Move sqrt out of Add()

bench/stats.go

@@ -9,9 +9,7 @@ type Stats struct {
 	Min            time.Duration
 	Max            time.Duration
 	Mean           time.Duration
-	SumSquareDelta float64
-	Variance       float64
-	StdDev         time.Duration
+	sumSquareDelta float64
 	Total          time.Duration
 	Num            int64
 }
@@ -32,15 +30,11 @@ func (s *Stats) Add(td time.Duration) {
 		s.Max = td
 	}
 
-	// these three comprise an online variance calculation
+	// online variance calculation
 	// https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Online_algorithm
 	delta := td - s.Mean
 	s.Mean += delta / time.Duration(s.Num)
-	s.SumSquareDelta += float64(delta * (td - s.Mean))
-
-	// these are the useful results, but don't need to be updated every iteration
-	s.Variance = s.SumSquareDelta / float64(s.Num)
-	s.StdDev = time.Duration(math.Sqrt(s.Variance))
+	s.sumSquareDelta += float64(delta * (td - s.Mean))
 }
 
 func (s *Stats) Avg() time.Duration {
@@ -50,5 +44,7 @@ func (s *Stats) Avg() time.Duration {
 func AddToResults(s *Stats, results map[string]interface{}) {
 	results["min"] = s.Min
 	results["max"] = s.Max
-	results["avg"] = s.Avg()
+	results["avg"] = s.Mean
+	variance := s.sumSquareDelta / float64(s.Num)
+	results["std"] = time.Duration(math.Sqrt(variance))
 }