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/*
 License:

 blueprint-sdk is licensed under the terms of Eclipse Public License(EPL) v1.0
 (http://www.eclipse.org/legal/epl-v10.html)


 Distribution:

 Maven Central - https://search.maven.org/artifact/io.github.lempel/blueprint-sdk
 MVN Repository - https://mvnrepository.com/artifact/io.github.lempel/blueprint-sdk
 */

package blueprint.sdk.util.queue;

import java.util.LinkedList;
import java.util.NoSuchElementException;
import java.util.UUID;

/**
 * Simple Message Queue.
 *
 * @author [email protected]
 * @since 2013. 9. 4.
 */
public class MessageQueue implements Queue {
    /**
     * actual queue
     */
    private final LinkedList queue = new LinkedList<>();

    /**
     * waiting consumers
     */
    private final LinkedList waiters = new LinkedList<>();

    @Override
    public void clear() {
        synchronized (queue) {
            queue.clear();
        }
    }

    @Override
    public void push(String element) {
        if (element == null) {
            throw new NullPointerException("Can't push null");
        }

        Element item = new Element();
        item.uuid = UUID.randomUUID().toString();
        item.content = element;

        synchronized (queue) {
            queue.push(item);
        }
        notifyWaiter();
    }

    /**
     * Retrieves an element from queue.
     *
     * @return queue element or null(queue is empty)
     */
    @SuppressWarnings("WeakerAccess")
    public String pop() {
        String result = null;

        try {
            Element element;
            synchronized (queue) {
                element = queue.pop();
            }

            if (element != null) {
                result = element.content;
            }
        } catch (NoSuchElementException ignored) {
            // just return null
        }

        return result;
    }

    @SuppressWarnings("SynchronizationOnLocalVariableOrMethodParameter")
    @Override
    public String take() {

        String result = pop();
        if (result == null) {
            Thread current = Thread.currentThread();
            synchronized (waiters) {
                waiters.add(current);
            }

            synchronized (current) {
                try {
                    current.wait();
                } catch (InterruptedException ignored) {
                }

                result = pop();
            }
        }

        return result;
    }

    /**
     * Wake up a blocked thread
     */
    @SuppressWarnings({"SynchronizationOnLocalVariableOrMethodParameter", "WeakerAccess"})
    protected void notifyWaiter() {
        try {
            Thread consumer;
            synchronized (waiters) {
                consumer = waiters.pop();
            }

            if (consumer != null) {
                synchronized (consumer) {
                    consumer.notifyAll();
                }
            }
        } catch (NoSuchElementException ignored) {
        }
    }

    /**
     * release all threads which is block by {@link MessageQueue#take()}
     */
    @SuppressWarnings("SynchronizationOnLocalVariableOrMethodParameter")
    public void release() {
        synchronized (waiters) {
            while (!waiters.isEmpty()) {
                Thread waiter = waiters.pop();
                if (waiter != null) {
                    synchronized (waiter) {
                        waiter.notifyAll();
                    }
                }
            }
        }
    }

    @Override
    public int size() {
        synchronized (queue) {
            return queue.size();
        }
    }
}

/**
 * Internal element of MessageQueue
 *
 * @author [email protected]
 * @since 2013. 8. 27.
 */
class Element {
    /**
     * UUID of content
     */
    public String uuid;
    /**
     * actual queue content
     */
    public String content;
}