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