| /* |
| * Copyright (C) 2012 Apple Inc. All rights reserved. |
| * |
| * Redistribution and use in source and binary forms, with or without |
| * modification, are permitted provided that the following conditions |
| * are met: |
| * 1. Redistributions of source code must retain the above copyright |
| * notice, this list of conditions and the following disclaimer. |
| * 2. Redistributions in binary form must reproduce the above copyright |
| * notice, this list of conditions and the following disclaimer in the |
| * documentation and/or other materials provided with the distribution. |
| * |
| * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY |
| * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE |
| * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR |
| * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR |
| * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, |
| * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, |
| * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR |
| * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY |
| * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT |
| * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE |
| * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
| */ |
| |
| #ifndef SimpleStats_h |
| #define SimpleStats_h |
| |
| #include <wtf/MathExtras.h> |
| #include <wtf/StdLibExtras.h> |
| |
| namespace WTF { |
| |
| // Simple and cheap way of tracking statistics if you're not worried about chopping on |
| // the sum of squares (i.e. the sum of squares is unlikely to exceed 2^52). |
| class SimpleStats { |
| public: |
| SimpleStats() |
| : m_count(0) |
| , m_sum(0) |
| , m_sumOfSquares(0) |
| { |
| } |
| |
| void add(double value) |
| { |
| m_count++; |
| m_sum += value; |
| m_sumOfSquares += value * value; |
| } |
| |
| bool operator!() const |
| { |
| return !m_count; |
| } |
| |
| double count() const |
| { |
| return m_count; |
| } |
| |
| double sum() const |
| { |
| return m_sum; |
| } |
| |
| double sumOfSquares() const |
| { |
| return m_sumOfSquares; |
| } |
| |
| double mean() const |
| { |
| return m_sum / m_count; |
| } |
| |
| // NB. This gives a biased variance as it divides by the number of samples rather |
| // than the degrees of freedom. This is fine once the count grows large, which in |
| // our case will happen rather quickly. |
| double variance() const |
| { |
| if (m_count < 2) |
| return 0; |
| |
| // Compute <x^2> - <x>^2 |
| double secondMoment = m_sumOfSquares / m_count; |
| double firstMoment = m_sum / m_count; |
| |
| double result = secondMoment - firstMoment * firstMoment; |
| |
| // It's possible to get -epsilon. Protect against this and turn it into |
| // +0. |
| if (result <= 0) |
| return 0; |
| |
| return result; |
| } |
| |
| // NB. This gives a biased standard deviation. See above. |
| double standardDeviation() const |
| { |
| return sqrt(variance()); |
| } |
| |
| private: |
| double m_count; |
| double m_sum; |
| double m_sumOfSquares; |
| }; |
| |
| } // namespace WTF |
| |
| #endif // SimpleStats_h |
| |