From fbae2f7fe95a05954f79c7dd48f3cb3b55ffce8d Mon Sep 17 00:00:00 2001
From: Alan Bernstein <abernstein@umbel.com>
Date: Thu, 17 Nov 2016 15:44:19 -0600
Subject: [PATCH 1/2] Add standard deviation to stats

---
 bench/stats.go | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/bench/stats.go b/bench/stats.go
index 0c1b027cf..dc8a86328 100644
--- a/bench/stats.go
+++ b/bench/stats.go
@@ -1,14 +1,19 @@
 package bench
 
 import (
+	"math"
 	"time"
 )
 
 type Stats struct {
-	Min   time.Duration
-	Max   time.Duration
-	Total time.Duration
-	Num   int64
+	Min            time.Duration
+	Max            time.Duration
+	Mean           time.Duration
+	SumSquareDelta float64
+	Variance       float64
+	StdDev         time.Duration
+	Total          time.Duration
+	Num            int64
 }
 
 func NewStats() *Stats {
@@ -26,6 +31,16 @@ func (s *Stats) Add(td time.Duration) {
 	if td > s.Max {
 		s.Max = td
 	}
+
+	// these three comprise an 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))
 }
 
 func (s *Stats) Avg() time.Duration {

From ff8766126803203c74b0b75fe7e019743790fe15 Mon Sep 17 00:00:00 2001
From: Alan Bernstein <abernstein@umbel.com>
Date: Fri, 18 Nov 2016 09:33:48 -0600
Subject: [PATCH 2/2] Move sqrt out of Add()

---
 bench/stats.go | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/bench/stats.go b/bench/stats.go
index dc8a86328..cb4395c13 100644
--- a/bench/stats.go
+++ b/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))
 }