traversal_3_average_step.cpp

struct RunningAverage
{
   double m_value;
   int m_count;
};

struct GetAverage
{
   RunningAverage operator()() const { return RunningAverage{ 0.0, 0 }; }
   RunningAverage operator()(RunningAverage const& avg, double value) const
   {
      return RunningAverage {
         (avg.m_value * avg.m_count + value) / (avg.m_count + 1),
         avg.m_count + 1 };
   }
};