001/*
002 *  Licensed to the Apache Software Foundation (ASF) under one
003 *  or more contributor license agreements.  See the NOTICE file
004 *  distributed with this work for additional information
005 *  regarding copyright ownership.  The ASF licenses this file
006 *  to you under the Apache License, Version 2.0 (the
007 *  "License"); you may not use this file except in compliance
008 *  with the License.  You may obtain a copy of the License at
009 *  
010 *    http://www.apache.org/licenses/LICENSE-2.0
011 *  
012 *  Unless required by applicable law or agreed to in writing,
013 *  software distributed under the License is distributed on an
014 *  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
015 *  KIND, either express or implied.  See the License for the
016 *  specific language governing permissions and limitations
017 *  under the License. 
018 *  
019 */
020package org.apache.directory.server.xdbm.search.impl;
021
022
023import java.util.Comparator;
024
025import org.apache.directory.server.xdbm.search.Evaluator;
026
027
028/**
029 * A helper class used to compare scan counts.
030 * 
031 * @author <a href="mailto:dev@directory.apache.org">Apache Directory Project</a>
032 */
033public class ScanCountComparator implements Comparator<Evaluator<?>>
034{
035    /**
036     * Compare two scan counts frpm two evaluators 
037     */
038    public int compare( Evaluator<?> e1, Evaluator<?> e2 )
039    {
040        Object count1 = e1.getExpression().get( DefaultOptimizer.COUNT_ANNOTATION );
041        Object count2 = e2.getExpression().get( DefaultOptimizer.COUNT_ANNOTATION );
042        long scanCount1 = Long.MAX_VALUE;
043        long scanCount2 = Long.MAX_VALUE;
044
045        if ( count1 != null )
046        {
047            scanCount1 = ( Long ) e1.getExpression().get( DefaultOptimizer.COUNT_ANNOTATION );
048        }
049
050        if ( count2 != null )
051        {
052            scanCount2 = ( Long ) e2.getExpression().get( DefaultOptimizer.COUNT_ANNOTATION );
053        }
054
055        if ( scanCount1 == scanCount2 )
056        {
057            return 0;
058        }
059
060        /*
061         * We want the Evaluator with the smallest scan count first
062         * since this node has the highest probability of failing, or
063         * rather the least probability of succeeding.  That way we
064         * can short the sub-expression evaluation process.
065         */
066        if ( scanCount1 < scanCount2 )
067        {
068            return -1;
069        }
070
071        return 1;
072    }
073}