/*

 * 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 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(' ');

        }

    }

}