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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 jsr166e;
import java.io.Serializable;
/**
* One or more variables that together maintain a running {@code double}
* maximum with initial value {@code Double.NEGATIVE_INFINITY}. When
* updates (method {@link #update}) are contended across threads, the
* set of variables may grow dynamically to reduce contention. Method
* {@link #max} (or, equivalently, {@link #doubleValue}) returns the
* current maximum across the variables maintaining updates.
*
* <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.
*
* <p><em>jsr166e note: This class is targeted to be placed in
* java.util.concurrent.atomic.</em>
*
* @since 1.8
* @author Doug Lea
*/
public class DoubleMaxUpdater extends Striped64 implements Serializable {
private static final long serialVersionUID = 7249069246863182397L;
/**
* Long representation of negative infinity. See class Double
* internal documentation for explanation.
*/
private static final long MIN_AS_LONG = 0xfff0000000000000L;
/**
* Update function. See class DoubleAdder for rationale
* for using conversions from/to long.
*/
final long fn(long v, long x) {
return Double.longBitsToDouble(v) > Double.longBitsToDouble(x) ? v : x;
}
/**
* Creates a new instance with initial value of {@code
* Double.NEGATIVE_INFINITY}.
*/
public DoubleMaxUpdater() {
base = MIN_AS_LONG;
}
/**
* Updates the maximum to be at least the given value.
*
* @param x the value to update
*/
public void update(double x) {
long lx = Double.doubleToRawLongBits(x);
Cell[] as; long b, v; HashCode hc; Cell a; int n;
if ((as = cells) != null ||
(Double.longBitsToDouble(b = base) < x && !casBase(b, lx))) {
boolean uncontended = true;
int h = (hc = threadHashCode.get()).code;
if (as == null || (n = as.length) < 1 ||
(a = as[(n - 1) & h]) == null ||
(Double.longBitsToDouble(v = a.value) < x &&
!(uncontended = a.cas(v, lx))))
retryUpdate(lx, hc, uncontended);
}
}
/**
* Returns the current maximum. 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 maximum
*/
public double max() {
Cell[] as = cells;
double max = Double.longBitsToDouble(base);
if (as != null) {
int n = as.length;
double v;
for (int i = 0; i < n; ++i) {
Cell a = as[i];
if (a != null && (v = Double.longBitsToDouble(a.value)) > max)
max = v;
}
}
return max;
}
/**
* Resets variables maintaining updates to {@code
* Double.NEGATIVE_INFINITY}. 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() {
internalReset(MIN_AS_LONG);
}
/**
* Equivalent in effect to {@link #max} 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 maximum
*/
public double maxThenReset() {
Cell[] as = cells;
double max = Double.longBitsToDouble(base);
base = MIN_AS_LONG;
if (as != null) {
int n = as.length;
for (int i = 0; i < n; ++i) {
Cell a = as[i];
if (a != null) {
double v = Double.longBitsToDouble(a.value);
a.value = MIN_AS_LONG;
if (v > max)
max = v;
}
}
}
return max;
}
/**
* Returns the String representation of the {@link #max}.
* @return the String representation of the {@link #max}
*/
public String toString() {
return Double.toString(max());
}
/**
* Equivalent to {@link #max}.
*
* @return the max
*/
public double doubleValue() {
return max();
}
/**
* Returns the {@link #max} as a {@code long} after a
* narrowing primitive conversion.
*/
public long longValue() {
return (long)max();
}
/**
* Returns the {@link #max} as an {@code int} after a
* narrowing primitive conversion.
*/
public int intValue() {
return (int)max();
}
/**
* Returns the {@link #max} as a {@code float}
* after a narrowing primitive conversion.
*/
public float floatValue() {
return (float)max();
}
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
s.defaultWriteObject();
s.writeDouble(max());
}
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
busy = 0;
cells = null;
base = Double.doubleToRawLongBits(s.readDouble());
}
}
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