From b6e22cd346eda9ad57c4b26d0bed10adf873a73f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=C5=81ukasz=20Mierzwa?= <l.mierzwa@gmail.com>
Date: Mon, 7 Aug 2023 16:55:17 +0100
Subject: [PATCH] Short-cut common memChunk operations
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memChunk is a linked list, speed up some common operations when there's no need to iterate all elements on the list.

Signed-off-by: Łukasz Mierzwa <l.mierzwa@gmail.com>
---
 tsdb/head.go | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/tsdb/head.go b/tsdb/head.go
index 2963d781d0..fefd31bd33 100644
--- a/tsdb/head.go
+++ b/tsdb/head.go
@@ -2265,6 +2265,10 @@ type memChunk struct {
 
 // len returns the length of memChunk list, including the element it was called on.
 func (mc *memChunk) len() (count int) {
+	if mc.prev == nil {
+		return 1
+	}
+
 	elem := mc
 	for elem != nil {
 		count++
@@ -2276,6 +2280,9 @@ func (mc *memChunk) len() (count int) {
 // oldest returns the oldest element on the list.
 // For single element list this will be the same memChunk oldest() was called on.
 func (mc *memChunk) oldest() (elem *memChunk) {
+	if mc.prev == nil {
+		return mc
+	}
 	elem = mc
 	for elem.prev != nil {
 		elem = elem.prev
@@ -2288,6 +2295,9 @@ func (mc *memChunk) atOffset(offset int) (elem *memChunk) {
 	if offset == 0 {
 		return mc
 	}
+	if offset == 1 {
+		return mc.prev
+	}
 	if offset < 0 {
 		return nil
 	}
@@ -2301,7 +2311,6 @@ func (mc *memChunk) atOffset(offset int) (elem *memChunk) {
 			break
 		}
 	}
-
 	return elem
 }