luni/src/main/java/java/util/concurrent/atomic/LongAccumulator.java - platform/libcore - Git at Google

 /*
  * 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.LongBinaryOperator;

 /**
  * One or more variables that together maintain a running {@code long}
  * 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 #longValue}) returns the current value
  * across the variables maintaining updates.
  *
  * <p>This class is usually preferable to {@link AtomicLong} when
  * multiple threads update a common value that is used for purposes such
  * as collecting statistics, not for fine-grained synchronization
  * control.  Under low update contention, the two classes have similar
  * characteristics. But under high contention, expected throughput of
  * this class is significantly higher, at the expense of higher space
  * consumption.
  *
  * <p>The order of accumulation within or across threads is not
  * guaranteed and cannot be depended upon, so this class is only
  * applicable to functions for which the order of accumulation does
  * not matter. 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 Long::max} along with {@code
  * Long.MIN_VALUE} as the identity.
  *
  * <p>Class {@link LongAdder} provides analogs of the functionality of
  * this class for the common special case of maintaining counts and
  * sums.  The call {@code new LongAdder()} is equivalent to {@code new
  * LongAccumulator((x, y) -> x + y, 0L}.
  *
  * <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 LongAccumulator extends Striped64 implements Serializable {
     private static final long serialVersionUID = 7249069246863182397L;

     private final LongBinaryOperator function;
     private final long identity;

     /**
      * 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 LongAccumulator(LongBinaryOperator accumulatorFunction,
                            long identity) {
         this.function = accumulatorFunction;
         base = this.identity = identity;
     }

     /**
      * Updates with the given value.
      *
      * @param x the value
      */
     public void accumulate(long x) {
         Cell[] as; long b, v, r; int m; Cell a;
         if ((as = cells) != null ||
             (r = function.applyAsLong(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 = function.applyAsLong(v = a.value, x)) == v ||
                   a.cas(v, r)))
                 longAccumulate(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 long get() {
         Cell[] as = cells;
         long result = base;
         if (as != null) {
             for (Cell a : as)
                 if (a != null)
                     result = function.applyAsLong(result, 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;
         base = identity;
         if (as != null) {
             for (Cell a : as)
                 if (a != null)
                     a.reset(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 long getThenReset() {
         Cell[] as = cells;
         long result = base;
         base = identity;
         if (as != null) {
             for (Cell a : as) {
                 if (a != null) {
                     long v = a.value;
                     a.reset(identity);
                     result = function.applyAsLong(result, v);
                 }
             }
         }
         return result;
     }

     /**
      * Returns the String representation of the current value.
      * @return the String representation of the current value
      */
     public String toString() {
         return Long.toString(get());
     }

     /**
      * Equivalent to {@link #get}.
      *
      * @return the current value
      */
     public long longValue() {
         return 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 widening primitive conversion.
      */
     public float floatValue() {
         return (float)get();
     }

     /**
      * Returns the {@linkplain #get current value} as a {@code double}
      * after a widening primitive conversion.
      */
     public double doubleValue() {
         return (double)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 long value;

         /**
          * The function used for updates.
          * @serial
          */
         private final LongBinaryOperator function;

         /**
          * The identity value.
          * @serial
          */
         private final long identity;

         SerializationProxy(long value,
                            LongBinaryOperator function,
                            long identity) {
             this.value = value;
             this.function = function;
             this.identity = identity;
         }

         /**
          * Returns a {@code LongAccumulator} object with initial state
          * held by this proxy.
          *
          * @return a {@code LongAccumulator} object with initial state
          * held by this proxy
          */
         private Object readResolve() {
             LongAccumulator a = new LongAccumulator(function, identity);
             a.base = value;
             return a;
         }
     }

     /**
      * Returns a
      * <a href="../../../../serialized-form.html#java.util.concurrent.atomic.LongAccumulator.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(get(), function, identity);
     }

     /**
      * @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");
     }

 }
	/*
	* 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.LongBinaryOperator;

	/**
	* One or more variables that together maintain a running {@code long}
	* 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 #longValue}) returns the current value
	* across the variables maintaining updates.
	*
	* <p>This class is usually preferable to {@link AtomicLong} when
	* multiple threads update a common value that is used for purposes such
	* as collecting statistics, not for fine-grained synchronization
	* control. Under low update contention, the two classes have similar
	* characteristics. But under high contention, expected throughput of
	* this class is significantly higher, at the expense of higher space
	* consumption.
	*
	* <p>The order of accumulation within or across threads is not
	* guaranteed and cannot be depended upon, so this class is only
	* applicable to functions for which the order of accumulation does
	* not matter. 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 Long::max} along with {@code
	* Long.MIN_VALUE} as the identity.
	*
	* <p>Class {@link LongAdder} provides analogs of the functionality of
	* this class for the common special case of maintaining counts and
	* sums. The call {@code new LongAdder()} is equivalent to {@code new
	* LongAccumulator((x, y) -> x + y, 0L}.
	*
	* <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 LongAccumulator extends Striped64 implements Serializable {
	private static final long serialVersionUID = 7249069246863182397L;

	private final LongBinaryOperator function;
	private final long identity;

	/**
	* 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 LongAccumulator(LongBinaryOperator accumulatorFunction,
	long identity) {
	this.function = accumulatorFunction;
	base = this.identity = identity;
	}

	/**
	* Updates with the given value.
	*
	* @param x the value
	*/
	public void accumulate(long x) {
	Cell[] as; long b, v, r; int m; Cell a;
	if ((as = cells) != null \|\|
	(r = function.applyAsLong(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 = function.applyAsLong(v = a.value, x)) == v \|\|
	a.cas(v, r)))
	longAccumulate(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 long get() {
	Cell[] as = cells;
	long result = base;
	if (as != null) {
	for (Cell a : as)
	if (a != null)
	result = function.applyAsLong(result, 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;
	base = identity;
	if (as != null) {
	for (Cell a : as)
	if (a != null)
	a.reset(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 long getThenReset() {
	Cell[] as = cells;
	long result = base;
	base = identity;
	if (as != null) {
	for (Cell a : as) {
	if (a != null) {
	long v = a.value;
	a.reset(identity);
	result = function.applyAsLong(result, v);
	}
	}
	}
	return result;
	}

	/**
	* Returns the String representation of the current value.
	* @return the String representation of the current value
	*/
	public String toString() {
	return Long.toString(get());
	}

	/**
	* Equivalent to {@link #get}.
	*
	* @return the current value
	*/
	public long longValue() {
	return 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 widening primitive conversion.
	*/
	public float floatValue() {
	return (float)get();
	}

	/**
	* Returns the {@linkplain #get current value} as a {@code double}
	* after a widening primitive conversion.
	*/
	public double doubleValue() {
	return (double)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 long value;

	/**
	* The function used for updates.
	* @serial
	*/
	private final LongBinaryOperator function;

	/**
	* The identity value.
	* @serial
	*/
	private final long identity;

	SerializationProxy(long value,
	LongBinaryOperator function,
	long identity) {
	this.value = value;
	this.function = function;
	this.identity = identity;
	}

	/**
	* Returns a {@code LongAccumulator} object with initial state
	* held by this proxy.
	*
	* @return a {@code LongAccumulator} object with initial state
	* held by this proxy
	*/
	private Object readResolve() {
	LongAccumulator a = new LongAccumulator(function, identity);
	a.base = value;
	return a;
	}
	}

	/**
	* Returns a
	* <a href="../../../../serialized-form.html#java.util.concurrent.atomic.LongAccumulator.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(get(), function, identity);
	}

	/**
	* @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");
	}

	}