package org.apache.helix.filestore;
   
   /*
    * 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.File;
  import java.io.FileNotFoundException;
  import java.io.IOException;
  import java.io.RandomAccessFile;
  import java.nio.charset.Charset;
  import java.util.ArrayList;
  import java.util.Collections;
  import java.util.List;
  import java.util.concurrent.locks.Condition;
  import java.util.concurrent.locks.Lock;
  import java.util.concurrent.locks.ReentrantLock;
  
  public class ChangeLogReader implements FileChangeWatcher
  {
    int MAX_ENTRIES_TO_READ = 100;
    private final String changeLogDir;
    Lock lock;
    private Condition condition;
  
    public ChangeLogReader(String changeLogDir)
    {
      this.changeLogDir = changeLogDir;
      lock = new ReentrantLock();
      condition = lock.newCondition();
  
    }
  
    /**
     * Blocking call
     * 
     * @param record
     * @return
     */
    public List<ChangeRecord> getChangeSince(ChangeRecord record)
    {
      List<ChangeRecord> changes = new ArrayList<ChangeRecord>();
      String fileName;
      long endOffset;
      if (record == null)
      {
        fileName = "log.1";
        endOffset = 0;
      } else
      {
        fileName = record.changeLogFileName;
        endOffset = record.endOffset;
      }
      try
      {
        lock.lock();
        
        File file;
        file = new File(changeLogDir + "/" + fileName);
        while (!file.exists() || file.length() <= endOffset)
        {
       // wait
          try
          {
            System.out.println("Waiting for new changes");
            condition.await();
            System.out.println("Detected changes");
          } catch (InterruptedException e)
          {
            e.printStackTrace();
          }
        }
        RandomAccessFile raf = new RandomAccessFile(
            changeLogDir + "/" + fileName, "r");
        raf.seek(endOffset);
        // out.writeLong(record.txid);
        // out.writeShort(record.type);
        // out.writeLong(record.timestamp);
        // out.writeUTF(record.file);
  
        int count = 0;
       do {
          ChangeRecord newRecord = new ChangeRecord();
          newRecord.changeLogFileName = fileName;
         newRecord.startOffset = raf.getFilePointer();
         newRecord.txid = raf.readLong();
         newRecord.type = raf.readShort();
         newRecord.timestamp = raf.readLong();
         newRecord.file = raf.readUTF();
         newRecord.endOffset = raf.getFilePointer();
         changes.add(newRecord);
         count++;
       }while (count < MAX_ENTRIES_TO_READ && raf.getFilePointer()< raf.length());
     } catch (FileNotFoundException e)
     {
       e.printStackTrace();
     } catch (IOException e)
     {
       e.printStackTrace();
     } finally
     {
       lock.unlock();
     }
     return changes;
   }
 
   @Override
   public void onEntryModified(String path)
   {
     try
     {
       lock.lock();
       condition.signalAll();
     } catch (Exception e)
     {
       // TODO: handle exception
     } finally
     {
       lock.unlock();
     }
   }
 
   @Override
   public void onEntryAdded(String path)
   {
     try
     {
       lock.lock();
       condition.signalAll();
     } catch (Exception e)
     {
       // TODO: handle exception
     } finally
     {
       lock.unlock();
     }
   }
 
   @Override
   public void onEntryDeleted(String path)
   {
     try
     {
       lock.lock();
       condition.signalAll();
     } catch (Exception e)
     {
       // TODO: handle exception
     } finally
     {
       lock.unlock();
     }
   }
 }