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
#	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. 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	base = this.identity = doubleToRawLongBits(identity);
69	}
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	if ((as = cells) != null
79	\|\| ((r = doubleToRawLongBits
80	(function.applyAsDouble(longBitsToDouble(b = base), x))) != b
81	&& !casBase(b, r))) {
82	boolean uncontended = true;
83	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	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	double result = longBitsToDouble(base);
107	if (as != null) {
108	for (Cell a : as)
109	if (a != null)
110	result = function.applyAsDouble
111	(result, longBitsToDouble(a.value));
112	}
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	double result = longBitsToDouble(base);
147	base = identity;
148	if (as != null) {
149	for (Cell a : as) {
150	if (a != null) {
151	double v = longBitsToDouble(a.value);
152	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	double d = longBitsToDouble(identity);
246	DoubleAccumulator a = new DoubleAccumulator(function, d);
247	a.base = doubleToRawLongBits(value);
248	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	}