1//
2//  ========================================================================
3//  Copyright (c) 1995-2014 Mort Bay Consulting Pty. Ltd.
4//  ------------------------------------------------------------------------
5//  All rights reserved. This program and the accompanying materials
6//  are made available under the terms of the Eclipse Public License v1.0
7//  and Apache License v2.0 which accompanies this distribution.
8//
9//      The Eclipse Public License is available at
10//      http://www.eclipse.org/legal/epl-v10.html
11//
12//      The Apache License v2.0 is available at
13//      http://www.opensource.org/licenses/apache2.0.php
14//
15//  You may elect to redistribute this code under either of these licenses.
16//  ========================================================================
17//
18
19package org.eclipse.jetty.util.statistic;
20
21import java.util.concurrent.atomic.AtomicLong;
22
23import org.eclipse.jetty.util.Atomics;
24
25
26/* ------------------------------------------------------------ */
27/**
28 * SampledStatistics
29 * <p>
30 * Provides max, total, mean, count, variance, and standard
31 * deviation of continuous sequence of samples.
32 * <p>
33 * Calculates estimates of mean, variance, and standard deviation
34 * characteristics of a sample using a non synchronized
35 * approximation of the on-line algorithm presented
36 * in Donald Knuth's Art of Computer Programming, Volume 2,
37 * Seminumerical Algorithms, 3rd edition, page 232,
38 * Boston: Addison-Wesley. that cites a 1962 paper by B.P. Welford
39 * that can be found by following the link http://www.jstor.org/pss/1266577
40 * <p>
41 * This algorithm is also described in Wikipedia at
42 * http://en.wikipedia.org/w/index.php?title=Algorithms_for_calculating_variance§ion=4#On-line_algorithm
43 */
44public class SampleStatistic
45{
46    protected final AtomicLong _max = new AtomicLong();
47    protected final AtomicLong _total = new AtomicLong();
48    protected final AtomicLong _count = new AtomicLong();
49    protected final AtomicLong _totalVariance100 = new AtomicLong();
50
51    public void reset()
52    {
53        _max.set(0);
54        _total.set(0);
55        _count.set(0);
56        _totalVariance100.set(0);
57    }
58
59    public void set(final long sample)
60    {
61        long total = _total.addAndGet(sample);
62        long count = _count.incrementAndGet();
63
64        if (count>1)
65        {
66            long mean10 = total*10/count;
67            long delta10 = sample*10 - mean10;
68            _totalVariance100.addAndGet(delta10*delta10);
69        }
70
71        Atomics.updateMax(_max, sample);
72    }
73
74    /**
75     * @return the max value
76     */
77    public long getMax()
78    {
79        return _max.get();
80    }
81
82    public long getTotal()
83    {
84        return _total.get();
85    }
86
87    public long getCount()
88    {
89        return _count.get();
90    }
91
92    public double getMean()
93    {
94        return (double)_total.get()/_count.get();
95    }
96
97    public double getVariance()
98    {
99        final long variance100 = _totalVariance100.get();
100        final long count = _count.get();
101
102        return count>1?((double)variance100)/100.0/(count-1):0.0;
103    }
104
105    public double getStdDev()
106    {
107        return Math.sqrt(getVariance());
108    }
109}
110