/*

 * Copyright (c) 2017, 2018, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code 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 General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

import java.util.concurrent.atomic.AtomicLong;

/**

 * Encapsulates operations with demand (Reactive Streams).

 *

 * <p> Demand is the aggregated number of elements requested by a Subscriber

 * which is yet to be delivered (fulfilled) by the Publisher.

 */

public final class Demand {

    private final AtomicLong val = new AtomicLong();

    /**

     * Increases this demand by the specified positive value.

     *

     * @param n

     *         increment {@code > 0}

     *

     * @return {@code true} iff prior to this operation this demand was fulfilled

     */

    public boolean increase(long n) {

        if (n <= 0) {

            throw new IllegalArgumentException(String.valueOf(n));

        }

        long prev = val.getAndAccumulate(n, (p, i) -> p + i < 0 ? Long.MAX_VALUE : p + i);

        return prev == 0;

    }

    /**

     * Tries to decrease this demand by the specified positive value.

     *

     * <p> The actual value this demand has been decreased by might be less than

     * {@code n}, including {@code 0} (no decrease at all).

     *

     * @param n

     *         decrement {@code > 0}

     *

     * @return a value {@code m} ({@code 0 <= m <= n}) this demand has been

     *         actually decreased by

     */

    public long decreaseAndGet(long n) {

        if (n <= 0) {

            throw new IllegalArgumentException(String.valueOf(n));

        }

        long p, d;

        do {

            d = val.get();

            p = Math.min(d, n);

        } while (!val.compareAndSet(d, d - p));

        return p;

    }

    /**

     * Tries to decrease this demand by {@code 1}.

     *

     * @return {@code true} iff this demand has been decreased by {@code 1}

     */

    public boolean tryDecrement() {

        return decreaseAndGet(1) == 1;

    }

    /**

     * @return {@code true} iff there is no unfulfilled demand

     */

    public boolean isFulfilled() {

        return val.get() == 0;

    }

    /**

     * Resets this object to its initial state.

     */

    public void reset() {

        val.set(0);

    }

    /**

     * Returns the current value of this demand.

     *

     * @return the current value of this demand

     */

    public long get() {

        return val.get();

    }

    @Override

    public String toString() {

        return String.valueOf(val.get());

    }

}