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/*
* 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;
/**
* One or more variables that together maintain an initially zero
* {@code double} sum. When updates (method {@link #add}) are
* contended across threads, the set of variables may grow dynamically
* to reduce contention. Method {@link #sum} (or, equivalently {@link
* #doubleValue}) returns the current total combined across the
* variables maintaining the sum. 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.
*
* 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.
*
*
This class extends {@link Number}, but does not 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 DoubleAdder extends Striped64 implements Serializable {
private static final long serialVersionUID = 7249069246863182397L;
/*
* Note that we must use "long" for underlying representations,
* because there is no compareAndSet for double, due to the fact
* that the bitwise equals used in any CAS implementation is not
* the same as double-precision equals. However, we use CAS only
* to detect and alleviate contention, for which bitwise equals
* works best anyway. In principle, the long/double conversions
* used here should be essentially free on most platforms since
* they just re-interpret bits.
*/
/**
* Creates a new adder with initial sum of zero.
*/
public DoubleAdder() {
}
/**
* Adds the given value.
*
* @param x the value to add
*/
public void add(double x) {
Cell[] cs; long b, v; int m; Cell c;
if ((cs = cells) != null ||
!casBase(b = base,
Double.doubleToRawLongBits
(Double.longBitsToDouble(b) + x))) {
int index = getProbe();
boolean uncontended = true;
if (cs == null || (m = cs.length - 1) < 0 ||
(c = cs[index & m]) == null ||
!(uncontended = c.cas(v = c.value,
Double.doubleToRawLongBits
(Double.longBitsToDouble(v) + x))))
doubleAccumulate(x, null, uncontended, index);
}
}
/**
* Returns the current sum. The returned value is NOT an
* atomic snapshot; invocation in the absence of concurrent
* updates returns an accurate result, but concurrent updates that
* occur while the sum is being calculated might not be
* incorporated. Also, because floating-point arithmetic is not
* strictly associative, the returned result need not be identical
* to the value that would be obtained in a sequential series of
* updates to a single variable.
*
* @return the sum
*/
public double sum() {
Cell[] cs = cells;
double sum = Double.longBitsToDouble(base);
if (cs != null) {
for (Cell c : cs)
if (c != null)
sum += Double.longBitsToDouble(c.value);
}
return sum;
}
/**
* Resets variables maintaining the sum to zero. This method may
* be a useful alternative to creating a new adder, 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 = 0L; // relies on fact that double 0 must have same rep as long
if (cs != null) {
for (Cell c : cs)
if (c != null)
c.reset();
}
}
/**
* Equivalent in effect to {@link #sum} 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
* not guaranteed to be the final value occurring before
* the reset.
*
* @return the sum
*/
public double sumThenReset() {
Cell[] cs = cells;
double sum = Double.longBitsToDouble(getAndSetBase(0L));
if (cs != null) {
for (Cell c : cs) {
if (c != null)
sum += Double.longBitsToDouble(c.getAndSet(0L));
}
}
return sum;
}
/**
* Returns the String representation of the {@link #sum}.
* @return the String representation of the {@link #sum}
*/
public String toString() {
return Double.toString(sum());
}
/**
* Equivalent to {@link #sum}.
*
* @return the sum
*/
public double doubleValue() {
return sum();
}
/**
* Returns the {@link #sum} as a {@code long} after a
* narrowing primitive conversion.
*/
public long longValue() {
return (long)sum();
}
/**
* Returns the {@link #sum} as an {@code int} after a
* narrowing primitive conversion.
*/
public int intValue() {
return (int)sum();
}
/**
* Returns the {@link #sum} as a {@code float}
* after a narrowing primitive conversion.
*/
public float floatValue() {
return (float)sum();
}
/**
* 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 sum().
* @serial
*/
private final double value;
SerializationProxy(DoubleAdder a) {
value = a.sum();
}
/**
* Returns a {@code DoubleAdder} object with initial state
* held by this proxy.
*
* @return a {@code DoubleAdder} object with initial state
* held by this proxy
*/
private Object readResolve() {
DoubleAdder a = new DoubleAdder();
a.base = Double.doubleToRawLongBits(value);
return a;
}
}
/**
* Returns a
*
* SerializationProxy
* representing the state of this instance.
*
* @return a {@link SerializationProxy}
* representing the state of this instance
*/
private Object writeReplace() {
return new SerializationProxy(this);
}
/**
* @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");
}
}