src/jsr166e/DoubleAdder.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 jsr166e;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

/**
 * One or more variables that together maintain an initially zero
 * {@code double} sum.  When updates (method {@link #add}) are
 * contended across threads, the set of variables may grow dynamically
 * to reduce contention.  Method {@link #sum} (or, equivalently {@link
 * #doubleValue}) returns the current total combined across the
 * variables maintaining the sum.
 *
 * <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.
 *
 * <p><em>jsr166e note: This class is targeted to be placed in
 * java.util.concurrent.atomic.</em>
 *
 * @since 1.8
 * @author Doug Lea
 */
public class DoubleAdder extends Striped64 implements Serializable {
    private static final long serialVersionUID = 7249069246863182397L;

    /**
     * Update function. Note that we must use "long" for underlying
     * representations, because there is no compareAndSet for double,
     * due to the fact that the bitwise equals used in any CAS
     * implementation is not the same as double-precision equals.
     * However, we use CAS only to detect and alleviate contention,
     * for which bitwise equals works best anyway. In principle, the
     * long/double conversions used here should be essentially free on
     * most platforms since they just re-interpret bits.
     *
     * Similar conversions are used in other methods.
     */
    final long fn(long v, long x) {
        return Double.doubleToRawLongBits
            (Double.longBitsToDouble(v) +
             Double.longBitsToDouble(x));
    }

    /**
     * Creates a new adder with initial sum of zero.
     */
    public DoubleAdder() {
    }

    /**
     * Adds the given value.
     *
     * @param x the value to add
     */
    public void add(double x) {
        Cell[] as; long b, v; int[] hc; Cell a; int n;
        if ((as = cells) != null ||
            !casBase(b = base,
                     Double.doubleToRawLongBits
                     (Double.longBitsToDouble(b) + x))) {
            boolean uncontended = true;
            if ((hc = threadHashCode.get()) == null ||
                as == null || (n = as.length) < 1 ||
                (a = as[(n - 1) & hc[0]]) == null ||
                !(uncontended = a.cas(v = a.value,
                                      Double.doubleToRawLongBits
                                      (Double.longBitsToDouble(v) + x))))
                retryUpdate(Double.doubleToRawLongBits(x), hc, uncontended);
        }
    }

    /**
     * Returns the current sum.  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 sum is being calculated might not be
     * incorporated.  Also, because floating-point arithmetic is not
     * strictly associative, the returned result need not be identical
     * to the value that would be obtained in a sequential series of
     * updates to a single variable.
     *
     * @return the sum
     */
    public double sum() {
        Cell[] as = cells;
        double sum = Double.longBitsToDouble(base);
        if (as != null) {
            int n = as.length;
            for (int i = 0; i < n; ++i) {
                Cell a = as[i];
                if (a != null)
                    sum += Double.longBitsToDouble(a.value);
            }
        }
        return sum;
    }

    /**
     * Resets variables maintaining the sum to zero.  This method may
     * be a useful alternative to creating a new adder, 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() {
        internalReset(0L);
    }

    /**
     * Equivalent in effect to {@link #sum} 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 sum
     */
    public double sumThenReset() {
        Cell[] as = cells;
        double sum = Double.longBitsToDouble(base);
        base = 0L;
        if (as != null) {
            int n = as.length;
            for (int i = 0; i < n; ++i) {
                Cell a = as[i];
                if (a != null) {
                    long v = a.value;
                    a.value = 0L;
                    sum += Double.longBitsToDouble(v);
                }
            }
        }
        return sum;
    }

    /**
     * Returns the String representation of the {@link #sum}.
     * @return the String representation of the {@link #sum}
     */
    public String toString() {
        return Double.toString(sum());
    }

    /**
     * Equivalent to {@link #sum}.
     *
     * @return the sum
     */
    public double doubleValue() {
        return sum();
    }

    /**
     * Returns the {@link #sum} as a {@code long} after a
     * narrowing primitive conversion.
     */
    public long longValue() {
        return (long)sum();
    }

    /**
     * Returns the {@link #sum} as an {@code int} after a
     * narrowing primitive conversion.
     */
    public int intValue() {
        return (int)sum();
    }

    /**
     * Returns the {@link #sum} as a {@code float}
     * after a narrowing primitive conversion.
     */
    public float floatValue() {
        return (float)sum();
    }

    private void writeObject(ObjectOutputStream s) throws IOException {
        s.defaultWriteObject();
        s.writeDouble(sum());
    }

    private void readObject(ObjectInputStream s)
            throws IOException, ClassNotFoundException {
        s.defaultReadObject();
        busy = 0;
        cells = null;
        base = Double.doubleToRawLongBits(s.readDouble());
    }

}
Revision:	1.12
Committed:	Mon May 5 20:20:15 2014 UTC (9 years, 11 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.11:	+7 -4 lines
Log Message:	import everything to reduce diffs with guava fork
#	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 jsr166e;
8
9	import java.io.IOException;
10	import java.io.ObjectInputStream;
11	import java.io.ObjectOutputStream;
12	import java.io.Serializable;
13
14	/**
15	* One or more variables that together maintain an initially zero
16	* {@code double} sum. When updates (method {@link #add}) are
17	* contended across threads, the set of variables may grow dynamically
18	* to reduce contention. Method {@link #sum} (or, equivalently {@link
19	* #doubleValue}) returns the current total combined across the
20	* variables maintaining the sum.
21	*
22	* <p>This class extends {@link Number}, but does <em>not</em> define
23	* methods such as {@code equals}, {@code hashCode} and {@code
24	* compareTo} because instances are expected to be mutated, and so are
25	* not useful as collection keys.
26	*
27	* <p><em>jsr166e note: This class is targeted to be placed in
28	* java.util.concurrent.atomic.</em>
29	*
30	* @since 1.8
31	* @author Doug Lea
32	*/
33	public class DoubleAdder extends Striped64 implements Serializable {
34	private static final long serialVersionUID = 7249069246863182397L;
35
36	/**
37	* Update function. Note that we must use "long" for underlying
38	* representations, because there is no compareAndSet for double,
39	* due to the fact that the bitwise equals used in any CAS
40	* implementation is not the same as double-precision equals.
41	* However, we use CAS only to detect and alleviate contention,
42	* for which bitwise equals works best anyway. In principle, the
43	* long/double conversions used here should be essentially free on
44	* most platforms since they just re-interpret bits.
45	*
46	* Similar conversions are used in other methods.
47	*/
48	final long fn(long v, long x) {
49	return Double.doubleToRawLongBits
50	(Double.longBitsToDouble(v) +
51	Double.longBitsToDouble(x));
52	}
53
54	/**
55	* Creates a new adder with initial sum of zero.
56	*/
57	public DoubleAdder() {
58	}
59
60	/**
61	* Adds the given value.
62	*
63	* @param x the value to add
64	*/
65	public void add(double x) {
66	Cell[] as; long b, v; int[] hc; Cell a; int n;
67	if ((as = cells) != null \|\|
68	!casBase(b = base,
69	Double.doubleToRawLongBits
70	(Double.longBitsToDouble(b) + x))) {
71	boolean uncontended = true;
72	if ((hc = threadHashCode.get()) == null \|\|
73	as == null \|\| (n = as.length) < 1 \|\|
74	(a = as[(n - 1) & hc[0]]) == null \|\|
75	!(uncontended = a.cas(v = a.value,
76	Double.doubleToRawLongBits
77	(Double.longBitsToDouble(v) + x))))
78	retryUpdate(Double.doubleToRawLongBits(x), hc, uncontended);
79	}
80	}
81
82	/**
83	* Returns the current sum. The returned value is <em>NOT</em> an
84	* atomic snapshot; invocation in the absence of concurrent
85	* updates returns an accurate result, but concurrent updates that
86	* occur while the sum is being calculated might not be
87	* incorporated. Also, because floating-point arithmetic is not
88	* strictly associative, the returned result need not be identical
89	* to the value that would be obtained in a sequential series of
90	* updates to a single variable.
91	*
92	* @return the sum
93	*/
94	public double sum() {
95	Cell[] as = cells;
96	double sum = Double.longBitsToDouble(base);
97	if (as != null) {
98	int n = as.length;
99	for (int i = 0; i < n; ++i) {
100	Cell a = as[i];
101	if (a != null)
102	sum += Double.longBitsToDouble(a.value);
103	}
104	}
105	return sum;
106	}
107
108	/**
109	* Resets variables maintaining the sum to zero. This method may
110	* be a useful alternative to creating a new adder, but is only
111	* effective if there are no concurrent updates. Because this
112	* method is intrinsically racy, it should only be used when it is
113	* known that no threads are concurrently updating.
114	*/
115	public void reset() {
116	internalReset(0L);
117	}
118
119	/**
120	* Equivalent in effect to {@link #sum} followed by {@link
121	* #reset}. This method may apply for example during quiescent
122	* points between multithreaded computations. If there are
123	* updates concurrent with this method, the returned value is
124	* <em>not</em> guaranteed to be the final value occurring before
125	* the reset.
126	*
127	* @return the sum
128	*/
129	public double sumThenReset() {
130	Cell[] as = cells;
131	double sum = Double.longBitsToDouble(base);
132	base = 0L;
133	if (as != null) {
134	int n = as.length;
135	for (int i = 0; i < n; ++i) {
136	Cell a = as[i];
137	if (a != null) {
138	long v = a.value;
139	a.value = 0L;
140	sum += Double.longBitsToDouble(v);
141	}
142	}
143	}
144	return sum;
145	}
146
147	/**
148	* Returns the String representation of the {@link #sum}.
149	* @return the String representation of the {@link #sum}
150	*/
151	public String toString() {
152	return Double.toString(sum());
153	}
154
155	/**
156	* Equivalent to {@link #sum}.
157	*
158	* @return the sum
159	*/
160	public double doubleValue() {
161	return sum();
162	}
163
164	/**
165	* Returns the {@link #sum} as a {@code long} after a
166	* narrowing primitive conversion.
167	*/
168	public long longValue() {
169	return (long)sum();
170	}
171
172	/**
173	* Returns the {@link #sum} as an {@code int} after a
174	* narrowing primitive conversion.
175	*/
176	public int intValue() {
177	return (int)sum();
178	}
179
180	/**
181	* Returns the {@link #sum} as a {@code float}
182	* after a narrowing primitive conversion.
183	*/
184	public float floatValue() {
185	return (float)sum();
186	}
187
188	private void writeObject(ObjectOutputStream s) throws IOException {
189	s.defaultWriteObject();
190	s.writeDouble(sum());
191	}
192
193	private void readObject(ObjectInputStream s)
194	throws IOException, ClassNotFoundException {
195	s.defaultReadObject();
196	busy = 0;
197	cells = null;
198	base = Double.doubleToRawLongBits(s.readDouble());
199	}
200
201	}