/*
 *  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 java.util.NoSuchElementException;

import javax.naming.NamingEnumeration;
import javax.naming.NamingException;

import org.apache.directory.server.core.partition.impl.btree.Tuple;
import org.apache.directory.shared.ldap.exception.LdapNamingException;
import org.apache.directory.shared.ldap.message.ResultCodeEnum;

import jdbm.btree.BTree;
import jdbm.helper.TupleBrowser;


/**
 * A NamingEnumeration that returns underlying keys in a BTree as values for a 
 * single key within a Tuple.  This is used specifically for situations when tables
 * support duplicate keys and a BTree is used for storing the values for that
 * key.  This enumeration thus advances a browser forwards or reverse returning
 * keys from a BTree as Tuple values.  The key provided in the constructor is always
 * returned as the key of the Tuple.
 * 
 * <p>
 * WARNING: The tuple returned is reused every time for efficiency and populated
 * a over and over again with the new value.  The key never changes.
 * </p>
 *
 * @author <a href="mailto:dev@directory.apache.org">Apache Directory Project</a>
 * @version $Rev$
 */
public class BTreeTupleEnumeration implements NamingEnumeration
{
    private final Object key;
    private final Tuple tuple = new Tuple();
    private final jdbm.helper.Tuple jdbmTuple = new jdbm.helper.Tuple();
    private final boolean stepForward;
    private TupleBrowser browser;
    private boolean success = false;
    
    
    BTreeTupleEnumeration( BTree tree, Comparator<Object> comparator, Object key, Object val, boolean isGreaterThan ) 
        throws LdapNamingException 
    {
        this.key = key;
        stepForward = isGreaterThan;
        
        try
        {
            browser = tree.browse( val );
            
            if ( ! isGreaterThan )
            {
                boolean gotNextOk = browser.getNext( jdbmTuple );
                if ( gotNextOk )
                {
                    Object nextVal = jdbmTuple.getKey();
                    int compared = comparator.compare( val, nextVal );
                    if ( compared != 0 )
                    {
                        browser.getPrevious( jdbmTuple );
                    }
                }
            }
            
            prefetch();
        }
        catch ( IOException e )
        {
            LdapNamingException lne = new LdapNamingException( "Failure on btree: " + e.getMessage(), 
                ResultCodeEnum.OTHER );
            lne.setRootCause( e );
            throw lne;
        }
    }

    
    BTreeTupleEnumeration( BTree tree, Object key ) throws NamingException 
    {
        this.key = key;
        stepForward = true;
        
        try
        {
            browser = tree.browse();
            prefetch();
        }
        catch ( IOException e )
        {
            LdapNamingException lne = new LdapNamingException( "Failure on btree: " + e.getMessage(), 
                ResultCodeEnum.OTHER );
            lne.setRootCause( e );
            throw lne;
        }
    }

    
    private void prefetch() throws IOException
    {
        if ( stepForward )
        {
            success = browser.getNext( jdbmTuple );
        }
        else
        {
            success = browser.getPrevious( jdbmTuple );
        }
    }
    
    
    public void close() throws NamingException
    {
        success = false;
    }


    public boolean hasMore() throws NamingException
    {
        return success;
    }


    public Object next() throws NamingException
    {
        if ( ! success )
        {
            throw new NoSuchElementException();
        }
        
        tuple.setKey( key );
        tuple.setValue( jdbmTuple.getKey() );
        try
        {
            prefetch();
        }
        catch ( IOException e )
        {
            LdapNamingException lne = new LdapNamingException( "Failure on btree: " + e.getMessage(), 
                ResultCodeEnum.OTHER );
            lne.setRootCause( e );
            throw lne;
        }
        return tuple;
    }


    public boolean hasMoreElements()
    {
        return success;
    }


    public Object nextElement()
    {
        try
        {
            return next();
        }
        catch ( NamingException e )
        {
            e.printStackTrace();
            throw new NoSuchElementException( "Got IO Failure on btree: " + e.getCause().getMessage() );
        }
    }
}