1 /*
2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation. Oracle designates this
7 * particular file as subject to the "Classpath" exception as provided
8 * by Oracle in the LICENSE file that accompanied this code.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 */
24
25 /*
26 * This file is available under and governed by the GNU General Public
27 * License version 2 only, as published by the Free Software Foundation.
28 * However, the following notice accompanied the original version of this
29 * file:
30 *
31 * Written by Doug Lea with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/publicdomain/zero/1.0/
34 */
35
36 package java.util.concurrent.atomic;
37 import java.io.IOException;
38 import java.io.Serializable;
39 import java.io.ObjectInputStream;
40 import java.io.ObjectOutputStream;
41 import java.util.function.DoubleBinaryOperator;
42
43 /**
44 * One or more variables that together maintain a running {@code double}
45 * value updated using a supplied function. When updates (method
46 * {@link #accumulate}) are contended across threads, the set of variables
47 * may grow dynamically to reduce contention. Method {@link #get}
48 * (or, equivalently, {@link #doubleValue}) returns the current value
49 * across the variables maintaining updates.
50 *
51 * <p>The supplied accumulator function must be side-effect-free. It
52 * may be re-applied when attempted updates fail due to contention
53 * among threads. The function is applied with the current value as
54 * its first argument, and the given update as the second argument.
55 * For example, to maintain a running maximum value, you could supply
56 * {@code (x, y) -> (y > x) ? y : x} along with {@code
57 * Double.MINIMUM_VALUE} as the identity. (Class {@link DoubleAdder}
58 * provides analogs of the functionality of this class for the common
59 * special case of maintaining sums.)
60 *
61 * <p>This class extends {@link Number}, but does <em>not</em> define
62 * methods such as {@code hashCode} and {@code compareTo} because
63 * instances are expected to be mutated, and so are not useful as
64 * collection keys.
65 *
66 * @since 1.8
67 * @author Doug Lea
68 */
69 public class DoubleAccumulator extends Striped64 implements Serializable {
70 private static final long serialVersionUID = 7249069246863182397L;
71
72 private final DoubleBinaryOperator function;
73 private final long identity; // use long representation
74
75 /**
76 * Creates a new instance using the given accumulator function
77 * and identity element.
78 */
79 public DoubleAccumulator(DoubleBinaryOperator accumulatorFunction,
80 double identity) {
81 this.function = accumulatorFunction;
82 base = this.identity = Double.doubleToRawLongBits(identity);
83 }
84
85 /**
86 * Updates with the given value.
87 *
88 * @param x the value
89 */
90 public void accumulate(double x) {
91 Cell[] as; long b, v, r; CellHashCode hc; Cell a; int m;
92 if ((as = cells) != null ||
93 (r = Double.doubleToRawLongBits
94 (function.operateAsDouble // ## rename when JDK8 syncs with lambda, applyAsDouble
95 (Double.longBitsToDouble(b = base), x))) != b && !casBase(b, r)) {
96 boolean uncontended = true;
97 if ((hc = threadCellHashCode.get()) == null ||
98 as == null || (m = as.length - 1) < 0 ||
99 (a = as[m & hc.code]) == null ||
100 !(uncontended =
101 (r = Double.doubleToRawLongBits
102 (function.operateAsDouble // ## rename when JDK8 syncs with lambda, applyAsDouble
103 (Double.longBitsToDouble(v = a.value), x))) == v ||
104 a.cas(v, r)))
105 doubleAccumulate(x, hc, function, uncontended);
106 }
107 }
108
109 /**
110 * Returns the current value. The returned value is
111 * <em>NOT</em> an atomic snapshot: Invocation in the absence of
112 * concurrent updates returns an accurate result, but concurrent
113 * updates that occur while the value is being calculated might
114 * not be incorporated.
115 *
116 * @return the current value
117 */
118 public double get() {
119 Cell[] as = cells; Cell a;
120 double result = Double.longBitsToDouble(base);
121 if (as != null) {
122 for (int i = 0; i < as.length; ++i) {
123 if ((a = as[i]) != null)
124 result = function.operateAsDouble // ## rename when JDK8 syncs with lambda, applyAsDouble
125 (result, Double.longBitsToDouble(a.value));
126 }
127 }
128 return result;
129 }
130
131 /**
132 * Resets variables maintaining updates the given value. This
133 * method may be a useful alternative to creating a new updater,
134 * but is only effective if there are no concurrent updates.
135 * Because this method is intrinsically racy, it should only be
136 * used when it is known that no threads are concurrently
137 * updating.
138 */
139 public void reset() {
140 Cell[] as = cells; Cell a;
141 base = identity;
142 if (as != null) {
143 for (int i = 0; i < as.length; ++i) {
144 if ((a = as[i]) != null)
145 a.value = identity;
146 }
147 }
148 }
149
150 /**
151 * Equivalent in effect to {@link #get} followed by {@link
152 * #reset}. This method may apply for example during quiescent
153 * points between multithreaded computations. If there are
154 * updates concurrent with this method, the returned value is
155 * <em>not</em> guaranteed to be the final value occurring before
156 * the reset.
157 *
158 * @return the value before reset
159 */
160 public double getThenReset() {
161 Cell[] as = cells; Cell a;
162 double result = Double.longBitsToDouble(base);
163 base = identity;
164 if (as != null) {
165 for (int i = 0; i < as.length; ++i) {
166 if ((a = as[i]) != null) {
167 double v = Double.longBitsToDouble(a.value);
168 a.value = identity;
169 result = function.operateAsDouble(result, v); // ## rename when JDK8 syncs with lambda, applyAsDouble
170 }
171 }
172 }
173 return result;
174 }
175
176 /**
177 * Returns the String representation of the {@link #sum}.
178 * @return the String representation of the {@link #sum}
179 */
180 public String toString() {
181 return Double.toString(get());
182 }
183
184 /**
185 * Equivalent to {@link #sum}.
186 *
187 * @return the sum
188 */
189 public double doubleValue() {
190 return get();
191 }
192
193 /**
194 * Returns the {@link #sum} as a {@code long} after a
195 * narrowing primitive conversion.
196 */
197 public long longValue() {
198 return (long)get();
199 }
200
201 /**
202 * Returns the {@link #sum} as an {@code int} after a
203 * narrowing primitive conversion.
204 */
205 public int intValue() {
206 return (int)get();
207 }
208
209 /**
210 * Returns the {@link #sum} as a {@code float}
211 * after a narrowing primitive conversion.
212 */
213 public float floatValue() {
214 return (float)get();
215 }
216
217 private void writeObject(java.io.ObjectOutputStream s)
218 throws java.io.IOException {
219 s.defaultWriteObject();
220 s.writeDouble(get());
221 }
222
223 private void readObject(ObjectInputStream s)
224 throws IOException, ClassNotFoundException {
225 s.defaultReadObject();
226 cellsBusy = 0;
227 cells = null;
228 base = Double.doubleToRawLongBits(s.readDouble());
229 }
230
231 }