/*

  * 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

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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;

 import sun.misc.Unsafe;

 import java.util.*;

 /**

  * An array of object references in which elements may be updated

  * atomically.  See the {@link java.util.concurrent.atomic} package

  * specification for description of the properties of atomic

  * variables.

  * @since 1.5

  * @author Doug Lea

  * @param <E> The base class of elements held in this array

  */

 public class AtomicReferenceArray<E> implements java.io.Serializable {

     private static final long serialVersionUID = -6209656149925076980L;

     private static final Unsafe unsafe = Unsafe.getUnsafe();

     private static final int base = unsafe.arrayBaseOffset(Object[].class);

     private static final int shift;

     private final Object[] array;

     static {

         int scale = unsafe.arrayIndexScale(Object[].class);

         if ((scale & (scale - 1)) != 0)

             throw new Error("data type scale not a power of two");

         shift = 31 - Integer.numberOfLeadingZeros(scale);

     }

     private long checkedByteOffset(int i) {

         if (i < 0 || i >= array.length)

             throw new IndexOutOfBoundsException("index " + i);

         return byteOffset(i);

     }

     private static long byteOffset(int i) {

         return ((long) i << shift) + base;

     }

     /**

      * Creates a new AtomicReferenceArray of the given length, with all

      * elements initially null.

      *

      * @param length the length of the array

      */

     public AtomicReferenceArray(int length) {

         array = new Object[length];

     }

     /**

      * Creates a new AtomicReferenceArray with the same length as, and

      * all elements copied from, the given array.

      *

      * @param array the array to copy elements from

      * @throws NullPointerException if array is null

      */

     public AtomicReferenceArray(E[] array) {

         // Visibility guaranteed by final field guarantees

         this.array = array.clone();

     }

     /**

      * Returns the length of the array.

      *

      * @return the length of the array

      */

     public final int length() {

         return array.length;

     }

     /**

      * Gets the current value at position {@code i}.

      *

      * @param i the index

      * @return the current value

      */

     public final E get(int i) {

         return getRaw(checkedByteOffset(i));

     }

     private E getRaw(long offset) {

         return (E) unsafe.getObjectVolatile(array, offset);

     }

     /**

      * Sets the element at position {@code i} to the given value.

      *

      * @param i the index

      * @param newValue the new value

      */

     public final void set(int i, E newValue) {

         unsafe.putObjectVolatile(array, checkedByteOffset(i), newValue);

     }

     /**

      * Eventually sets the element at position {@code i} to the given value.

      *

      * @param i the index

      * @param newValue the new value

      * @since 1.6

      */

     public final void lazySet(int i, E newValue) {

         unsafe.putOrderedObject(array, checkedByteOffset(i), newValue);

     }

     /**

      * Atomically sets the element at position {@code i} to the given

      * value and returns the old value.

      *

      * @param i the index

      * @param newValue the new value

      * @return the previous value

      */

     public final E getAndSet(int i, E newValue) {

         long offset = checkedByteOffset(i);

         while (true) {

             E current = (E) getRaw(offset);

             if (compareAndSetRaw(offset, current, newValue))

                 return current;

         }

     }

     /**

      * Atomically sets the element at position {@code i} to the given

      * updated value if the current value {@code ==} the expected value.

      *

      * @param i the index

      * @param expect the expected value

      * @param update the new value

      * @return true if successful. False return indicates that

      * the actual value was not equal to the expected value.

      */

     public final boolean compareAndSet(int i, E expect, E update) {

         return compareAndSetRaw(checkedByteOffset(i), expect, update);

     }

     private boolean compareAndSetRaw(long offset, E expect, E update) {

         return unsafe.compareAndSwapObject(array, offset, expect, update);

     }

     /**

      * Atomically sets the element at position {@code i} to the given

      * updated value if the current value {@code ==} the expected value.

      *

      * <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 i the index

      * @param expect the expected value

      * @param update the new value

      * @return true if successful.

      */

     public final boolean weakCompareAndSet(int i, E expect, E update) {

         return compareAndSet(i, expect, update);

     }

     /**

      * Returns the String representation of the current values of array.

      * @return the String representation of the current values of array

      */

     public String toString() {

            int iMax = array.length - 1;

         if (iMax == -1)

             return "[]";

         StringBuilder b = new StringBuilder();

         b.append('[');

         for (int i = 0; ; i++) {

             b.append(getRaw(byteOffset(i)));

             if (i == iMax)

                 return b.append(']').toString();

             b.append(',').append(' ');

         }

     }

 }