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The DSI utilities are a mishmash of classes accumulated during the last twenty years in projects developed at the DSI (Dipartimento di Scienze dell'Informazione, i.e., Information Sciences Department), now DI (Dipartimento di Informatica, i.e., Informatics Department), of the Universita` degli Studi di Milano.
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package it.unimi.dsi.util.concurrent;

/*
 * DSI utilities
 *
 * Copyright (C) 2017-2019 Sebastiano Vigna
 *
 *  This library is free software; you can redistribute it and/or modify it
 *  under the terms of the GNU Lesser General Public License as published by the Free
 *  Software Foundation; either version 3 of the License, or (at your option)
 *  any later version.
 *
 *  This library is distributed in the hope that it will be useful, but
 *  WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 *  or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License
 *  for more details.
 *
 *  You should have received a copy of the GNU Lesser General Public License
 *  along with this program; if not, see .
 *
 */

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

import it.unimi.dsi.fastutil.HashCommon;

/** A blocking queue holding a fixed amount of timestamped items. A typical use
 * case is that of multiple threads analyzing an input divided in record, one record per
 * thread, and generating some output that must be written preserving the input order. The
 * threads {@linkplain #put(Object, long) enqueue} their output to an instance of this class,
 * and a flushing thread {@linkplain #take() dequeues} it in input order.
 *
 * The {@link #put(Object, long)}
 * must be called with an object and a timestamp. Timestamps must be a contiguous interval of the
 * natural numbers starting at zero, and objects will be returned in timestamp order. Failure to
 * comply with the contract (i.e., missing timestamps) will cause the queue to block forever.
 *
 * 
{@link #put(Object, long)} might block if there is not enough space to keep track of the
 * object (i.e., if its timestamp is too far in time w.r.t. the timestamp that would be
 * returned next by the queue). {@link #take()} might block if the object with the next
 * timestamp has not been {@link #put(Object, long)} yet.
 *
 * 
The implementation is based on a circular, fixed-size buffer, so
 * all methods of this class complete in constant time.
 */
public class ReorderingBlockingQueue {
    /** The backing array. */
    private final Object[] a;
    /** The length of {@link #a} minus one, cached. */
    private final int mask;
    /** The current position into {@link #a} (the position of the next object to be returned). */
    private int start;
    /** The timestamp of the next object to be returned. */
    private long timeStamp;
    /** The number of elements in the queue. */
    private int count;
    /** The main lock. */
    private final ReentrantLock lock;
    /** Condition for waiting takes. */
    private final Condition nextObjectReady;
    /** Condition for waiting puts. */
    private final Condition newSpaceAvailable;

    /** Creates a {@code ReorderingBlockingQueue} with the given fixed
     * capacity.
     *
     * @param capacity the capacity of this queue (will be rounded to the next power of two).
     */
	public ReorderingBlockingQueue(final int capacity) {
		if (capacity <= 0) throw new IllegalArgumentException();
		a = new Object[HashCommon.nextPowerOfTwo(capacity)];
		mask = a.length - 1;
		lock = new ReentrantLock(false);
		nextObjectReady = lock.newCondition();
		newSpaceAvailable = lock.newCondition();
	}

    /** Inserts an element with given timestamp, waiting for space to become available
     * if the timestamp of the element minus the current timestamp of the queue exceeds
     * the queue capacity.
     *
     * @param e an element.
     * @param timeStamp the timestamp of {@code e}.
     * @throws NullPointerException if {@code e} is null;
     */
	public void put(final E e, final long timeStamp) throws InterruptedException {
		if (e == null) throw new NullPointerException();
		final ReentrantLock lock = this.lock;
		lock.lockInterruptibly();
		try {
			// mask is a.length - 1
			while(timeStamp - this.timeStamp > mask) newSpaceAvailable.await();
			final int timeOffset = (int)(timeStamp - this.timeStamp);
			assert a[start + timeOffset & mask] == null : a[start + timeOffset & mask];
			a[start + timeOffset & mask] = e;
			++count;
			if (timeOffset == 0) nextObjectReady.signal();
		}
		finally {
			lock.unlock();
		}
	}

    /** Returns the element with the next timestamp, waiting until it is available.
     *
     * Note that because of the reordering semantics, an invocation of this method
     * on a {@linkplain #isEmpty() nonempty} queue might block nonetheless.
     *
     * @return the element with the next timestamp.
     */
	public E take() throws InterruptedException {
		final ReentrantLock lock = this.lock;
		lock.lockInterruptibly();
		try {
			while (a[start] == null) nextObjectReady.await();
			@SuppressWarnings("unchecked")
			final E x = (E)a[start];
			a[start] = null;
			start = start + 1 & mask;
			--count;
			timeStamp++;
			newSpaceAvailable.signalAll();
			return x;
		}
		finally {
			lock.unlock();
		}
	}

    /** Returns the number of elements in this queue.
     *
     * @return the number of elements in this queue
     * @see #isEmpty()
     */
    public int size() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return count;
        } finally {
            lock.unlock();
        }
    }

    /** Returns whether this queue is empty.
     *
     * @return whether this queue is empty.
     */
    public boolean isEmpty() {
    	return size() == 0;
    }
}