From c6dc1090553b71092603c34ac6e8e84c2bd6192b Mon Sep 17 00:00:00 2001
From: Charles Masson <charles.masson@datadoghq.com>
Date: Tue, 18 Apr 2023 19:00:13 +0200
Subject: [PATCH] Fix merge after clear

---
 ddsketch/store/buffered_paginated.go |  3 +++
 ddsketch/store/store_test.go         | 17 +++++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/ddsketch/store/buffered_paginated.go b/ddsketch/store/buffered_paginated.go
index 5fad7fb..11f4310 100644
--- a/ddsketch/store/buffered_paginated.go
+++ b/ddsketch/store/buffered_paginated.go
@@ -379,6 +379,9 @@ func (s *BufferedPaginatedStore) MergeWith(other Store) {
 	if ok && s.pageLenLog2 == o.pageLenLog2 {
 		// Merge pages.
 		for oPageOffset, oPage := range o.pages {
+			if len(oPage) == 0 {
+				continue
+			}
 			oPageIndex := o.minPageIndex + oPageOffset
 			page := s.page(oPageIndex, true)
 			for i, oCount := range oPage {
diff --git a/ddsketch/store/store_test.go b/ddsketch/store/store_test.go
index 7278100..6a6e1c8 100644
--- a/ddsketch/store/store_test.go
+++ b/ddsketch/store/store_test.go
@@ -300,6 +300,23 @@ func TestMergeFuzzy(t *testing.T) {
 	}
 }
 
+func TestMergeAfterClear(t *testing.T) {
+	for _, testCase := range testCases {
+		t.Run(testCase.name, func(t *testing.T) {
+			s1 := testCase.newStore()
+			s2 := testCase.newStore()
+
+			for i := 0; i < 1000; i++ {
+				s2.Add(i)
+			}
+			s2.Clear()
+
+			s1.MergeWith(s2)
+			s2.MergeWith(s1)
+		})
+	}
+}
+
 func testStore(t *testing.T, store Store, normalizedBins []Bin) {
 	assertEncodeBins(t, store, normalizedBins)
 	testCopy(t, store, normalizedBins)