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/*
* 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
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