gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/Order.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;

import java.util.Comparator;

import org.apache.tinkerpop.gremlin.util.GremlinValueComparator;

/**
 * Provides {@code Comparator} instances for ordering traversers.
 *
 * @author Marko A. Rodriguez (http://markorodriguez.com)
 * @author Stephen Mallette (http://stephen.genoprime.com)
 */
public enum Order implements Comparator<Object> {

    /**
     * Order in a random fashion. While this enum implements {@code Comparator}, the {@code compare(a,b)} method is not
     * supported as a direct call. This change to the implementation of {@code compare(a,b)} occurred at 3.5.0 but
     * this implementation was never used directly within the TinkerPop code base.
     *
     * @since 3.0.0-incubating
     */
    shuffle {
        @Override
        public int compare(final Object first, final Object second) {
            throw new UnsupportedOperationException("Order.shuffle should not be used as an actual Comparator - it is a marker only");
        }

        @Override
        public Order reversed() {
            return shuffle;
        }
    },

    /**
     * Order in ascending fashion.
     *
     * @since 3.3.4
     */
    asc {
        @Override
        public int compare(final Object first, final Object second) {
            return ORDER.compare(first, second);
        }

        @Override
        public Order reversed() {
            return desc;
        }
    },

    /**
     * Order in descending fashion.
     *
     * @since 3.3.4
     */
    desc {
        @Override
        public int compare(final Object first, final Object second) {
            return ORDER.compare(second, first);
        }

        @Override
        public Order reversed() {
            return asc;
        }
    };

    private static final Comparator<Object> ORDER = Comparator.comparing(
            // first transform (strip Traverser layer and convert Enums)
            Order::transform, GremlinValueComparator.ORDERABILITY);

    /**
     * Strip the Traverser layer and convert Enum to string.
     */
    private static Object transform(Object o) {
        // we want to sort the underlying object contained by the traverser
        if (o instanceof Traverser)
            o = ((Traverser) o).get();
        // need to convert enum to string representations for comparison or else you can get cast exceptions.
        // this typically happens when sorting local on the keys of maps that contain T
        if (o instanceof Enum)
            o = ((Enum) o).name();
        return o;
    }

    /**
     * {@inheritDoc}
     */
    public abstract int compare(final Object first, final Object second);

    /**
     * Produce the opposite representation of the current {@code Order} enum.
     */
    @Override
    public abstract Order reversed();
}