1/*
2 * Copyright (C) 2012 Apple Inc. All rights reserved.
3 *
4 * Redistribution and use in source and binary forms, with or without
5 * modification, are permitted provided that the following conditions
6 * are met:
7 * 1. Redistributions of source code must retain the above copyright
8 * notice, this list of conditions and the following disclaimer.
9 * 2. Redistributions in binary form must reproduce the above copyright
10 * notice, this list of conditions and the following disclaimer in the
11 * documentation and/or other materials provided with the distribution.
12 *
13 * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
14 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
15 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
16 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR
17 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
18 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
19 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
20 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
21 * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
22 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
23 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
24 */
25
26#ifndef SimpleStats_h
27#define SimpleStats_h
28
29#include <wtf/MathExtras.h>
30#include <wtf/StdLibExtras.h>
31
32namespace WTF {
33
34// Simple and cheap way of tracking statistics if you're not worried about chopping on
35// the sum of squares (i.e. the sum of squares is unlikely to exceed 2^52).
36class SimpleStats {
37public:
38 SimpleStats()
39 : m_count(0)
40 , m_sum(0)
41 , m_sumOfSquares(0)
42 {
43 }
44
45 void add(double value)
46 {
47 m_count++;
48 m_sum += value;
49 m_sumOfSquares += value * value;
50 }
51
52 bool operator!() const
53 {
54 return !m_count;
55 }
56
57 double count() const
58 {
59 return m_count;
60 }
61
62 double sum() const
63 {
64 return m_sum;
65 }
66
67 double sumOfSquares() const
68 {
69 return m_sumOfSquares;
70 }
71
72 double mean() const
73 {
74 return m_sum / m_count;
75 }
76
77 // NB. This gives a biased variance as it divides by the number of samples rather
78 // than the degrees of freedom. This is fine once the count grows large, which in
79 // our case will happen rather quickly.
80 double variance() const
81 {
82 if (m_count < 2)
83 return 0;
84
85 // Compute <x^2> - <x>^2
86 double secondMoment = m_sumOfSquares / m_count;
87 double firstMoment = m_sum / m_count;
88
89 double result = secondMoment - firstMoment * firstMoment;
90
91 // It's possible to get -epsilon. Protect against this and turn it into
92 // +0.
93 if (result <= 0)
94 return 0;
95
96 return result;
97 }
98
99 // NB. This gives a biased standard deviation. See above.
100 double standardDeviation() const
101 {
102 return sqrt(variance());
103 }
104
105private:
106 double m_count;
107 double m_sum;
108 double m_sumOfSquares;
109};
110
111} // namespace WTF
112
113#endif // SimpleStats_h
114
115