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.Serializable;
38 import java.util.function.DoubleBinaryOperator;
39
40 /**
41 * One or more variables that together maintain a running {@code double}
42 * value updated using a supplied function. When updates (method
43 * {@link #accumulate}) are contended across threads, the set of variables
44 * may grow dynamically to reduce contention. Method {@link #get}
45 * (or, equivalently, {@link #doubleValue}) returns the current value
46 * across the variables maintaining updates.
47 *
48 * <p>This class is usually preferable to alternatives when multiple
49 * threads update a common value that is used for purposes such as
50 * summary statistics that are frequently updated but less frequently
51 * read.
52 *
53 * <p>The supplied accumulator function should be side-effect-free,
54 * since it may be re-applied when attempted updates fail due to
55 * contention among threads. The function is applied with the current
56 * value as its first argument, and the given update as the second
57 * argument. For example, to maintain a running maximum value, you
58 * could supply {@code Double::max} along with {@code
59 * Double.NEGATIVE_INFINITY} as the identity. The order of
60 * accumulation within or across threads is not guaranteed. Thus, this
61 * class may not be applicable if numerical stability is required,
62 * especially when combining values of substantially different orders
63 * of magnitude.
64 *
65 * <p>Class {@link DoubleAdder} provides analogs of the functionality
66 * of this class for the common special case of maintaining sums. The
67 * call {@code new DoubleAdder()} is equivalent to {@code new
68 * DoubleAccumulator((x, y) -> x + y, 0.0}.
69 *
70 * <p>This class extends {@link Number}, but does <em>not</em> define
71 * methods such as {@code equals}, {@code hashCode} and {@code
72 * compareTo} because instances are expected to be mutated, and so are
73 * not useful as collection keys.
74 *
75 * @since 1.8
76 * @author Doug Lea
77 */
78 public class DoubleAccumulator extends Striped64 implements Serializable {
79 private static final long serialVersionUID = 7249069246863182397L;
80
81 private final DoubleBinaryOperator function;
82 private final long identity; // use long representation
83
84 /**
85 * Creates a new instance using the given accumulator function
86 * and identity element.
87 */
88 public DoubleAccumulator(DoubleBinaryOperator accumulatorFunction,
89 double identity) {
90 this.function = accumulatorFunction;
91 base = this.identity = Double.doubleToRawLongBits(identity);
92 }
93
94 /**
95 * Updates with the given value.
96 *
97 * @param x the value
98 */
99 public void accumulate(double x) {
100 Cell[] as; long b, v, r; int m; Cell a;
101 if ((as = cells) != null ||
102 (r = Double.doubleToRawLongBits
103 (function.operateAsDouble
104 (Double.longBitsToDouble(b = base), x))) != b && !casBase(b, r)) {
105 boolean uncontended = true;
106 if (as == null || (m = as.length - 1) < 0 ||
107 (a = as[getProbe() & m]) == null ||
108 !(uncontended =
109 (r = Double.doubleToRawLongBits
110 (function.operateAsDouble
111 (Double.longBitsToDouble(v = a.value), x))) == v ||
112 a.cas(v, r)))
113 doubleAccumulate(x, function, uncontended);
114 }
115 }
116
117 /**
118 * Returns the current value. The returned value is <em>NOT</em>
119 * an atomic snapshot; invocation in the absence of concurrent
120 * updates returns an accurate result, but concurrent updates that
121 * occur while the value is being calculated might not be
122 * incorporated.
123 *
124 * @return the current value
125 */
126 public double get() {
127 Cell[] as = cells; Cell a;
128 double result = Double.longBitsToDouble(base);
129 if (as != null) {
130 for (int i = 0; i < as.length; ++i) {
131 if ((a = as[i]) != null)
132 result = function.operateAsDouble
133 (result, Double.longBitsToDouble(a.value));
134 }
135 }
136 return result;
137 }
138
139 /**
140 * Resets variables maintaining updates to the identity value.
141 * This method may be a useful alternative to creating a new
142 * updater, but is only effective if there are no concurrent
143 * updates. Because this method is intrinsically racy, it should
144 * only be used when it is known that no threads are concurrently
145 * updating.
146 */
147 public void reset() {
148 Cell[] as = cells; Cell a;
149 base = identity;
150 if (as != null) {
151 for (int i = 0; i < as.length; ++i) {
152 if ((a = as[i]) != null)
153 a.value = identity;
154 }
155 }
156 }
157
158 /**
159 * Equivalent in effect to {@link #get} followed by {@link
160 * #reset}. This method may apply for example during quiescent
161 * points between multithreaded computations. If there are
162 * updates concurrent with this method, the returned value is
163 * <em>not</em> guaranteed to be the final value occurring before
164 * the reset.
165 *
166 * @return the value before reset
167 */
168 public double getThenReset() {
169 Cell[] as = cells; Cell a;
170 double result = Double.longBitsToDouble(base);
171 base = identity;
172 if (as != null) {
173 for (int i = 0; i < as.length; ++i) {
174 if ((a = as[i]) != null) {
175 double v = Double.longBitsToDouble(a.value);
176 a.value = identity;
177 result = function.operateAsDouble(result, v);
178 }
179 }
180 }
181 return result;
182 }
183
184 /**
185 * Returns the String representation of the current value.
186 * @return the String representation of the current value
187 */
188 public String toString() {
189 return Double.toString(get());
190 }
191
192 /**
193 * Equivalent to {@link #get}.
194 *
195 * @return the current value
196 */
197 public double doubleValue() {
198 return get();
199 }
200
201 /**
202 * Returns the {@linkplain #get current value} as a {@code long}
203 * after a narrowing primitive conversion.
204 */
205 public long longValue() {
206 return (long)get();
207 }
208
209 /**
210 * Returns the {@linkplain #get current value} as an {@code int}
211 * after a narrowing primitive conversion.
212 */
213 public int intValue() {
214 return (int)get();
215 }
216
217 /**
218 * Returns the {@linkplain #get current value} as a {@code float}
219 * after a narrowing primitive conversion.
220 */
221 public float floatValue() {
222 return (float)get();
223 }
224
225 private void writeObject(java.io.ObjectOutputStream s)
226 throws java.io.IOException {
227 s.defaultWriteObject();
228 s.writeDouble(get());
229 }
230
231 private void readObject(java.io.ObjectInputStream s)
232 throws java.io.IOException, ClassNotFoundException {
233 s.defaultReadObject();
234 cellsBusy = 0;
235 cells = null;
236 base = Double.doubleToRawLongBits(s.readDouble());
237 }
238
239 }