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/*
 * 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.server.op;

import org.apache.tinkerpop.gremlin.server.OpProcessor;
import org.apache.tinkerpop.gremlin.server.Settings;
import org.apache.tinkerpop.gremlin.server.op.standard.StandardOpProcessor;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.Optional;
import java.util.ServiceLoader;

/**
 * Uses {@link ServiceLoader} to load {@link OpProcessor} instances into a cache.
 *
 * @author Stephen Mallette (http://stephen.genoprime.com)
 */
public final class OpLoader {
    private static final Logger logger = LoggerFactory.getLogger(OpLoader.class);

    private static final Map processors = new HashMap<>();

    static {
        ServiceLoader.load(OpProcessor.class).forEach(op -> {
            final String name = op.getName();
            logger.info("Adding the {} OpProcessor.", name.equals(StandardOpProcessor.OP_PROCESSOR_NAME) ? "standard" : name);
            if (processors.containsKey(name))
                throw new RuntimeException(String.format("There is a naming conflict with the %s OpProcessor implementations.", name));

            processors.put(name, op);
        });
    }

    private static volatile boolean initialized = false;

    /**
     * Initialize the {@code OpLoader} with server settings. This method should only be called once at startup but is
     * designed to be idempotent.
     */
    public static synchronized void init(final Settings settings) {
        if (!initialized) {
            processors.values().forEach(processor -> processor.init(settings));
            initialized = true;
        }
    }

    /**
     * Gets an {@link OpProcessor} by its name. If it cannot be found an {@link Optional#empty()} is returned.
     */
    public static Optional getProcessor(final String name) {
        return Optional.ofNullable(processors.get(name));
    }

    /**
     * Gets a read-only map of the processors where the key is the {@link OpProcessor} name and the value is the
     * instance created by {@link ServiceLoader}.
     */
    public static Map getProcessors() {
        return Collections.unmodifiableMap(processors);
    }

    /**
     * Reset the processors so that they can be re-initialized with different settings which is useful in testing
     * scenarios.
     */
    public synchronized static void reset() {
        initialized = false;
    }
}