concurrent/atomic/DoubleAccumulator.java

/*
 * 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 static java.lang.Double.doubleToRawLongBits;
import static java.lang.Double.longBitsToDouble;

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 = 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 = doubleToRawLongBits
                (function.applyAsDouble(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 = doubleToRawLongBits
                       (function.applyAsDouble
                        (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;
        double result = longBitsToDouble(base);
        if (as != null) {
            for (Cell a : as)
                if (a != null)
                    result = function.applyAsDouble
                        (result, 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;
        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 double getThenReset() {
        Cell[] as = cells;
        double result = longBitsToDouble(base);
        base = identity;
        if (as != null) {
            for (Cell a : as) {
                if (a != null) {
                    double v = longBitsToDouble(a.value);
                    a.reset(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, represented as a long, as converted by
         * {@link Double#doubleToRawLongBits}.  The original identity
         * can be recovered using {@link Double#longBitsToDouble}.
         * @serial
         */
        private final long identity;

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

        /**
         * 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 = longBitsToDouble(identity);
            DoubleAccumulator a = new DoubleAccumulator(function, d);
            a.base = 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(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");
    }

}
Revision:	1.2
Committed:	Thu Sep 15 06:57:06 2016 UTC (7 years, 8 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.1:	+22 -18 lines
Log Message:	improve readability; fix errorprone warning OperatorPrecedence
#	User	Rev	Content
1	jsr166	1.1	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3			* Expert Group and released to the public domain, as explained at
4			* http://creativecommons.org/publicdomain/zero/1.0/
5			*/
6
7			package java.util.concurrent.atomic;
8
9	jsr166	1.2	import static java.lang.Double.doubleToRawLongBits;
10			import static java.lang.Double.longBitsToDouble;
11
12	jsr166	1.1	import java.io.Serializable;
13			import java.util.function.DoubleBinaryOperator;
14
15			/**
16			* One or more variables that together maintain a running {@code double}
17			* value updated using a supplied function. When updates (method
18			* {@link #accumulate}) are contended across threads, the set of variables
19			* may grow dynamically to reduce contention. Method {@link #get}
20			* (or, equivalently, {@link #doubleValue}) returns the current value
21			* across the variables maintaining updates.
22			*
23			* <p>This class is usually preferable to alternatives when multiple
24			* threads update a common value that is used for purposes such as
25			* summary statistics that are frequently updated but less frequently
26			* read.
27			*
28			* <p>The supplied accumulator function should be side-effect-free,
29			* since it may be re-applied when attempted updates fail due to
30			* contention among threads. The function is applied with the current
31			* value as its first argument, and the given update as the second
32			* argument. For example, to maintain a running maximum value, you
33			* could supply {@code Double::max} along with {@code
34			* Double.NEGATIVE_INFINITY} as the identity. The order of
35			* accumulation within or across threads is not guaranteed. Thus, this
36			* class may not be applicable if numerical stability is required,
37			* especially when combining values of substantially different orders
38			* of magnitude.
39			*
40			* <p>Class {@link DoubleAdder} provides analogs of the functionality
41			* of this class for the common special case of maintaining sums. The
42			* call {@code new DoubleAdder()} is equivalent to {@code new
43			* DoubleAccumulator((x, y) -> x + y, 0.0)}.
44			*
45			* <p>This class extends {@link Number}, but does <em>not</em> define
46			* methods such as {@code equals}, {@code hashCode} and {@code
47			* compareTo} because instances are expected to be mutated, and so are
48			* not useful as collection keys.
49			*
50			* @since 1.8
51			* @author Doug Lea
52			*/
53			public class DoubleAccumulator extends Striped64 implements Serializable {
54			private static final long serialVersionUID = 7249069246863182397L;
55
56			private final DoubleBinaryOperator function;
57			private final long identity; // use long representation
58
59			/**
60			* Creates a new instance using the given accumulator function
61			* and identity element.
62			* @param accumulatorFunction a side-effect-free function of two arguments
63			* @param identity identity (initial value) for the accumulator function
64			*/
65			public DoubleAccumulator(DoubleBinaryOperator accumulatorFunction,
66			double identity) {
67			this.function = accumulatorFunction;
68	jsr166	1.2	base = this.identity = doubleToRawLongBits(identity);
69	jsr166	1.1	}
70
71			/**
72			* Updates with the given value.
73			*
74			* @param x the value
75			*/
76			public void accumulate(double x) {
77			Cell[] as; long b, v, r; int m; Cell a;
78	jsr166	1.2	if ((as = cells) != null
79			\|\| ((r = doubleToRawLongBits
80			(function.applyAsDouble(longBitsToDouble(b = base), x))) != b
81			&& !casBase(b, r))) {
82	jsr166	1.1	boolean uncontended = true;
83	jsr166	1.2	if (as == null
84			\|\| (m = as.length - 1) < 0
85			\|\| (a = as[getProbe() & m]) == null
86			\|\| !(uncontended =
87			((r = doubleToRawLongBits
88			(function.applyAsDouble
89			(longBitsToDouble(v = a.value), x))) == v)
90			\|\| a.cas(v, r)))
91	jsr166	1.1	doubleAccumulate(x, function, uncontended);
92			}
93			}
94
95			/**
96			* Returns the current value. The returned value is <em>NOT</em>
97			* an atomic snapshot; invocation in the absence of concurrent
98			* updates returns an accurate result, but concurrent updates that
99			* occur while the value is being calculated might not be
100			* incorporated.
101			*
102			* @return the current value
103			*/
104			public double get() {
105			Cell[] as = cells;
106	jsr166	1.2	double result = longBitsToDouble(base);
107	jsr166	1.1	if (as != null) {
108			for (Cell a : as)
109			if (a != null)
110			result = function.applyAsDouble
111	jsr166	1.2	(result, longBitsToDouble(a.value));
112	jsr166	1.1	}
113			return result;
114			}
115
116			/**
117			* Resets variables maintaining updates to the identity value.
118			* This method may be a useful alternative to creating a new
119			* updater, but is only effective if there are no concurrent
120			* updates. Because this method is intrinsically racy, it should
121			* only be used when it is known that no threads are concurrently
122			* updating.
123			*/
124			public void reset() {
125			Cell[] as = cells;
126			base = identity;
127			if (as != null) {
128			for (Cell a : as)
129			if (a != null)
130			a.reset(identity);
131			}
132			}
133
134			/**
135			* Equivalent in effect to {@link #get} followed by {@link
136			* #reset}. This method may apply for example during quiescent
137			* points between multithreaded computations. If there are
138			* updates concurrent with this method, the returned value is
139			* <em>not</em> guaranteed to be the final value occurring before
140			* the reset.
141			*
142			* @return the value before reset
143			*/
144			public double getThenReset() {
145			Cell[] as = cells;
146	jsr166	1.2	double result = longBitsToDouble(base);
147	jsr166	1.1	base = identity;
148			if (as != null) {
149			for (Cell a : as) {
150			if (a != null) {
151	jsr166	1.2	double v = longBitsToDouble(a.value);
152	jsr166	1.1	a.reset(identity);
153			result = function.applyAsDouble(result, v);
154			}
155			}
156			}
157			return result;
158			}
159
160			/**
161			* Returns the String representation of the current value.
162			* @return the String representation of the current value
163			*/
164			public String toString() {
165			return Double.toString(get());
166			}
167
168			/**
169			* Equivalent to {@link #get}.
170			*
171			* @return the current value
172			*/
173			public double doubleValue() {
174			return get();
175			}
176
177			/**
178			* Returns the {@linkplain #get current value} as a {@code long}
179			* after a narrowing primitive conversion.
180			*/
181			public long longValue() {
182			return (long)get();
183			}
184
185			/**
186			* Returns the {@linkplain #get current value} as an {@code int}
187			* after a narrowing primitive conversion.
188			*/
189			public int intValue() {
190			return (int)get();
191			}
192
193			/**
194			* Returns the {@linkplain #get current value} as a {@code float}
195			* after a narrowing primitive conversion.
196			*/
197			public float floatValue() {
198			return (float)get();
199			}
200
201			/**
202			* Serialization proxy, used to avoid reference to the non-public
203			* Striped64 superclass in serialized forms.
204			* @serial include
205			*/
206			private static class SerializationProxy implements Serializable {
207			private static final long serialVersionUID = 7249069246863182397L;
208
209			/**
210			* The current value returned by get().
211			* @serial
212			*/
213			private final double value;
214
215			/**
216			* The function used for updates.
217			* @serial
218			*/
219			private final DoubleBinaryOperator function;
220
221			/**
222			* The identity value, represented as a long, as converted by
223			* {@link Double#doubleToRawLongBits}. The original identity
224			* can be recovered using {@link Double#longBitsToDouble}.
225			* @serial
226			*/
227			private final long identity;
228
229			SerializationProxy(double value,
230			DoubleBinaryOperator function,
231			long identity) {
232			this.value = value;
233			this.function = function;
234			this.identity = identity;
235			}
236
237			/**
238			* Returns a {@code DoubleAccumulator} object with initial state
239			* held by this proxy.
240			*
241			* @return a {@code DoubleAccumulator} object with initial state
242			* held by this proxy
243			*/
244			private Object readResolve() {
245	jsr166	1.2	double d = longBitsToDouble(identity);
246	jsr166	1.1	DoubleAccumulator a = new DoubleAccumulator(function, d);
247	jsr166	1.2	a.base = doubleToRawLongBits(value);
248	jsr166	1.1	return a;
249			}
250			}
251
252			/**
253			* Returns a
254			* <a href="../../../../serialized-form.html#java.util.concurrent.atomic.DoubleAccumulator.SerializationProxy">
255			* SerializationProxy</a>
256			* representing the state of this instance.
257			*
258			* @return a {@link SerializationProxy}
259			* representing the state of this instance
260			*/
261			private Object writeReplace() {
262			return new SerializationProxy(get(), function, identity);
263			}
264
265			/**
266			* @param s the stream
267			* @throws java.io.InvalidObjectException always
268			*/
269			private void readObject(java.io.ObjectInputStream s)
270			throws java.io.InvalidObjectException {
271			throw new java.io.InvalidObjectException("Proxy required");
272			}
273
274			}