function is identical to the {@link #seek(Cell)} function except
    * that scanner.seek(seekKey) is changed to scanner.reseek(seekKey).
    */
   @Override
   public boolean reseek(Cell seekKey) throws IOException {
     return generalizedSeek(false,    // This is not a lazy seek
                            seekKey,
                            true,     // forward (true because this is reseek)
                            false);   // Not using Bloom filters
   }
 
   /**
    * {@inheritDoc}
    */
   @Override
   public boolean requestSeek(Cell key, boolean forward,
       boolean useBloom) throws IOException {
     return generalizedSeek(true, key, forward, useBloom);
   }
 
   /**
    * @param isLazy whether we are trying to seek to exactly the given row/col.
    *          Enables Bloom filter and most-recent-file-first optimizations for
    *          multi-column get/scan queries.
    * @param seekKey key to seek to
    * @param forward whether to seek forward (also known as reseek)
    * @param useBloom whether to optimize seeks using Bloom filters
    */
   private boolean generalizedSeek(boolean isLazy, Cell seekKey,
       boolean forward, boolean useBloom) throws IOException {
     if (!isLazy && useBloom) {
       throw new IllegalArgumentException("Multi-column Bloom filter " +
           "optimization requires a lazy seek");
     }
 
     if (current == null) {
       return false;
     }
     heap.add(current);
     current = null;
 
     KeyValueScanner scanner;
     while ((scanner = heap.poll()) != null) {
       Cell topKey = scanner.peek();
       if (comparator.getComparator().compare(seekKey, topKey) <= 0) {
         // Top KeyValue is at-or-after Seek KeyValue. We only know that all
         // scanners are at or after seekKey (because fake keys of
         // scanners where a lazy-seek operation has been done are not greater
         // than their real next keys) but we still need to enforce our
         // invariant that the top scanner has done a real seek. This way
         // StoreScanner and RegionScanner do not have to worry about fake keys.
         heap.add(scanner);
         current = pollRealKV();
         return current != null;
       }
 
       boolean seekResult;
       if (isLazy && heap.size() > 0) {
         // If there is only one scanner left, we don't do lazy seek.
         seekResult = scanner.requestSeek(seekKey, forward, useBloom);
       } else {
         seekResult = NonLazyKeyValueScanner.doRealSeek(
             scanner, seekKey, forward);
       }
 
       if (!seekResult) {
         scanner.close();
       } else {
         heap.add(scanner);
       }
     }
 
     // Heap is returning empty, scanner is done
     return false;
   }
 
   /**
    * Fetches the top sub-scanner from the priority queue, ensuring that a real
    * seek has been done on it. Works by fetching the top sub-scanner, and if it
    * has not done a real seek, making it do so (which will modify its top KV),
    * putting it back, and repeating this until success. Relies on the fact that
    * on a lazy seek we set the current key of a StoreFileScanner to a KV that
    * is not greater than the real next KV to be read from that file, so the
    * scanner that bubbles up to the top of the heap will have global next KV in
    * this scanner heap if (1) it has done a real seek and (2) its KV is the top
    * among all top KVs (some of which are fake) in the scanner heap.
    */
   protected KeyValueScanner pollRealKV() throws IOException {
     KeyValueScanner kvScanner = heap.poll();
     if (kvScanner == null) {
       return null;
     }
 
     while (kvScanner != null && !kvScanner.realSeekDone()) {
       if (kvScanner.peek() != null) {
         try {
           kvScanner.enforceSeek();
         } catch (IOException ioe) {
           kvScanner.close();
           throw ioe;
         }
         Cell curKV = kvScanner.peek();
         if (curKV != null) {
           KeyValueScanner nextEarliestScanner = heap.peek();
           if (nextEarliestScanner == null) {
             // The heap is empty. Return the only possible scanner.
             return kvScanner;
           }
 
           // Compare the current scanner to the next scanner. We try to avoid
           // putting the current one back into the heap if possible.
           Cell nextKV = nextEarliestScanner.peek();
           if (nextKV == null || comparator.compare(curKV, nextKV) < 0) {
             // We already have the scanner with the earliest KV, so return it.
             return kvScanner;
           }
 
           // Otherwise, put the scanner back into the heap and let it compete
           // against all other scanners (both those that have done a "real
           // seek" and a "lazy seek").
           heap.add(kvScanner);
         } else {
           // Close the scanner because we did a real seek and found out there
           // are no more KVs.
           kvScanner.close();
         }
       } else {
         // Close the scanner because it has already run out of KVs even before
         // we had to do a real seek on it.
         kvScanner.close();
       }
       kvScanner = heap.poll();
     }
 
     return kvScanner;
   }
 
   /**
    * @return the current Heap
    */
   public PriorityQueue<KeyValueScanner> getHeap() {
     return this.heap;
   }
 
   @Override
   public long getSequenceID() {
     return 0;
   }
 
   KeyValueScanner getCurrentForTesting() {
     return current;
   }
 
   @Override
   public Cell getNextIndexedKey() {
     // here we return the next index key from the top scanner
     return current == null ? null : current.getNextIndexedKey();
   }
 }