/**
    *
    * 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.util.LinkedList;
  import java.util.concurrent.atomic.AtomicLong;
  
  import com.google.common.annotations.VisibleForTesting;
  
  import com.google.common.base.Objects;
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  import org.apache.hadoop.hbase.util.Bytes;
  import org.apache.hadoop.hbase.util.ClassSize;
  
  
  /**
   * Manages the read/write consistency. This provides an interface for readers to determine what
   * entries to ignore, and a mechanism for writers to obtain new write numbers, then "commit"
   * the new writes for readers to read (thus forming atomic transactions).
   */
  @InterfaceAudience.Private
  public class MultiVersionConcurrencyControl {
    private static final Log LOG = LogFactory.getLog(MultiVersionConcurrencyControl.class);
  
    final AtomicLong readPoint = new AtomicLong(0);
    final AtomicLong writePoint = new AtomicLong(0);
    private final Object readWaiters = new Object();
    /**
     * Represents no value, or not set.
     */
    public static final long NONE = -1;
  
    // This is the pending queue of writes.
    //
    // TODO(eclark): Should this be an array of fixed size to
    // reduce the number of allocations on the write path?
    // This could be equal to the number of handlers + a small number.
    // TODO: St.Ack 20150903 Sounds good to me.
    private final LinkedList<WriteEntry> writeQueue = new LinkedList<WriteEntry>();
  
    public MultiVersionConcurrencyControl() {
      super();
    }
  
    /**
     * Construct and set read point. Write point is uninitialized.
     */
    public MultiVersionConcurrencyControl(long startPoint) {
      tryAdvanceTo(startPoint, NONE);
    }
  
    /**
     * Step the MVCC forward on to a new read/write basis.
     * @param newStartPoint
     */
    public void advanceTo(long newStartPoint) {
      while (true) {
        long seqId = this.getWritePoint();
        if (seqId >= newStartPoint) break;
        if (this.tryAdvanceTo(/* newSeqId = */ newStartPoint, /* expected = */ seqId)) break;
      }
    }
  
    /**
     * Step the MVCC forward on to a new read/write basis.
     * @param newStartPoint Point to move read and write points to.
     * @param expected If not -1 (#NONE)
     * @return Returns false if <code>expected</code> is not equal to the
     * current <code>readPoint</code> or if <code>startPoint</code> is less than current
     * <code>readPoint</code>
     */
    boolean tryAdvanceTo(long newStartPoint, long expected) {
      synchronized (writeQueue) {
        long currentRead = this.readPoint.get();
        long currentWrite = this.writePoint.get();
        if (currentRead != currentWrite) {
          throw new RuntimeException("Already used this mvcc; currentRead=" + currentRead +
            ", currentWrite=" + currentWrite + "; too late to tryAdvanceTo");
        }
        if (expected != NONE && expected != currentRead) {
          return false;
       }
 
       if (newStartPoint < currentRead) {
         return false;
       }
 
       readPoint.set(newStartPoint);
       writePoint.set(newStartPoint);
     }
     return true;
   }
 
   /**
    * Start a write transaction. Create a new {@link WriteEntry} with a new write number and add it
    * to our queue of ongoing writes. Return this WriteEntry instance.
    * To complete the write transaction and wait for it to be visible, call
    * {@link #completeAndWait(WriteEntry)}. If the write failed, call
    * {@link #complete(WriteEntry)} so we can clean up AFTER removing ALL trace of the failed write
    * transaction.
    * @see #complete(WriteEntry)
    * @see #completeAndWait(WriteEntry)
    */
   public WriteEntry begin() {
     synchronized (writeQueue) {
       long nextWriteNumber = writePoint.incrementAndGet();
       WriteEntry e = new WriteEntry(nextWriteNumber);
       writeQueue.add(e);
       return e;
     }
   }
 
   /**
    * Wait until the read point catches up to the write point; i.e. wait on all outstanding mvccs
    * to complete.
    */
   public void await() {
     // Add a write and then wait on reads to catch up to it.
     completeAndWait(begin());
   }
 
   /**
    * Complete a {@link WriteEntry} that was created by {@link #begin()} then wait until the
    * read point catches up to our write.
    *
    * At the end of this call, the global read point is at least as large as the write point
    * of the passed in WriteEntry.  Thus, the write is visible to MVCC readers.
    */
   public void completeAndWait(WriteEntry e) {
     complete(e);
     waitForRead(e);
   }
 
   /**
    * Mark the {@link WriteEntry} as complete and advance the read point as much as possible.
    * Call this even if the write has FAILED (AFTER backing out the write transaction
    * changes completely) so we can clean up the outstanding transaction.
    *
    * How much is the read point advanced?
    * 
    * Let S be the set of all write numbers that are completed. Set the read point to the highest
    * numbered write of S.
    *
    * @param writeEntry
    *
    * @return true if e is visible to MVCC readers (that is, readpoint >= e.writeNumber)
    */
   public boolean complete(WriteEntry writeEntry) {
     synchronized (writeQueue) {
       writeEntry.markCompleted();
 
       long nextReadValue = NONE;
       boolean ranOnce = false;
       while (!writeQueue.isEmpty()) {
         ranOnce = true;
         WriteEntry queueFirst = writeQueue.getFirst();
 
         if (nextReadValue > 0) {
           if (nextReadValue + 1 != queueFirst.getWriteNumber()) {
             throw new RuntimeException("Invariant in complete violated, nextReadValue="
                 + nextReadValue + ", writeNumber=" + queueFirst.getWriteNumber());
           }
         }
 
         if (queueFirst.isCompleted()) {
           nextReadValue = queueFirst.getWriteNumber();
           writeQueue.removeFirst();
         } else {
           break;
         }
       }
 
       if (!ranOnce) {
         throw new RuntimeException("There is no first!");
       }
 
       if (nextReadValue > 0) {
         synchronized (readWaiters) {
           readPoint.set(nextReadValue);
           readWaiters.notifyAll();
         }
       }
       return readPoint.get() >= writeEntry.getWriteNumber();
     }
   }
 
   /**
    * Wait for the global readPoint to advance up to the passed in write entry number.
    */
   void waitForRead(WriteEntry e) {
     boolean interrupted = false;
     int count = 0;
     synchronized (readWaiters) {
       while (readPoint.get() < e.getWriteNumber()) {
         if (count % 100 == 0 && count > 0) {
           LOG.warn("STUCK: " + this);
         }
         count++;
         try {
           readWaiters.wait(10);
         } catch (InterruptedException ie) {
           // We were interrupted... finish the loop -- i.e. cleanup --and then
           // on our way out, reset the interrupt flag.
           interrupted = true;
         }
       }
     }
     if (interrupted) {
       Thread.currentThread().interrupt();
     }
   }
 
   @VisibleForTesting
   public String toString() {
     return Objects.toStringHelper(this)
         .add("readPoint", readPoint)
         .add("writePoint", writePoint).toString();
   }
 
   public long getReadPoint() {
     return readPoint.get();
   }
 
   @VisibleForTesting
   public long getWritePoint() {
     return writePoint.get();
   }
 
   /**
    * Write number and whether write has completed given out at start of a write transaction.
    * Every created WriteEntry must be completed by calling mvcc#complete or #completeAndWait.
    */
   @InterfaceAudience.Private
   public static class WriteEntry {
     private final long writeNumber;
     private boolean completed = false;
 
     WriteEntry(long writeNumber) {
       this.writeNumber = writeNumber;
     }
 
     void markCompleted() {
       this.completed = true;
     }
 
     boolean isCompleted() {
       return this.completed;
     }
 
     public long getWriteNumber() {
       return this.writeNumber;
     }
 
     @Override
     public String toString() {
       return this.writeNumber + ", " + this.completed;
     }
   }
 
   public static final long FIXED_SIZE = ClassSize.align(
       ClassSize.OBJECT +
       2 * Bytes.SIZEOF_LONG +
       2 * ClassSize.REFERENCE);
 }