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

/**
 * 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 double-precision 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(ObjectInputStream s)
        throws IOException, ClassNotFoundException {
        s.defaultReadObject();
        busy = 0;
        cells = null;
        base = Double.doubleToRawLongBits(s.readDouble());
    }

}
Revision:	1.7
Committed:	Sun Jan 6 18:50:00 2013 UTC (11 years, 3 months ago) by jsr166
Branch:	MAIN
Changes since 1.6:	+1 -1 lines
Log Message:	javadoc style
#	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.IOException;
9	import java.io.Serializable;
10	import java.io.ObjectInputStream;
11	import java.io.ObjectOutputStream;
12
13	/**
14	* One or more variables that together maintain an initially zero
15	* {@code double} sum. When updates (method {@link #add}) are
16	* contended across threads, the set of variables may grow dynamically
17	* to reduce contention. Method {@link #sum} (or, equivalently {@link
18	* #doubleValue}) returns the current total combined across the
19	* variables maintaining the sum.
20	*
21	* <p>This class extends {@link Number}, but does <em>not</em> define
22	* methods such as {@code equals}, {@code hashCode} and {@code
23	* compareTo} because instances are expected to be mutated, and so are
24	* not useful as collection keys.
25	*
26	* <p><em>jsr166e note: This class is targeted to be placed in
27	* java.util.concurrent.atomic.</em>
28	*
29	* @since 1.8
30	* @author Doug Lea
31	*/
32	public class DoubleAdder extends Striped64 implements Serializable {
33	private static final long serialVersionUID = 7249069246863182397L;
34
35	/**
36	* Update function. Note that we must use "long" for underlying
37	* representations, because there is no compareAndSet for double,
38	* due to the fact that the bitwise equals used in any CAS
39	* implementation is not the same as double-precision equals.
40	* However, we use CAS only to detect and alleviate contention,
41	* for which bitwise equals works best anyway. In principle, the
42	* long/double conversions used here should be essentially free on
43	* most platforms since they just re-interpret bits.
44	*
45	* Similar conversions are used in other methods.
46	*/
47	final long fn(long v, long x) {
48	return Double.doubleToRawLongBits
49	(Double.longBitsToDouble(v) +
50	Double.longBitsToDouble(x));
51	}
52
53	/**
54	* Creates a new adder with initial sum of zero.
55	*/
56	public DoubleAdder() {
57	}
58
59	/**
60	* Adds the given value.
61	*
62	* @param x the value to add
63	*/
64	public void add(double x) {
65	Cell[] as; long b, v; HashCode hc; Cell a; int n;
66	if ((as = cells) != null \|\|
67	!casBase(b = base,
68	Double.doubleToRawLongBits
69	(Double.longBitsToDouble(b) + x))) {
70	boolean uncontended = true;
71	int h = (hc = threadHashCode.get()).code;
72	if (as == null \|\| (n = as.length) < 1 \|\|
73	(a = as[(n - 1) & h]) == null \|\|
74	!(uncontended = a.cas(v = a.value,
75	Double.doubleToRawLongBits
76	(Double.longBitsToDouble(v) + x))))
77	retryUpdate(Double.doubleToRawLongBits(x), hc, uncontended);
78	}
79	}
80
81	/**
82	* Returns the current sum. The returned value is <em>NOT</em> an
83	* atomic snapshot: invocation in the absence of concurrent
84	* updates returns an accurate result, but concurrent updates that
85	* occur while the sum is being calculated might not be
86	* incorporated. Also, because double-precision arithmetic is not
87	* strictly associative, the returned result need not be identical
88	* to the value that would be obtained in a sequential series of
89	* updates to a single variable.
90	*
91	* @return the sum
92	*/
93	public double sum() {
94	Cell[] as = cells;
95	double sum = Double.longBitsToDouble(base);
96	if (as != null) {
97	int n = as.length;
98	for (int i = 0; i < n; ++i) {
99	Cell a = as[i];
100	if (a != null)
101	sum += Double.longBitsToDouble(a.value);
102	}
103	}
104	return sum;
105	}
106
107	/**
108	* Resets variables maintaining the sum to zero. This method may
109	* be a useful alternative to creating a new adder, but is only
110	* effective if there are no concurrent updates. Because this
111	* method is intrinsically racy, it should only be used when it is
112	* known that no threads are concurrently updating.
113	*/
114	public void reset() {
115	internalReset(0L);
116	}
117
118	/**
119	* Equivalent in effect to {@link #sum} followed by {@link
120	* #reset}. This method may apply for example during quiescent
121	* points between multithreaded computations. If there are
122	* updates concurrent with this method, the returned value is
123	* <em>not</em> guaranteed to be the final value occurring before
124	* the reset.
125	*
126	* @return the sum
127	*/
128	public double sumThenReset() {
129	Cell[] as = cells;
130	double sum = Double.longBitsToDouble(base);
131	base = 0L;
132	if (as != null) {
133	int n = as.length;
134	for (int i = 0; i < n; ++i) {
135	Cell a = as[i];
136	if (a != null) {
137	long v = a.value;
138	a.value = 0L;
139	sum += Double.longBitsToDouble(v);
140	}
141	}
142	}
143	return sum;
144	}
145
146	/**
147	* Returns the String representation of the {@link #sum}.
148	* @return the String representation of the {@link #sum}
149	*/
150	public String toString() {
151	return Double.toString(sum());
152	}
153
154	/**
155	* Equivalent to {@link #sum}.
156	*
157	* @return the sum
158	*/
159	public double doubleValue() {
160	return sum();
161	}
162
163	/**
164	* Returns the {@link #sum} as a {@code long} after a
165	* narrowing primitive conversion.
166	*/
167	public long longValue() {
168	return (long)sum();
169	}
170
171	/**
172	* Returns the {@link #sum} as an {@code int} after a
173	* narrowing primitive conversion.
174	*/
175	public int intValue() {
176	return (int)sum();
177	}
178
179	/**
180	* Returns the {@link #sum} as a {@code float}
181	* after a narrowing primitive conversion.
182	*/
183	public float floatValue() {
184	return (float)sum();
185	}
186
187	private void writeObject(java.io.ObjectOutputStream s)
188	throws java.io.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	}