gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/strategy/optimization/RepeatUnrollStrategy.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.
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package org.apache.tinkerpop.gremlin.process.traversal.strategy.optimization;

import org.apache.tinkerpop.gremlin.process.traversal.Step;
import org.apache.tinkerpop.gremlin.process.traversal.Traversal;
import org.apache.tinkerpop.gremlin.process.traversal.TraversalStrategy;
import org.apache.tinkerpop.gremlin.process.traversal.lambda.LoopTraversal;
import org.apache.tinkerpop.gremlin.process.traversal.step.Barrier;
import org.apache.tinkerpop.gremlin.process.traversal.step.branch.RepeatStep;
import org.apache.tinkerpop.gremlin.process.traversal.step.map.NoOpBarrierStep;
import org.apache.tinkerpop.gremlin.process.traversal.strategy.AbstractTraversalStrategy;
import org.apache.tinkerpop.gremlin.process.traversal.util.TraversalHelper;

/**
 * @author Marko A. Rodriguez (http://markorodriguez.com)
 */
public final class RepeatUnrollStrategy extends AbstractTraversalStrategy<TraversalStrategy.OptimizationStrategy> implements TraversalStrategy.OptimizationStrategy {

    private static final RepeatUnrollStrategy INSTANCE = new RepeatUnrollStrategy();

    private RepeatUnrollStrategy() {
    }

    @Override
    public void apply(final Traversal.Admin<?, ?> traversal) {
        if (TraversalHelper.onGraphComputer(traversal))
            return;

        for (int i = 0; i < traversal.getSteps().size(); i++) {
            if (traversal.getSteps().get(i) instanceof RepeatStep) {
                final RepeatStep<?> repeatStep = (RepeatStep) traversal.getSteps().get(i);
                if (null == repeatStep.getEmitTraversal() && repeatStep.getUntilTraversal() instanceof LoopTraversal && ((LoopTraversal) repeatStep.getUntilTraversal()).getMaxLoops() > 0) {
                    final Traversal.Admin<?, ?> repeatTraversal = repeatStep.getGlobalChildren().get(0);
                    final int repeatLength = repeatTraversal.getSteps().size() - 1;
                    repeatTraversal.removeStep(repeatLength); // removes the RepeatEndStep
                    int insertIndex = i;
                    final int loops = (int) ((LoopTraversal) repeatStep.getUntilTraversal()).getMaxLoops();
                    for (int j = 0; j < loops; j++) {
                        TraversalHelper.insertTraversal(insertIndex, repeatTraversal.clone(), traversal);
                        insertIndex = insertIndex + repeatLength;
                        if (j != (loops - 1) || !(traversal.getSteps().get(insertIndex).getNextStep() instanceof Barrier)) // only add a final NoOpBarrier is subsequent step is not a barrier
                            traversal.addStep(++insertIndex, new NoOpBarrierStep<>(traversal, 5000));
                    }
                    // label last step if repeat() was labeled
                    if (!repeatStep.getLabels().isEmpty()) {
                        final Step<?, ?> lastStep = traversal.getSteps().get(insertIndex);
                        repeatStep.getLabels().forEach(lastStep::addLabel);
                    }

                    traversal.removeStep(i); // remove the RepeatStep
                }
            }
        }
    }


    public static RepeatUnrollStrategy instance() {
        return INSTANCE;
    }
}