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
#	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			* 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			}