/*

 * 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 static java.lang.Double.doubleToRawLongBits;

import static java.lang.Double.longBitsToDouble;

import java.io.Serializable;

import java.util.function.DoubleBinaryOperator;

/**

 * One or more variables that together maintain a running {@code double}

 * value updated using a supplied function.  When updates (method

 * {@link #accumulate}) are contended across threads, the set of variables

 * may grow dynamically to reduce contention.  Method {@link #get}

 * (or, equivalently, {@link #doubleValue}) returns the current value

 * across the variables maintaining updates.

 *

 * <p>This class is usually preferable to alternatives when multiple

 * threads update a common value that is used for purposes such as

 * summary statistics that are frequently updated but less frequently

 * read.

 *

 * <p>The supplied accumulator function should be side-effect-free,

 * since it may be re-applied when attempted updates fail due to

 * contention among threads.  For predictable results, the accumulator

 * function should be commutative and associative within the floating

 * point tolerance required in usage contexts. The function is applied

 * with an existing value (or identity) as one argument, and a given

 * update as the other argument. For example, to maintain a running

 * maximum value, you could supply {@code Double::max} along with

 * {@code Double.NEGATIVE_INFINITY} as the identity. The order of

 * accumulation within or across threads is not guaranteed. Thus, this

 * class may not be applicable if numerical stability is required,

 * especially when combining values of substantially different orders

 * of magnitude.

 *

 * <p>Class {@link DoubleAdder} provides analogs of the functionality

 * of this class for the common special case of maintaining sums.  The

 * call {@code new DoubleAdder()} is equivalent to {@code new

 * DoubleAccumulator((x, y) -> x + y, 0.0)}.

 *

 * <p>This class extends {@link Number}, but does <em>not</em> define

 * methods such as {@code equals}, {@code hashCode} and {@code

 * compareTo} because instances are expected to be mutated, and so are

 * not useful as collection keys.

 *

 * @since 1.8

 * @author Doug Lea

 */

public class DoubleAccumulator extends Striped64 implements Serializable {

    private static final long serialVersionUID = 7249069246863182397L;

    private final DoubleBinaryOperator function;

    private final long identity; // use long representation

    /**

     * Creates a new instance using the given accumulator function

     * and identity element.

     * @param accumulatorFunction a side-effect-free function of two arguments

     * @param identity identity (initial value) for the accumulator function

     */

    public DoubleAccumulator(DoubleBinaryOperator accumulatorFunction,

                             double identity) {

        this.function = accumulatorFunction;

        base = this.identity = doubleToRawLongBits(identity);

    }

    /**

     * Updates with the given value.

     *

     * @param x the value

     */

    public void accumulate(double x) {

        Cell[] cs; long b, v, r; int m; Cell c;

        if ((cs = cells) != null

            || ((r = doubleToRawLongBits

                (function.applyAsDouble(longBitsToDouble(b = base), x))) != b

                && !casBase(b, r))) {

            boolean uncontended = true;

            if (cs == null

                || (m = cs.length - 1) < 0

                || (c = cs[getProbe() & m]) == null

                || !(uncontended =

                     ((r = doubleToRawLongBits

                       (function.applyAsDouble

                        (longBitsToDouble(v = c.value), x))) == v)

                     || c.cas(v, r)))

                doubleAccumulate(x, function, uncontended);

        }

    }

    /**

     * Returns the current value.  The returned value is <em>NOT</em>

     * an atomic snapshot; invocation in the absence of concurrent

     * updates returns an accurate result, but concurrent updates that

     * occur while the value is being calculated might not be

     * incorporated.

     *

     * @return the current value

     */

    public double get() {

        Cell[] cs = cells;

        double result = longBitsToDouble(base);

        if (cs != null) {

            for (Cell c : cs)

                if (c != null)

                    result = function.applyAsDouble

                        (result, longBitsToDouble(c.value));

        }

        return result;

    }

    /**

     * Resets variables maintaining updates to the identity value.

     * This method may be a useful alternative to creating a new

     * updater, but is only effective if there are no concurrent

     * updates.  Because this method is intrinsically racy, it should

     * only be used when it is known that no threads are concurrently

     * updating.

     */

    public void reset() {

        Cell[] cs = cells;

        base = identity;

        if (cs != null) {

            for (Cell c : cs)

                if (c != null)

                    c.reset(identity);

        }

    }

    /**

     * Equivalent in effect to {@link #get} followed by {@link

     * #reset}. This method may apply for example during quiescent

     * points between multithreaded computations.  If there are

     * updates concurrent with this method, the returned value is

     * <em>not</em> guaranteed to be the final value occurring before

     * the reset.

     *

     * @return the value before reset

     */

    public double getThenReset() {

        Cell[] cs = cells;

        double result = longBitsToDouble(getAndSetBase(identity));

        if (cs != null) {

            for (Cell c : cs) {

                if (c != null) {

                    double v = longBitsToDouble(c.getAndSet(identity));

                    result = function.applyAsDouble(result, v);

                }

            }

        }

        return result;

    }

    /**

     * Returns the String representation of the current value.

     * @return the String representation of the current value

     */

    public String toString() {

        return Double.toString(get());

    }

    /**

     * Equivalent to {@link #get}.

     *

     * @return the current value

     */

    public double doubleValue() {

        return get();

    }

    /**

     * Returns the {@linkplain #get current value} as a {@code long}

     * after a narrowing primitive conversion.

     */

    public long longValue() {

        return (long)get();

    }

    /**

     * Returns the {@linkplain #get current value} as an {@code int}

     * after a narrowing primitive conversion.

     */

    public int intValue() {

        return (int)get();

    }

    /**

     * Returns the {@linkplain #get current value} as a {@code float}

     * after a narrowing primitive conversion.

     */

    public float floatValue() {

        return (float)get();

    }

    /**

     * Serialization proxy, used to avoid reference to the non-public

     * Striped64 superclass in serialized forms.

     * @serial include

     */

    private static class SerializationProxy implements Serializable {

        private static final long serialVersionUID = 7249069246863182397L;

        /**

         * The current value returned by get().

         * @serial

         */

        private final double value;

        /**

         * The function used for updates.

         * @serial

         */

        private final DoubleBinaryOperator function;

        /**

         * The identity value, represented as a long, as converted by

         * {@link Double#doubleToRawLongBits}.  The original identity

         * can be recovered using {@link Double#longBitsToDouble}.

         * @serial

         */

        private final long identity;

        SerializationProxy(double value,

                           DoubleBinaryOperator function,

                           long identity) {

            this.value = value;

            this.function = function;

            this.identity = identity;

        }

        /**

         * Returns a {@code DoubleAccumulator} object with initial state

         * held by this proxy.

         *

         * @return a {@code DoubleAccumulator} object with initial state

         * held by this proxy

         */

        private Object readResolve() {

            double d = longBitsToDouble(identity);

            DoubleAccumulator a = new DoubleAccumulator(function, d);

            a.base = doubleToRawLongBits(value);

            return a;

        }

    }

    /**

     * Returns a

     * <a href="{@docRoot}/serialized-form.html#java.util.concurrent.atomic.DoubleAccumulator.SerializationProxy">

     * SerializationProxy</a>

     * representing the state of this instance.

     *

     * @return a {@link SerializationProxy}

     * representing the state of this instance

     */

    private Object writeReplace() {

        return new SerializationProxy(get(), function, identity);

    }

    /**

     * @param s the stream

     * @throws java.io.InvalidObjectException always

     */

    private void readObject(java.io.ObjectInputStream s)

        throws java.io.InvalidObjectException {

        throw new java.io.InvalidObjectException("Proxy required");

    }

}