gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/MinGlobalStep.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.tinkerpop.gremlin.process.traversal.step.map;

import org.apache.tinkerpop.gremlin.process.computer.KeyValue;
import org.apache.tinkerpop.gremlin.process.computer.MapReduce;
import org.apache.tinkerpop.gremlin.process.computer.traversal.TraversalVertexProgram;
import org.apache.tinkerpop.gremlin.process.computer.util.StaticMapReduce;
import org.apache.tinkerpop.gremlin.process.traversal.Traversal;
import org.apache.tinkerpop.gremlin.process.traversal.Traverser;
import org.apache.tinkerpop.gremlin.process.traversal.step.MapReducer;
import org.apache.tinkerpop.gremlin.process.traversal.step.util.ReducingBarrierStep;
import org.apache.tinkerpop.gremlin.process.traversal.traverser.TraverserRequirement;
import org.apache.tinkerpop.gremlin.structure.Vertex;
import org.apache.tinkerpop.gremlin.util.function.ConstantSupplier;
import org.apache.tinkerpop.gremlin.util.iterator.IteratorUtils;

import java.io.Serializable;
import java.util.Collections;
import java.util.Iterator;
import java.util.Set;
import java.util.function.BiFunction;

import static org.apache.tinkerpop.gremlin.process.traversal.NumberHelper.min;

/**
 * @author Marko A. Rodriguez (http://markorodriguez.com)
 */
public final class MinGlobalStep<S extends Number> extends ReducingBarrierStep<S, S> implements MapReducer {

    public MinGlobalStep(final Traversal.Admin traversal) {
        super(traversal);
        this.setSeedSupplier(new ConstantSupplier<>(null));
        this.setBiFunction(MinGlobalBiFunction.instance());
    }

    @Override
    public Set<TraverserRequirement> getRequirements() {
        return Collections.singleton(TraverserRequirement.OBJECT);
    }

    @Override
    public MapReduce<MapReduce.NullObject, Number, MapReduce.NullObject, Number, Number> getMapReduce() {
        return MinGlobalMapReduce.instance();
    }

    /////

    private static class MinGlobalBiFunction<S extends Number> implements BiFunction<S, Traverser<S>, S>, Serializable {

        private static final MinGlobalBiFunction INSTANCE = new MinGlobalBiFunction();

        private MinGlobalBiFunction() {

        }

        @Override
        public S apply(final S mutatingSeed, final Traverser<S> traverser) {
            final S value = traverser.get();
            return mutatingSeed != null ? (S) min(mutatingSeed, traverser.get()) : value;
        }

        public static <S extends Number> MinGlobalBiFunction<S> instance() {
            return INSTANCE;
        }
    }

    ///////////

    private static class MinGlobalMapReduce extends StaticMapReduce<MapReduce.NullObject, Number, MapReduce.NullObject, Number, Number> {

        private static final MinGlobalMapReduce INSTANCE = new MinGlobalMapReduce();

        private MinGlobalMapReduce() {

        }

        @Override
        public boolean doStage(final MapReduce.Stage stage) {
            return true;
        }

        @Override
        public void map(final Vertex vertex, final MapEmitter<NullObject, Number> emitter) {
            final Iterator<Number> values = IteratorUtils.map(vertex.<Set<Traverser.Admin<Number>>>property(TraversalVertexProgram.HALTED_TRAVERSERS).orElse(Collections.emptySet()).iterator(), Traverser.Admin::get);
            if (values.hasNext())
                emitter.emit(getMin(values));
        }

        @Override
        public void combine(final NullObject key, final Iterator<Number> values, final ReduceEmitter<NullObject, Number> emitter) {
            this.reduce(key, values, emitter);
        }

        @Override
        public void reduce(final NullObject key, final Iterator<Number> values, final ReduceEmitter<NullObject, Number> emitter) {
            if (values.hasNext()) emitter.emit(getMin(values));
        }

        private Number getMin(final Iterator<Number> numbers) {
            Number min = null;
            while (numbers.hasNext()) {
                final Number value = numbers.next();
                min = min != null ? min(value, min) : value;
            }
            return min;
        }

        @Override
        public String getMemoryKey() {
            return REDUCING;
        }

        @Override
        public Number generateFinalResult(final Iterator<KeyValue<NullObject, Number>> keyValues) {
            return keyValues.hasNext() ? keyValues.next().getValue() : Double.NaN;
        }

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