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>
 *
 * @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:	1.5
Committed:	Sun Nov 18 03:07:22 2012 UTC (11 years, 5 months ago) by jsr166
Branch:	MAIN
Changes since 1.4:	+1 -1 lines
Log Message:	properly close <em> javadoc tags
#	User	Rev	Content
1	dl	1.1	/*
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	jsr166	1.5	* java.util.concurrent.atomic.</em>
28	dl	1.1	*
29	jsr166	1.2	* @since 1.8
30	dl	1.1	* @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	jsr166	1.3	* @return the String representation of the {@link #sum}
149	dl	1.1	*/
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	jsr166	1.4	* narrowing primitive conversion.
166	dl	1.1	*/
167			public long longValue() {
168			return (long)sum();
169			}
170
171			/**
172			* Returns the {@link #sum} as an {@code int} after a
173	jsr166	1.4	* narrowing primitive conversion.
174	dl	1.1	*/
175			public int intValue() {
176			return (int)sum();
177			}
178
179			/**
180			* Returns the {@link #sum} as a {@code float}
181	jsr166	1.4	* after a narrowing primitive conversion.
182	dl	1.1	*/
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			}