/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
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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;
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();
}
/**
* 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
* @serial
*/
private final long
identity;
SerializationProxy(
DoubleAccumulator a) {
function =
a.
function;
identity =
a.
identity;
value =
a.
get();
}
/**
* 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 =
Double.
longBitsToDouble(
identity);
DoubleAccumulator a = new
DoubleAccumulator(
function,
d);
a.
base =
Double.
doubleToRawLongBits(
value);
return
a;
}
}
/**
* Returns a
* <a href="../../../../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(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");
}
}