/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
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/*
*
*
*
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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/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.
*
* <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>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
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[]
as; long
b,
v; int
m;
Cell a;
if ((
as =
cells) != null ||
!
casBase(
b =
base,
Double.
doubleToRawLongBits
(
Double.
longBitsToDouble(
b) +
x))) {
boolean
uncontended = true;
if (
as == null || (
m =
as.length - 1) < 0 ||
(
a =
as[
getProbe() &
m]) == null ||
!(
uncontended =
a.
cas(
v =
a.
value,
Double.
doubleToRawLongBits
(
Double.
longBitsToDouble(
v) +
x))))
doubleAccumulate(
x, null,
uncontended);
}
}
/**
* Returns the current sum. 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 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[]
as =
cells;
Cell a;
double
sum =
Double.
longBitsToDouble(
base);
if (
as != null) {
for (int
i = 0;
i <
as.length; ++
i) {
if ((
a =
as[
i]) != null)
sum +=
Double.
longBitsToDouble(
a.
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[]
as =
cells;
Cell a;
base = 0L; // relies on fact that double 0 must have same rep as long
if (
as != null) {
for (int
i = 0;
i <
as.length; ++
i) {
if ((
a =
as[
i]) != null)
a.
value = 0L;
}
}
}
/**
* 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
* <em>not</em> guaranteed to be the final value occurring before
* the reset.
*
* @return the sum
*/
public double
sumThenReset() {
Cell[]
as =
cells;
Cell a;
double
sum =
Double.
longBitsToDouble(
base);
base = 0L;
if (
as != null) {
for (int
i = 0;
i <
as.length; ++
i) {
if ((
a =
as[
i]) != null) {
long
v =
a.
value;
a.
value = 0L;
sum +=
Double.
longBitsToDouble(
v);
}
}
}
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
* <a href="../../../../serialized-form.html#java.util.concurrent.atomic.DoubleAdder.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(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");
}
}