text: /*
   * 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;
  
  import java.lang.invoke.MethodHandles;
  import java.lang.invoke.VarHandle;
  
  /**
   * An {@code AtomicStampedReference} maintains an object reference
   * along with an integer "stamp", that can be updated atomically.
   *
   * <p>Implementation note: This implementation maintains stamped
   * references by creating internal objects representing "boxed"
   * [reference, integer] pairs.
   *
   * @since 1.5
   * @author Doug Lea
   * @param <V> The type of object referred to by this reference
   */
  public class AtomicStampedReference<V> {
  
      private static class Pair<T> {
          final T reference;
          final int stamp;
          private Pair(T reference, int stamp) {
              this.reference = reference;
              this.stamp = stamp;
          }
          static <T> Pair<T> of(T reference, int stamp) {
              return new Pair<T>(reference, stamp);
          }
      }
  
      private volatile Pair<V> pair;
  
      /**
       * Creates a new {@code AtomicStampedReference} with the given
       * initial values.
       *
       * @param initialRef the initial reference
       * @param initialStamp the initial stamp
       */
      public AtomicStampedReference(V initialRef, int initialStamp) {
          pair = Pair.of(initialRef, initialStamp);
      }
  
      /**
       * 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 stamp.
       *
       * @return the current value of the stamp
       */
      public int getStamp() {
          return pair.stamp;
      }
  
      /**
       * Returns the current values of both the reference and the stamp.
       * Typical usage is {@code int[1] holder; ref = v.get(holder); }.
       *
       * @param stampHolder an array of size of at least one.  On return,
       * {@code stampHolder[0]} will hold the value of the stamp.
       * @return the current value of the reference
       */
      public V get(int[] stampHolder) {
          Pair<V> pair = this.pair;
          stampHolder[0] = pair.stamp;
          return pair.reference;
      }
  
      /**
       * Atomically sets the value of both the reference and stamp to
       * the given update values if the current reference is {@code ==}
       * to the expected reference and the current stamp is equal to the
       * expected stamp. This operation may fail spuriously 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 expectedStamp the expected value of the stamp
       * @param newStamp the new value for the stamp
       * @return {@code true} if successful
       */
      public boolean weakCompareAndSet(V   expectedReference,
                                       V   newReference,
                                       int expectedStamp,
                                       int newStamp) {
          return compareAndSet(expectedReference, newReference,
                               expectedStamp, newStamp);
      }
  
      /**
       * Atomically sets the value of both the reference and stamp
       * to the given update values if the
       * current reference is {@code ==} to the expected reference
       * and the current stamp is equal to the expected stamp.
       *
       * @param expectedReference the expected value of the reference
       * @param newReference the new value for the reference
       * @param expectedStamp the expected value of the stamp
       * @param newStamp the new value for the stamp
       * @return {@code true} if successful
       */
      public boolean compareAndSet(V   expectedReference,
                                   V   newReference,
                                   int expectedStamp,
                                   int newStamp) {
          Pair<V> current = pair;
          return
              expectedReference == current.reference &&
              expectedStamp == current.stamp &&
              ((newReference == current.reference &&
                newStamp == current.stamp) ||
               casPair(current, Pair.of(newReference, newStamp)));
      }
  
      /**
       * Unconditionally sets the value of both the reference and stamp.
       *
       * @param newReference the new value for the reference
       * @param newStamp the new value for the stamp
       */
      public void set(V newReference, int newStamp) {
          Pair<V> current = pair;
          if (newReference != current.reference || newStamp != current.stamp)
              this.pair = Pair.of(newReference, newStamp);
      }
  
      /**
       * Atomically sets the value of the stamp 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 newStamp the new value for the stamp
       * @return {@code true} if successful
       */
      public boolean attemptStamp(V expectedReference, int newStamp) {
          Pair<V> current = pair;
          return
              expectedReference == current.reference &&
              (newStamp == current.stamp ||
               casPair(current, Pair.of(expectedReference, newStamp)));
      }
  
      // VarHandle mechanics
      private static final VarHandle PAIR;
      static {
          try {
              MethodHandles.Lookup l = MethodHandles.lookup();
              PAIR = l.findVarHandle(AtomicStampedReference.class, "pair",
                                     Pair.class);
          } catch (ReflectiveOperationException e) {
              throw new ExceptionInInitializerError(e);
          }
      }
  
      private boolean casPair(Pair<V> cmp, Pair<V> val) {
          return PAIR.compareAndSet(this, cmp, val);
      }
  }
  