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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.codelibs.elasticsearch.taste.common;

import java.util.Collection;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.locks.ReentrantLock;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import com.google.common.collect.Lists;
import com.google.common.collect.Sets;

/**
 * A helper class for implementing {@link Refreshable}. This object is typically included in an implementation
 * {@link Refreshable} to implement {@link Refreshable#refresh(Collection)}. It execute the class's own
 * supplied update logic, after updating all the object's dependencies. This also ensures that dependencies
 * are not updated multiple times.
 */
public final class RefreshHelper implements Refreshable {

    private static final Logger log = LoggerFactory
            .getLogger(RefreshHelper.class);

    private final List dependencies;

    private final ReentrantLock refreshLock;

    private final Callable refreshRunnable;

    /**
     * @param refreshRunnable
     *          encapsulates the containing object's own refresh logic
     */
    public RefreshHelper(final Callable refreshRunnable) {
        dependencies = Lists.newArrayListWithCapacity(3);
        refreshLock = new ReentrantLock();
        this.refreshRunnable = refreshRunnable;
    }

    /** Add a dependency to be refreshed first when the encapsulating object does. */
    public void addDependency(final Refreshable refreshable) {
        if (refreshable != null) {
            dependencies.add(refreshable);
        }
    }

    public void removeDependency(final Refreshable refreshable) {
        if (refreshable != null) {
            dependencies.remove(refreshable);
        }
    }

    /**
     * Typically this is called in {@link Refreshable#refresh(java.util.Collection)} and is the entire body of
     * that method.
     */
    @Override
    public void refresh(Collection alreadyRefreshed) {
        if (refreshLock.tryLock()) {
            try {
                alreadyRefreshed = buildRefreshed(alreadyRefreshed);
                for (final Refreshable dependency : dependencies) {
                    maybeRefresh(alreadyRefreshed, dependency);
                }
                if (refreshRunnable != null) {
                    try {
                        refreshRunnable.call();
                    } catch (final Exception e) {
                        log.warn("Unexpected exception while refreshing", e);
                    }
                }
            } finally {
                refreshLock.unlock();
            }
        }
    }

    /**
     * Creates a new and empty {@link Collection} if the method parameter is {@code null}.
     *
     * @param currentAlreadyRefreshed
     *          {@link Refreshable}s to refresh later on
     * @return an empty {@link Collection} if the method param was {@code null} or the unmodified method
     *         param.
     */
    public static Collection buildRefreshed(
            final Collection currentAlreadyRefreshed) {
        return currentAlreadyRefreshed == null ? Sets
                . newHashSetWithExpectedSize(3)
                : currentAlreadyRefreshed;
    }

    /**
     * Adds the specified {@link Refreshable} to the given collection of {@link Refreshable}s if it is not
     * already there and immediately refreshes it.
     *
     * @param alreadyRefreshed
     *          the collection of {@link Refreshable}s
     * @param refreshable
     *          the {@link Refreshable} to potentially add and refresh
     */
    public static void maybeRefresh(
            final Collection alreadyRefreshed,
            final Refreshable refreshable) {
        if (!alreadyRefreshed.contains(refreshable)) {
            alreadyRefreshed.add(refreshable);
            log.info("Added refreshable: {}", refreshable);
            refreshable.refresh(alreadyRefreshed);
            log.info("Refreshed: {}", alreadyRefreshed);
        }
    }
}