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 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>
 *
 * @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
     * primitive conversion.
     */
    public long longValue() {
        return (long)sum();
    }

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

    /**
     * Returns the {@link #sum} as a {@code float}
     * after a 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:	1.1
Committed:	Tue Aug 2 18:04:12 2011 UTC (12 years, 8 months ago) by dl
Branch:	MAIN
Log Message:	Refactor and introduce new forms of update
#	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 hashCode} and {@code compareTo} because
23	* instances are expected to be mutated, and so are not useful as
24	* collection keys.
25	*
26	* <p><em>jsr166e note: This class is targeted to be placed in
27	* java.util.concurrent.atomic<em>
28	*
29	* @author Doug Lea
30	*/
31	public class DoubleAdder extends Striped64 implements Serializable {
32	private static final long serialVersionUID = 7249069246863182397L;
33
34	/**
35	* Update function. Note that we must use "long" for underlying
36	* representations, because there is no compareAndSet for double,
37	* due to the fact that the bitwise equals used in any CAS
38	* implementation is not the same as double-precision equals.
39	* However, we use CAS only to detect and alleviate contention,
40	* for which bitwise equals works best anyway. In principle, the
41	* long/double conversions used here should be essentially free on
42	* most platforms since they just re-interpret bits.
43	*
44	* Similar conversions are used in other methods.
45	*/
46	final long fn(long v, long x) {
47	return Double.doubleToRawLongBits
48	(Double.longBitsToDouble(v) +
49	Double.longBitsToDouble(x));
50	}
51
52	/**
53	* Creates a new adder with initial sum of zero.
54	*/
55	public DoubleAdder() {
56	}
57
58	/**
59	* Adds the given value.
60	*
61	* @param x the value to add
62	*/
63	public void add(double x) {
64	Cell[] as; long b, v; HashCode hc; Cell a; int n;
65	if ((as = cells) != null \|\|
66	!casBase(b = base,
67	Double.doubleToRawLongBits
68	(Double.longBitsToDouble(b) + x))) {
69	boolean uncontended = true;
70	int h = (hc = threadHashCode.get()).code;
71	if (as == null \|\| (n = as.length) < 1 \|\|
72	(a = as[(n - 1) & h]) == null \|\|
73	!(uncontended = a.cas(v = a.value,
74	Double.doubleToRawLongBits
75	(Double.longBitsToDouble(v) + x))))
76	retryUpdate(Double.doubleToRawLongBits(x), hc, uncontended);
77	}
78	}
79
80	/**
81	* Returns the current sum. The returned value is <em>NOT</em> an
82	* atomic snapshot: Invocation in the absence of concurrent
83	* updates returns an accurate result, but concurrent updates that
84	* occur while the sum is being calculated might not be
85	* incorporated. Also, because double-precision arithmetic is not
86	* strictly associative, the returned result need not be identical
87	* to the value that would be obtained in a sequential series of
88	* updates to a single variable.
89	*
90	* @return the sum
91	*/
92	public double sum() {
93	Cell[] as = cells;
94	double sum = Double.longBitsToDouble(base);
95	if (as != null) {
96	int n = as.length;
97	for (int i = 0; i < n; ++i) {
98	Cell a = as[i];
99	if (a != null)
100	sum += Double.longBitsToDouble(a.value);
101	}
102	}
103	return sum;
104	}
105
106	/**
107	* Resets variables maintaining the sum to zero. This method may
108	* be a useful alternative to creating a new adder, but is only
109	* effective if there are no concurrent updates. Because this
110	* method is intrinsically racy, it should only be used when it is
111	* known that no threads are concurrently updating.
112	*/
113	public void reset() {
114	internalReset(0L);
115	}
116
117	/**
118	* Equivalent in effect to {@link #sum} followed by {@link
119	* #reset}. This method may apply for example during quiescent
120	* points between multithreaded computations. If there are
121	* updates concurrent with this method, the returned value is
122	* <em>not</em> guaranteed to be the final value occurring before
123	* the reset.
124	*
125	* @return the sum
126	*/
127	public double sumThenReset() {
128	Cell[] as = cells;
129	double sum = Double.longBitsToDouble(base);
130	base = 0L;
131	if (as != null) {
132	int n = as.length;
133	for (int i = 0; i < n; ++i) {
134	Cell a = as[i];
135	if (a != null) {
136	long v = a.value;
137	a.value = 0L;
138	sum += Double.longBitsToDouble(v);
139	}
140	}
141	}
142	return sum;
143	}
144
145	/**
146	* Returns the String representation of the {@link #sum}.
147	* @return the String representation of the {@link #sum}.
148	*/
149	public String toString() {
150	return Double.toString(sum());
151	}
152
153	/**
154	* Equivalent to {@link #sum}.
155	*
156	* @return the sum
157	*/
158	public double doubleValue() {
159	return sum();
160	}
161
162	/**
163	* Returns the {@link #sum} as a {@code long} after a
164	* primitive conversion.
165	*/
166	public long longValue() {
167	return (long)sum();
168	}
169
170	/**
171	* Returns the {@link #sum} as an {@code int} after a
172	* primitive conversion.
173	*/
174	public int intValue() {
175	return (int)sum();
176	}
177
178	/**
179	* Returns the {@link #sum} as a {@code float}
180	* after a primitive conversion.
181	*/
182	public float floatValue() {
183	return (float)sum();
184	}
185
186	private void writeObject(java.io.ObjectOutputStream s)
187	throws java.io.IOException {
188	s.defaultWriteObject();
189	s.writeDouble(sum());
190	}
191
192	private void readObject(ObjectInputStream s)
193	throws IOException, ClassNotFoundException {
194	s.defaultReadObject();
195	busy = 0;
196	cells = null;
197	base = Double.doubleToRawLongBits(s.readDouble());
198	}
199
200	}