jena-arq/src/main/java/org/apache/jena/sparql/algebra/op/OpSequence.java

/*
 * 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.jena.sparql.algebra.op;

import java.util.List ;

import org.apache.jena.sparql.algebra.Op ;
import org.apache.jena.sparql.algebra.OpVisitor ;
import org.apache.jena.sparql.algebra.Transform ;
import org.apache.jena.sparql.sse.Tags ;
import org.apache.jena.sparql.util.NodeIsomorphismMap ;

/** A "sequence" is a join-like operation where it is known that
 * the output of one step can be fed into the input of the next 
 * (that is, no scoping issues arise). */

public class OpSequence extends OpN
{
    public static OpSequence create() { return new OpSequence() ; } 
    
    public static Op create(Op left, Op right)
    { 
        // Avoid stages of nothing
        if ( left == null && right == null )
            return null ;
        // Avoid stages of one.
        if ( left == null )
            return right ;
        if ( right == null )
            return left ;
        // If left already an OpSequence ... maybe?
        if ( left instanceof OpSequence )
        {
            OpSequence opSequence = (OpSequence)left ;
            opSequence.add(right) ;
            return opSequence ; 
        }
//        if ( right instanceof OpSequence )
//        {
//            OpSequence opSequence = (OpSequence)right ;
//            // Add front.
//            opSequence.getElements().add(0, left) ;
//            return opSequence ; 
//        }
        
        OpSequence stage = new OpSequence() ;
        stage.add(left) ;
        stage.add(right) ;
        return stage ;
    }
    
    private OpSequence()           { super() ; }
    private OpSequence(List<Op> elts)  { super(elts) ; }
    
    @Override
    public String getName() { return Tags.tagSequence ; }

    @Override
    public void visit(OpVisitor opVisitor) { opVisitor.visit(this) ; }
    
    @Override
    public boolean equalTo(Op op, NodeIsomorphismMap labelMap)
    {
        if ( ! ( op instanceof OpSequence) ) return false ;
        OpSequence other = (OpSequence) op ;
        return super.equalsSubOps(other, labelMap) ;
    }

    @Override
    public Op apply(Transform transform, List<Op> elts)
    { return transform.transform(this, elts) ; }

    @Override
    public OpN copy(List<Op> elts)
    {
        return new OpSequence(elts) ; 
    }
}