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dl |
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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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package jsr166e; |
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import java.io.IOException; |
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import java.io.Serializable; |
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import java.io.ObjectInputStream; |
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import java.io.ObjectOutputStream; |
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/** |
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* One or more variables that together maintain a running {@code double} |
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* maximum with initial value {@code Double.NEGATIVE_INFINITY}. When |
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* updates (method {@link #update}) are contended across threads, the |
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* set of variables may grow dynamically to reduce contention. Method |
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* {@link #max} (or, equivalently, {@link #doubleValue}) returns the |
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* current maximum across the variables maintaining updates. |
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* |
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* <p>This class extends {@link Number}, but does <em>not</em> define |
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* methods such as {@code hashCode} and {@code compareTo} because |
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* instances are expected to be mutated, and so are not useful as |
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* collection keys. |
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* |
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* <p><em>jsr166e note: This class is targeted to be placed in |
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* java.util.concurrent.atomic<em> |
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* |
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* @author Doug Lea |
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*/ |
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public class DoubleMaxUpdater extends Striped64 implements Serializable { |
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private static final long serialVersionUID = 7249069246863182397L; |
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/** |
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* Long representation of negative infinity. See class Double |
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* internal documentation for explanation. |
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*/ |
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private static final long MIN_AS_LONG = 0xfff0000000000000L; |
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/** |
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* Update function. See class DoubleAdder for rationale |
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* for using conversions from/to long. |
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*/ |
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final long fn(long v, long x) { |
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return Double.longBitsToDouble(v) > Double.longBitsToDouble(x) ? v : x; |
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} |
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/** |
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* Creates a new instance with initial value of {@code |
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* Double.NEGATIVE_INFINITY}. |
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*/ |
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public DoubleMaxUpdater() { |
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base = MIN_AS_LONG; |
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} |
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/** |
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* Updates the maximum to be at least the given value. |
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* |
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* @param x the value to update |
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*/ |
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public void update(double x) { |
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long lx = Double.doubleToRawLongBits(x); |
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Cell[] as; long b, v; HashCode hc; Cell a; int n; |
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if ((as = cells) != null || |
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(Double.longBitsToDouble(b = base) < x && !casBase(b, lx))) { |
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boolean uncontended = true; |
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int h = (hc = threadHashCode.get()).code; |
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if (as == null || (n = as.length) < 1 || |
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(a = as[(n - 1) & h]) == null || |
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(Double.longBitsToDouble(v = a.value) < x && |
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!(uncontended = a.cas(v, lx)))) |
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retryUpdate(lx, hc, uncontended); |
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} |
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} |
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/** |
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* Returns the current maximum. The returned value is |
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* <em>NOT</em> an atomic snapshot: Invocation in the absence of |
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* concurrent updates returns an accurate result, but concurrent |
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* updates that occur while the value is being calculated might |
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* not be incorporated. |
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* |
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* @return the maximum |
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*/ |
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public double max() { |
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Cell[] as = cells; |
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double max = Double.longBitsToDouble(base); |
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if (as != null) { |
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int n = as.length; |
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double v; |
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for (int i = 0; i < n; ++i) { |
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Cell a = as[i]; |
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if (a != null && (v = Double.longBitsToDouble(a.value)) > max) |
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max = v; |
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} |
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} |
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return max; |
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} |
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/** |
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* Resets variables maintaining updates to {@code |
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* Double.NEGATIVE_INFINITY}. This method may be a useful |
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* alternative to creating a new updater, but is only effective if |
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* there are no concurrent updates. Because this method is |
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* intrinsically racy, it should only be used when it is known |
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* that no threads are concurrently updating. |
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*/ |
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public void reset() { |
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internalReset(MIN_AS_LONG); |
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} |
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/** |
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* Equivalent in effect to {@link #max} followed by {@link |
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* #reset}. This method may apply for example during quiescent |
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* points between multithreaded computations. If there are |
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* updates concurrent with this method, the returned value is |
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* <em>not</em> guaranteed to be the final value occurring before |
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* the reset. |
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* |
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* @return the maximum |
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*/ |
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public double maxThenReset() { |
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Cell[] as = cells; |
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double max = Double.longBitsToDouble(base); |
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base = MIN_AS_LONG; |
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if (as != null) { |
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int n = as.length; |
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for (int i = 0; i < n; ++i) { |
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Cell a = as[i]; |
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if (a != null) { |
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double v = Double.longBitsToDouble(a.value); |
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a.value = MIN_AS_LONG; |
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if (v > max) |
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max = v; |
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} |
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} |
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} |
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return max; |
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} |
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/** |
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* Returns the String representation of the {@link #max}. |
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* @return the String representation of the {@link #max}. |
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*/ |
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public String toString() { |
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return Double.toString(max()); |
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} |
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/** |
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* Equivalent to {@link #max}. |
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* |
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* @return the max |
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*/ |
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public double doubleValue() { |
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return max(); |
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} |
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/** |
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* Returns the {@link #max} as a {@code long} after a |
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* primitive conversion. |
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*/ |
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public long longValue() { |
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return (long)max(); |
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} |
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/** |
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* Returns the {@link #max} as an {@code int} after a |
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* primitive conversion. |
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*/ |
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public int intValue() { |
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return (int)max(); |
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} |
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/** |
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* Returns the {@link #max} as a {@code float} |
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* after a primitive conversion. |
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*/ |
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public float floatValue() { |
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return (float)max(); |
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} |
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private void writeObject(java.io.ObjectOutputStream s) |
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throws java.io.IOException { |
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s.defaultWriteObject(); |
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s.writeDouble(max()); |
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} |
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private void readObject(ObjectInputStream s) |
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throws IOException, ClassNotFoundException { |
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s.defaultReadObject(); |
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busy = 0; |
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cells = null; |
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base = Double.doubleToRawLongBits(s.readDouble()); |
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} |
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} |