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.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; HashCode hc; Cell a; int n;
        if ((as = cells) != null ||
            !casBase(b = base,
                     Double.doubleToRawLongBits
                     (Double.longBitsToDouble(b) + x))) {
            boolean uncontended = true;
            int h = (hc = threadHashCode.get()).code;
            if (as == null || (n = as.length) < 1 ||
                (a = as[(n - 1) & h]) == 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(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        s.defaultWriteObject();
        s.writeDouble(sum());
    }

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

}
Revision:	1.10
Committed:	Mon Jan 14 20:30:22 2013 UTC (11 years, 3 months ago) by jsr166
Branch:	MAIN
Changes since 1.9:	+1 -1 lines
Log Message:	javadoc wordsmithing
#	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	import java.io.Serializable;
9
10	/**
11	* One or more variables that together maintain an initially zero
12	* {@code double} sum. When updates (method {@link #add}) are
13	* contended across threads, the set of variables may grow dynamically
14	* to reduce contention. Method {@link #sum} (or, equivalently {@link
15	* #doubleValue}) returns the current total combined across the
16	* variables maintaining the sum.
17	*
18	* <p>This class extends {@link Number}, but does <em>not</em> define
19	* methods such as {@code equals}, {@code hashCode} and {@code
20	* compareTo} because instances are expected to be mutated, and so are
21	* not useful as collection keys.
22	*
23	* <p><em>jsr166e note: This class is targeted to be placed in
24	* java.util.concurrent.atomic.</em>
25	*
26	* @since 1.8
27	* @author Doug Lea
28	*/
29	public class DoubleAdder extends Striped64 implements Serializable {
30	private static final long serialVersionUID = 7249069246863182397L;
31
32	/**
33	* Update function. Note that we must use "long" for underlying
34	* representations, because there is no compareAndSet for double,
35	* due to the fact that the bitwise equals used in any CAS
36	* implementation is not the same as double-precision equals.
37	* However, we use CAS only to detect and alleviate contention,
38	* for which bitwise equals works best anyway. In principle, the
39	* long/double conversions used here should be essentially free on
40	* most platforms since they just re-interpret bits.
41	*
42	* Similar conversions are used in other methods.
43	*/
44	final long fn(long v, long x) {
45	return Double.doubleToRawLongBits
46	(Double.longBitsToDouble(v) +
47	Double.longBitsToDouble(x));
48	}
49
50	/**
51	* Creates a new adder with initial sum of zero.
52	*/
53	public DoubleAdder() {
54	}
55
56	/**
57	* Adds the given value.
58	*
59	* @param x the value to add
60	*/
61	public void add(double x) {
62	Cell[] as; long b, v; HashCode hc; Cell a; int n;
63	if ((as = cells) != null \|\|
64	!casBase(b = base,
65	Double.doubleToRawLongBits
66	(Double.longBitsToDouble(b) + x))) {
67	boolean uncontended = true;
68	int h = (hc = threadHashCode.get()).code;
69	if (as == null \|\| (n = as.length) < 1 \|\|
70	(a = as[(n - 1) & h]) == null \|\|
71	!(uncontended = a.cas(v = a.value,
72	Double.doubleToRawLongBits
73	(Double.longBitsToDouble(v) + x))))
74	retryUpdate(Double.doubleToRawLongBits(x), hc, uncontended);
75	}
76	}
77
78	/**
79	* Returns the current sum. The returned value is <em>NOT</em> an
80	* atomic snapshot; invocation in the absence of concurrent
81	* updates returns an accurate result, but concurrent updates that
82	* occur while the sum is being calculated might not be
83	* incorporated. Also, because floating-point arithmetic is not
84	* strictly associative, the returned result need not be identical
85	* to the value that would be obtained in a sequential series of
86	* updates to a single variable.
87	*
88	* @return the sum
89	*/
90	public double sum() {
91	Cell[] as = cells;
92	double sum = Double.longBitsToDouble(base);
93	if (as != null) {
94	int n = as.length;
95	for (int i = 0; i < n; ++i) {
96	Cell a = as[i];
97	if (a != null)
98	sum += Double.longBitsToDouble(a.value);
99	}
100	}
101	return sum;
102	}
103
104	/**
105	* Resets variables maintaining the sum to zero. This method may
106	* be a useful alternative to creating a new adder, but is only
107	* effective if there are no concurrent updates. Because this
108	* method is intrinsically racy, it should only be used when it is
109	* known that no threads are concurrently updating.
110	*/
111	public void reset() {
112	internalReset(0L);
113	}
114
115	/**
116	* Equivalent in effect to {@link #sum} followed by {@link
117	* #reset}. This method may apply for example during quiescent
118	* points between multithreaded computations. If there are
119	* updates concurrent with this method, the returned value is
120	* <em>not</em> guaranteed to be the final value occurring before
121	* the reset.
122	*
123	* @return the sum
124	*/
125	public double sumThenReset() {
126	Cell[] as = cells;
127	double sum = Double.longBitsToDouble(base);
128	base = 0L;
129	if (as != null) {
130	int n = as.length;
131	for (int i = 0; i < n; ++i) {
132	Cell a = as[i];
133	if (a != null) {
134	long v = a.value;
135	a.value = 0L;
136	sum += Double.longBitsToDouble(v);
137	}
138	}
139	}
140	return sum;
141	}
142
143	/**
144	* Returns the String representation of the {@link #sum}.
145	* @return the String representation of the {@link #sum}
146	*/
147	public String toString() {
148	return Double.toString(sum());
149	}
150
151	/**
152	* Equivalent to {@link #sum}.
153	*
154	* @return the sum
155	*/
156	public double doubleValue() {
157	return sum();
158	}
159
160	/**
161	* Returns the {@link #sum} as a {@code long} after a
162	* narrowing primitive conversion.
163	*/
164	public long longValue() {
165	return (long)sum();
166	}
167
168	/**
169	* Returns the {@link #sum} as an {@code int} after a
170	* narrowing primitive conversion.
171	*/
172	public int intValue() {
173	return (int)sum();
174	}
175
176	/**
177	* Returns the {@link #sum} as a {@code float}
178	* after a narrowing primitive conversion.
179	*/
180	public float floatValue() {
181	return (float)sum();
182	}
183
184	private void writeObject(java.io.ObjectOutputStream s)
185	throws java.io.IOException {
186	s.defaultWriteObject();
187	s.writeDouble(sum());
188	}
189
190	private void readObject(java.io.ObjectInputStream s)
191	throws java.io.IOException, ClassNotFoundException {
192	s.defaultReadObject();
193	busy = 0;
194	cells = null;
195	base = Double.doubleToRawLongBits(s.readDouble());
196	}
197
198	}