/*
    * 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.directory.server.core.partition.impl.btree.jdbm;
  
  
  import java.io.IOException;
  import java.util.Comparator;
  
  import jdbm.btree.BTree;
  import jdbm.helper.Tuple;
  import jdbm.helper.TupleBrowser;
  
  import org.apache.directory.api.ldap.model.constants.Loggers;
  import org.apache.directory.api.ldap.model.cursor.AbstractCursor;
  import org.apache.directory.api.ldap.model.cursor.CursorException;
  import org.apache.directory.api.ldap.model.cursor.InvalidCursorPositionException;
  import org.apache.directory.api.ldap.model.exception.LdapException;
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  
  /**
   * Cursor over the keys of a JDBM BTree.  Obviously does not return duplicate
   * keys since JDBM does not natively support multiple values for the same key.
   *
   * @author <a href="mailto:dev@directory.apache.org">Apache Directory Project</a>
   */
  public class KeyBTreeCursor<E> extends AbstractCursor<E>
  {
      /** A dedicated log for cursors */
      private static final Logger LOG_CURSOR = LoggerFactory.getLogger( Loggers.CURSOR_LOG.getName() );
  
      /** Speedup for logs */
      private static final boolean IS_DEBUG = LOG_CURSOR.isDebugEnabled();
  
      private final Tuple tuple = new Tuple();
  
      private final BTree btree;
      private final Comparator<E> comparator;
      private boolean valueAvailable;
      private TupleBrowser browser;
  
  
      /**
       * Creates a Cursor over the keys of a JDBM BTree.
       *
       * @param btree the JDBM BTree to build a Cursor over
       * @param comparator the Comparator used to determine key ordering
       */
      public KeyBTreeCursor( BTree btree, Comparator<E> comparator )
      {
          if ( IS_DEBUG )
          {
              LOG_CURSOR.debug( "Creating KeyBTreeCursor {}", this );
          }
  
          this.btree = btree;
          this.comparator = comparator;
      }
  
  
      private void clearValue()
      {
          tuple.setKey( null );
          tuple.setValue( null );
          valueAvailable = false;
      }
  
  
      public boolean available()
      {
          return valueAvailable;
      }
  
  
      /**
       * {@inheritDoc}
       */
      public void before( E element ) throws LdapException, CursorException
      {
          checkNotClosed();
          try
          {
             browser = btree.browse( element );
         }
         catch ( IOException e )
         {
             throw new CursorException( e );
         }
         clearValue();
     }
 
 
     /**
      * {@inheritDoc}
      */
     @SuppressWarnings("unchecked")
     public void after( E element ) throws LdapException, CursorException
     {
         try
         {
             browser = btree.browse( element );
 
             /*
              * While the next value is less than or equal to the element keep
              * advancing forward to the next item.  If we cannot advance any
              * further then stop and return.  If we find a value greater than
              * the element then we stop, backup, and return so subsequent calls
              * to getNext() will return a value greater than the element.
              */
             while ( browser.getNext( tuple ) )
             {
                 checkNotClosed();
                 E next = ( E ) tuple.getKey();
                 int nextCompared = comparator.compare( next, element );
 
                 if ( nextCompared > 0 )
                 {
                     /*
                      * If we just have values greater than the element argument
                      * then we are before the first element and must backup to
                      * before the first element state for the JDBM browser which 
                      * apparently the browser supports.
                      */
                     browser.getPrevious( tuple );
                     clearValue();
                     return;
                 }
             }
 
             clearValue();
             // just return
         }
         catch ( IOException e )
         {
             throw new CursorException( e );
         }
     }
 
 
     /**
      * {@inheritDoc}
      */
     public void beforeFirst() throws LdapException, CursorException
     {
         checkNotClosed();
         try
         {
             browser = btree.browse();
             clearValue();
         }
         catch ( IOException e )
         {
             throw new CursorException( e );
         }
     }
 
 
     /**
      * {@inheritDoc}
      */
     public void afterLast() throws LdapException, CursorException
     {
         checkNotClosed();
         try
         {
             browser = btree.browse( null );
         }
         catch ( IOException e )
         {
             throw new CursorException( e );
         }
     }
 
 
     /**
      * {@inheritDoc}
      */
     public boolean first() throws LdapException, CursorException
     {
         beforeFirst();
         return next();
     }
 
 
     /**
      * {@inheritDoc}
      */
     public boolean last() throws LdapException, CursorException
     {
         afterLast();
         return previous();
     }
 
 
     /**
      * {@inheritDoc}
      */
     public boolean previous() throws LdapException, CursorException
     {
         checkNotClosed();
 
         try
         {
             if ( browser == null )
             {
                 browser = btree.browse( null );
             }
 
             if ( browser.getPrevious( tuple ) )
             {
                 valueAvailable = true;
                 return true;
             }
             else
             {
                 clearValue();
                 return false;
             }
         }
         catch ( IOException e )
         {
             throw new CursorException( e );
         }
     }
 
 
     /**
      * {@inheritDoc}
      */
     public boolean next() throws LdapException, CursorException
     {
         checkNotClosed();
 
         try
         {
             if ( browser == null )
             {
                 browser = btree.browse();
             }
 
             if ( browser.getNext( tuple ) )
             {
                 valueAvailable = true;
                 return true;
             }
             else
             {
                 clearValue();
 
                 return false;
             }
         }
         catch ( IOException e )
         {
             throw new CursorException( e );
         }
     }
 
 
     /**
      * {@inheritDoc}
      */
     public E get() throws CursorException
     {
         checkNotClosed();
 
         if ( valueAvailable )
         {
             return ( E ) tuple.getKey();
         }
 
         throw new InvalidCursorPositionException();
     }
 
 
     /**
      * {@inheritDoc}
      */
     @Override
     public void close() throws IOException
     {
         if ( IS_DEBUG )
         {
             LOG_CURSOR.debug( "Closing KeyBTreeCursor {}", this );
         }
 
         super.close();
     }
 
 
     /**
      * {@inheritDoc}
      */
     @Override
     public void close( Exception cause ) throws IOException
     {
         if ( IS_DEBUG )
         {
             LOG_CURSOR.debug( "Closing KeyBTreeCursor {}", this );
         }
 
         super.close( cause );
     }
 }