/*
* 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 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. The function is applied with the current
* value as its first argument, and the given update as the second
* 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 = Double.doubleToRawLongBits(identity);
}
/**
* Updates with the given value.
*
* @param x the value
*/
public void accumulate(double x) {
Cell[] as; long b, v, r; int m; Cell a;
if ((as = cells) != null ||
(r = Double.doubleToRawLongBits
(function.applyAsDouble
(Double.longBitsToDouble(b = base), x))) != b && !casBase(b, r)) {
boolean uncontended = true;
if (as == null || (m = as.length - 1) < 0 ||
(a = as[getProbe() & m]) == null ||
!(uncontended =
(r = Double.doubleToRawLongBits
(function.applyAsDouble
(Double.longBitsToDouble(v = a.value), x))) == v ||
a.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[] as = cells; Cell a;
double result = Double.longBitsToDouble(base);
if (as != null) {
for (int i = 0; i < as.length; ++i) {
if ((a = as[i]) != null)
result = function.applyAsDouble
(result, Double.longBitsToDouble(a.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[] as = cells; Cell a;
base = identity;
if (as != null) {
for (int i = 0; i < as.length; ++i) {
if ((a = as[i]) != null)
a.value = 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[] as = cells; Cell a;
double result = Double.longBitsToDouble(base);
base = identity;
if (as != null) {
for (int i = 0; i < as.length; ++i) {
if ((a = as[i]) != null) {
double v = Double.longBitsToDouble(a.value);
a.value = 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();
}
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
s.defaultWriteObject();
s.writeDouble(get());
}
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
cellsBusy = 0;
cells = null;
base = Double.doubleToRawLongBits(s.readDouble());
}
}