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.  For predictable results, the accumulator
 * function should be commutative and associative within the floating
 * point tolerance required in usage contexts. The function is applied
 * with an existing value (or identity) as one argument, and a given
 * update as the other 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;

    @SuppressWarnings("serial") // Not statically typed as Serializable
    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[] cs; long b, v, r; int m; Cell c;
        if ((cs = cells) != null
            || ((r = doubleToRawLongBits
                (function.applyAsDouble(longBitsToDouble(b = base), x))) != b
                && !casBase(b, r))) {
            int index = getProbe();
            boolean uncontended = true;
            if (cs == null
                || (m = cs.length - 1) < 0
                || (c = cs[index & m]) == null
                || !(uncontended =
                     ((r = doubleToRawLongBits
                       (function.applyAsDouble
                        (longBitsToDouble(v = c.value), x))) == v)
                     || c.cas(v, r)))
                doubleAccumulate(x, function, uncontended, index);
        }
    }

    /**
     * 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[] cs = cells;
        double result = longBitsToDouble(base);
        if (cs != null) {
            for (Cell c : cs)
                if (c != null)
                    result = function.applyAsDouble
                        (result, longBitsToDouble(c.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[] cs = cells;
        base = identity;
        if (cs != null) {
            for (Cell c : cs)
                if (c != null)
                    c.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[] cs = cells;
        double result = longBitsToDouble(getAndSetBase(identity));
        if (cs != null) {
            for (Cell c : cs) {
                if (c != null) {
                    double v = longBitsToDouble(c.getAndSet(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
         */
        @SuppressWarnings("serial") // Not statically typed as Serializable
        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="{@docRoot}/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:	1.44
Committed:	Fri Nov 27 17:43:34 2020 UTC (3 years, 6 months ago) by dl
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.43:	+3 -2 lines
Log Message:	Incorporate snippets code improvements from Pavel Rappo
#	Content
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	import static java.lang.Double.doubleToRawLongBits;
10	import static java.lang.Double.longBitsToDouble;
11
12	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. For predictable results, the accumulator
31	* function should be commutative and associative within the floating
32	* point tolerance required in usage contexts. The function is applied
33	* with an existing value (or identity) as one argument, and a given
34	* update as the other argument. For example, to maintain a running
35	* maximum value, you could supply {@code Double::max} along with
36	* {@code Double.NEGATIVE_INFINITY} as the identity. The order of
37	* accumulation within or across threads is not guaranteed. Thus, this
38	* class may not be applicable if numerical stability is required,
39	* especially when combining values of substantially different orders
40	* of magnitude.
41	*
42	* <p>Class {@link DoubleAdder} provides analogs of the functionality
43	* of this class for the common special case of maintaining sums. The
44	* call {@code new DoubleAdder()} is equivalent to {@code new
45	* DoubleAccumulator((x, y) -> x + y, 0.0)}.
46	*
47	* <p>This class extends {@link Number}, but does <em>not</em> define
48	* methods such as {@code equals}, {@code hashCode} and {@code
49	* compareTo} because instances are expected to be mutated, and so are
50	* not useful as collection keys.
51	*
52	* @since 1.8
53	* @author Doug Lea
54	*/
55	public class DoubleAccumulator extends Striped64 implements Serializable {
56	private static final long serialVersionUID = 7249069246863182397L;
57
58	@SuppressWarnings("serial") // Not statically typed as Serializable
59	private final DoubleBinaryOperator function;
60	private final long identity; // use long representation
61
62	/**
63	* Creates a new instance using the given accumulator function
64	* and identity element.
65	* @param accumulatorFunction a side-effect-free function of two arguments
66	* @param identity identity (initial value) for the accumulator function
67	*/
68	public DoubleAccumulator(DoubleBinaryOperator accumulatorFunction,
69	double identity) {
70	this.function = accumulatorFunction;
71	base = this.identity = doubleToRawLongBits(identity);
72	}
73
74	/**
75	* Updates with the given value.
76	*
77	* @param x the value
78	*/
79	public void accumulate(double x) {
80	Cell[] cs; long b, v, r; int m; Cell c;
81	if ((cs = cells) != null
82	\|\| ((r = doubleToRawLongBits
83	(function.applyAsDouble(longBitsToDouble(b = base), x))) != b
84	&& !casBase(b, r))) {
85	int index = getProbe();
86	boolean uncontended = true;
87	if (cs == null
88	\|\| (m = cs.length - 1) < 0
89	\|\| (c = cs[index & m]) == null
90	\|\| !(uncontended =
91	((r = doubleToRawLongBits
92	(function.applyAsDouble
93	(longBitsToDouble(v = c.value), x))) == v)
94	\|\| c.cas(v, r)))
95	doubleAccumulate(x, function, uncontended, index);
96	}
97	}
98
99	/**
100	* Returns the current value. The returned value is <em>NOT</em>
101	* an atomic snapshot; invocation in the absence of concurrent
102	* updates returns an accurate result, but concurrent updates that
103	* occur while the value is being calculated might not be
104	* incorporated.
105	*
106	* @return the current value
107	*/
108	public double get() {
109	Cell[] cs = cells;
110	double result = longBitsToDouble(base);
111	if (cs != null) {
112	for (Cell c : cs)
113	if (c != null)
114	result = function.applyAsDouble
115	(result, longBitsToDouble(c.value));
116	}
117	return result;
118	}
119
120	/**
121	* Resets variables maintaining updates to the identity value.
122	* This method may be a useful alternative to creating a new
123	* updater, but is only effective if there are no concurrent
124	* updates. Because this method is intrinsically racy, it should
125	* only be used when it is known that no threads are concurrently
126	* updating.
127	*/
128	public void reset() {
129	Cell[] cs = cells;
130	base = identity;
131	if (cs != null) {
132	for (Cell c : cs)
133	if (c != null)
134	c.reset(identity);
135	}
136	}
137
138	/**
139	* Equivalent in effect to {@link #get} followed by {@link
140	* #reset}. This method may apply for example during quiescent
141	* points between multithreaded computations. If there are
142	* updates concurrent with this method, the returned value is
143	* <em>not</em> guaranteed to be the final value occurring before
144	* the reset.
145	*
146	* @return the value before reset
147	*/
148	public double getThenReset() {
149	Cell[] cs = cells;
150	double result = longBitsToDouble(getAndSetBase(identity));
151	if (cs != null) {
152	for (Cell c : cs) {
153	if (c != null) {
154	double v = longBitsToDouble(c.getAndSet(identity));
155	result = function.applyAsDouble(result, v);
156	}
157	}
158	}
159	return result;
160	}
161
162	/**
163	* Returns the String representation of the current value.
164	* @return the String representation of the current value
165	*/
166	public String toString() {
167	return Double.toString(get());
168	}
169
170	/**
171	* Equivalent to {@link #get}.
172	*
173	* @return the current value
174	*/
175	public double doubleValue() {
176	return get();
177	}
178
179	/**
180	* Returns the {@linkplain #get current value} as a {@code long}
181	* after a narrowing primitive conversion.
182	*/
183	public long longValue() {
184	return (long)get();
185	}
186
187	/**
188	* Returns the {@linkplain #get current value} as an {@code int}
189	* after a narrowing primitive conversion.
190	*/
191	public int intValue() {
192	return (int)get();
193	}
194
195	/**
196	* Returns the {@linkplain #get current value} as a {@code float}
197	* after a narrowing primitive conversion.
198	*/
199	public float floatValue() {
200	return (float)get();
201	}
202
203	/**
204	* Serialization proxy, used to avoid reference to the non-public
205	* Striped64 superclass in serialized forms.
206	* @serial include
207	*/
208	private static class SerializationProxy implements Serializable {
209	private static final long serialVersionUID = 7249069246863182397L;
210
211	/**
212	* The current value returned by get().
213	* @serial
214	*/
215	private final double value;
216
217	/**
218	* The function used for updates.
219	* @serial
220	*/
221	@SuppressWarnings("serial") // Not statically typed as Serializable
222	private final DoubleBinaryOperator function;
223
224	/**
225	* The identity value, represented as a long, as converted by
226	* {@link Double#doubleToRawLongBits}. The original identity
227	* can be recovered using {@link Double#longBitsToDouble}.
228	* @serial
229	*/
230	private final long identity;
231
232	SerializationProxy(double value,
233	DoubleBinaryOperator function,
234	long identity) {
235	this.value = value;
236	this.function = function;
237	this.identity = identity;
238	}
239
240	/**
241	* Returns a {@code DoubleAccumulator} object with initial state
242	* held by this proxy.
243	*
244	* @return a {@code DoubleAccumulator} object with initial state
245	* held by this proxy
246	*/
247	private Object readResolve() {
248	double d = longBitsToDouble(identity);
249	DoubleAccumulator a = new DoubleAccumulator(function, d);
250	a.base = doubleToRawLongBits(value);
251	return a;
252	}
253	}
254
255	/**
256	* Returns a
257	* <a href="{@docRoot}/serialized-form.html#java.util.concurrent.atomic.DoubleAccumulator.SerializationProxy">
258	* SerializationProxy</a>
259	* representing the state of this instance.
260	*
261	* @return a {@link SerializationProxy}
262	* representing the state of this instance
263	*/
264	private Object writeReplace() {
265	return new SerializationProxy(get(), function, identity);
266	}
267
268	/**
269	* @param s the stream
270	* @throws java.io.InvalidObjectException always
271	*/
272	private void readObject(java.io.ObjectInputStream s)
273	throws java.io.InvalidObjectException {
274	throw new java.io.InvalidObjectException("Proxy required");
275	}
276
277	}