/*

 * 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

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/*

 * This file is available under and governed by the GNU General Public

 * License version 2 only, as published by the Free Software Foundation.

 * However, the following notice accompanied the original version of this

 * file:

 *

 * Written by Doug Lea with assistance from members of JCP JSR-166

 * Expert Group and released to the public domain, as explained at

 * http://creativecommons.org/publicdomain/zero/1.0/

 */

package java.util.concurrent.atomic;

/**

 * An {@code AtomicMarkableReference} maintains an object reference

 * along with a mark bit, that can be updated atomically.

 *

 * <p>Implementation note: This implementation maintains markable

 * references by creating internal objects representing "boxed"

 * [reference, boolean] pairs.

 *

 * @since 1.5

 * @author Doug Lea

 * @param <V> The type of object referred to by this reference

 */

public class AtomicMarkableReference<V> {

    private static class Pair<T> {

        final T reference;

        final boolean mark;

        private Pair(T reference, boolean mark) {

            this.reference = reference;

            this.mark = mark;

        }

        static <T> Pair<T> of(T reference, boolean mark) {

            return new Pair<T>(reference, mark);

        }

    }

    private volatile Pair<V> pair;

    /**

     * Creates a new {@code AtomicMarkableReference} with the given

     * initial values.

     *

     * @param initialRef the initial reference

     * @param initialMark the initial mark

     */

    public AtomicMarkableReference(V initialRef, boolean initialMark) {

        pair = Pair.of(initialRef, initialMark);

    }

    /**

     * Returns the current value of the reference.

     *

     * @return the current value of the reference

     */

    public V getReference() {

        return pair.reference;

    }

    /**

     * Returns the current value of the mark.

     *

     * @return the current value of the mark

     */

    public boolean isMarked() {

        return pair.mark;

    }

    /**

     * Returns the current values of both the reference and the mark.

     * Typical usage is {@code boolean[1] holder; ref = v.get(holder); }.

     *

     * @param markHolder an array of size of at least one. On return,

     * {@code markholder[0]} will hold the value of the mark.

     * @return the current value of the reference

     */

    public V get(boolean[] markHolder) {

        Pair<V> pair = this.pair;

        markHolder[0] = pair.mark;

        return pair.reference;

    }

    /**

     * Atomically sets the value of both the reference and mark

     * to the given update values if the

     * current reference is {@code ==} to the expected reference

     * and the current mark is equal to the expected mark.

     *

     * <p>May <a href="package-summary.html#Spurious">fail spuriously</a>

     * and does not provide ordering guarantees, so is only rarely an

     * appropriate alternative to {@code compareAndSet}.

     *

     * @param expectedReference the expected value of the reference

     * @param newReference the new value for the reference

     * @param expectedMark the expected value of the mark

     * @param newMark the new value for the mark

     * @return true if successful

     */

    public boolean weakCompareAndSet(V       expectedReference,

                                     V       newReference,

                                     boolean expectedMark,

                                     boolean newMark) {

        return compareAndSet(expectedReference, newReference,

                             expectedMark, newMark);

    }

    /**

     * Atomically sets the value of both the reference and mark

     * to the given update values if the

     * current reference is {@code ==} to the expected reference

     * and the current mark is equal to the expected mark.

     *

     * @param expectedReference the expected value of the reference

     * @param newReference the new value for the reference

     * @param expectedMark the expected value of the mark

     * @param newMark the new value for the mark

     * @return true if successful

     */

    public boolean compareAndSet(V       expectedReference,

                                 V       newReference,

                                 boolean expectedMark,

                                 boolean newMark) {

        Pair<V> current = pair;

        return

            expectedReference == current.reference &&

            expectedMark == current.mark &&

            ((newReference == current.reference &&

              newMark == current.mark) ||

             casPair(current, Pair.of(newReference, newMark)));

    }

    /**

     * Unconditionally sets the value of both the reference and mark.

     *

     * @param newReference the new value for the reference

     * @param newMark the new value for the mark

     */

    public void set(V newReference, boolean newMark) {

        Pair<V> current = pair;

        if (newReference != current.reference || newMark != current.mark)

            this.pair = Pair.of(newReference, newMark);

    }

    /**

     * Atomically sets the value of the mark to the given update value

     * if the current reference is {@code ==} to the expected

     * reference.  Any given invocation of this operation may fail

     * (return {@code false}) spuriously, but repeated invocation

     * when the current value holds the expected value and no other

     * thread is also attempting to set the value will eventually

     * succeed.

     *

     * @param expectedReference the expected value of the reference

     * @param newMark the new value for the mark

     * @return true if successful

     */

    public boolean attemptMark(V expectedReference, boolean newMark) {

        Pair<V> current = pair;

        return

            expectedReference == current.reference &&

            (newMark == current.mark ||

             casPair(current, Pair.of(expectedReference, newMark)));

    }

    // Unsafe mechanics

    private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();

    private static final long pairOffset =

        objectFieldOffset(UNSAFE, "pair", AtomicMarkableReference.class);

    private boolean casPair(Pair<V> cmp, Pair<V> val) {

        return UNSAFE.compareAndSwapObject(this, pairOffset, cmp, val);

    }

    static long objectFieldOffset(sun.misc.Unsafe UNSAFE,

                                  String field, Class<?> klazz) {

        try {

            return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));

        } catch (NoSuchFieldException e) {

            // Convert Exception to corresponding Error

            NoSuchFieldError error = new NoSuchFieldError(field);

            error.initCause(e);

            throw error;

        }

    }

}