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/*

 * 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/licenses/publicdomain

 */

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

    private final Object[] array;

    private long rawIndex(int i) {

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

            throw new IndexOutOfBoundsException("index " + i);

        return base + i * scale;

    }

    /**

     * Creates a new AtomicReferenceArray of given length.

     * @param length the length of the array

     */

    public AtomicReferenceArray(int length) {

        array = new Object[length];

        // must perform at least one volatile write to conform to JMM

        if (length > 0)

            unsafe.putObjectVolatile(array, rawIndex(0), null);

    }

    /**

     * 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) {

        if (array == null)

            throw new NullPointerException();

        int length = array.length;

        this.array = new Object[length];

        if (length > 0) {

            int last = length-1;

            for (int i = 0; i < last; ++i)

                this.array[i] = array[i];

            // Do the last write as volatile

            E e = array[last];

            unsafe.putObjectVolatile(this.array, rawIndex(last), e);

        }

    }

    /**

     * 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 <tt>i</tt>.

     *

     * @param i the index

     * @return the current value

     */

    public final E get(int i) {

        return (E) unsafe.getObjectVolatile(array, rawIndex(i));

    }

    /**

     * Sets the element at position <tt>i</tt> to the given value.

     *

     * @param i the index

     * @param newValue the new value

     */

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

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

    }

    /**

     * Eventually sets the element at position <tt>i</tt> 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, rawIndex(i), newValue);

    }

    /**

     * Atomically sets the element at position <tt>i</tt> 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) {

        while (true) {

            E current = get(i);

            if (compareAndSet(i, current, newValue))

                return current;

        }

    }

    /**

     * Atomically sets the element at position <tt>i</tt> to the given

     * updated value if the current value <tt>==</tt> 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 unsafe.compareAndSwapObject(array, rawIndex(i),

                                         expect, update);

    }

    /**

     * Atomically sets the element at position <tt>i</tt> to the given

     * updated value if the current value <tt>==</tt> the expected value.

     * May fail spuriously and does not provide ordering guarantees,

     * so is only rarely an appropriate alternative to <tt>compareAndSet</tt>.

     *

     * @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() {

        if (array.length > 0) // force volatile read

            get(0);

        return Arrays.toString(array);

    }

}