content: /*
   * 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.
   */
  
  /*
   * 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><a href="package-summary.html#weakCompareAndSet">May fail
       * spuriously and does not provide ordering guarantees</a>, 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 {@code 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 {@code 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 {@code 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 U = sun.misc.Unsafe.getUnsafe();
      private static final long PAIR;
      static {
          try {
              PAIR = U.objectFieldOffset
                  (AtomicMarkableReference.class.getDeclaredField("pair"));
          } catch (ReflectiveOperationException e) {
              throw new Error(e);
          }
      }
  
      private boolean casPair(Pair<V> cmp, Pair<V> val) {
          return U.compareAndSwapObject(this, PAIR, cmp, val);
      }
  }
  