From c91fd6c67b9114b2b5823b25dec55d4f22a43172 Mon Sep 17 00:00:00 2001
From: Bryan Boreham <bjboreham@gmail.com>
Date: Mon, 9 Jul 2018 08:07:17 +0000
Subject: [PATCH] Optimise merge of LatestMaps with same keys, different
 timestamps

Swap the two maps so we are merging older into later.
---
 extras/generate_latest_map     |  8 ++++++--
 report/latest_map_generated.go | 16 ++++++++++++----
 2 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/extras/generate_latest_map b/extras/generate_latest_map
index e2bc3e2ea..0f831799c 100755
--- a/extras/generate_latest_map
+++ b/extras/generate_latest_map
@@ -79,13 +79,17 @@ function generate_latest_map() {
     // Tries to return one of its inputs, if that already holds the correct result.
     func (m ${latest_map_type}) Merge(n ${latest_map_type}) ${latest_map_type} {
         switch {
-        case m == nil:
+        case len(m) == 0:
             return n
-        case n == nil:
+        case len(n) == 0:
             return m
         }
         if len(n) > len(m) {
             m, n = n, m //swap so m is always at least as long as n
+        } else if len(n) == len(m) && m[0].Timestamp.Before(n[0].Timestamp) {
+            // Optimise common case where we merge two nodes with the same contents
+            // sampled at different times.
+            m, n = n, m // swap equal-length arrays so first element of m is newer
         }
 
         i, j := 0, 0
diff --git a/report/latest_map_generated.go b/report/latest_map_generated.go
index ff0e9272b..626b80988 100644
--- a/report/latest_map_generated.go
+++ b/report/latest_map_generated.go
@@ -47,13 +47,17 @@ func (m StringLatestMap) Size() int {
 // Tries to return one of its inputs, if that already holds the correct result.
 func (m StringLatestMap) Merge(n StringLatestMap) StringLatestMap {
 	switch {
-	case m == nil:
+	case len(m) == 0:
 		return n
-	case n == nil:
+	case len(n) == 0:
 		return m
 	}
 	if len(n) > len(m) {
 		m, n = n, m //swap so m is always at least as long as n
+	} else if len(n) == len(m) && m[0].Timestamp.Before(n[0].Timestamp) {
+		// Optimise common case where we merge two nodes with the same contents
+		// sampled at different times.
+		m, n = n, m // swap equal-length arrays so first element of m is newer
 	}
 
 	i, j := 0, 0
@@ -290,13 +294,17 @@ func (m NodeControlDataLatestMap) Size() int {
 // Tries to return one of its inputs, if that already holds the correct result.
 func (m NodeControlDataLatestMap) Merge(n NodeControlDataLatestMap) NodeControlDataLatestMap {
 	switch {
-	case m == nil:
+	case len(m) == 0:
 		return n
-	case n == nil:
+	case len(n) == 0:
 		return m
 	}
 	if len(n) > len(m) {
 		m, n = n, m //swap so m is always at least as long as n
+	} else if len(n) == len(m) && m[0].Timestamp.Before(n[0].Timestamp) {
+		// Optimise common case where we merge two nodes with the same contents
+		// sampled at different times.
+		m, n = n, m // swap equal-length arrays so first element of m is newer
 	}
 
 	i, j := 0, 0