From e1115ae58d069b3f7fd19ffc6b635a6c98882148 Mon Sep 17 00:00:00 2001
From: Bryan Boreham <bjboreham@gmail.com>
Date: Mon, 26 Jun 2023 18:35:22 +0100
Subject: [PATCH] labels: improve Get method for stringlabels build (#12485)

Inline one call to `decodeString`, and skip decoding the value string
until we find a match for the name.
Do a quick check on the first character in each string,
and exit early if we've gone past - labels are sorted in order.

Also improve tests and benchmark:
* labels: test Get with varying lengths - it's not typical for Prometheus labels to all be the same length.
* extend benchmark with label not found

---------

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>
---
 model/labels/labels_stringlabels.go | 24 +++++++++++++++++++-----
 model/labels/labels_test.go         |  6 ++++--
 2 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/model/labels/labels_stringlabels.go b/model/labels/labels_stringlabels.go
index eb98fd6fb7..223aa6ebf7 100644
--- a/model/labels/labels_stringlabels.go
+++ b/model/labels/labels_stringlabels.go
@@ -273,13 +273,27 @@ func (ls Labels) Copy() Labels {
 // Get returns the value for the label with the given name.
 // Returns an empty string if the label doesn't exist.
 func (ls Labels) Get(name string) string {
+	if name == "" { // Avoid crash in loop if someone asks for "".
+		return "" // Prometheus does not store blank label names.
+	}
 	for i := 0; i < len(ls.data); {
-		var lName, lValue string
-		lName, i = decodeString(ls.data, i)
-		lValue, i = decodeString(ls.data, i)
-		if lName == name {
-			return lValue
+		var size int
+		size, i = decodeSize(ls.data, i)
+		if ls.data[i] == name[0] {
+			lName := ls.data[i : i+size]
+			i += size
+			if lName == name {
+				lValue, _ := decodeString(ls.data, i)
+				return lValue
+			}
+		} else {
+			if ls.data[i] > name[0] { // Stop looking if we've gone past.
+				break
+			}
+			i += size
 		}
+		size, i = decodeSize(ls.data, i)
+		i += size
 	}
 	return ""
 }
diff --git a/model/labels/labels_test.go b/model/labels/labels_test.go
index f0f05734be..d91be27cbc 100644
--- a/model/labels/labels_test.go
+++ b/model/labels/labels_test.go
@@ -443,7 +443,8 @@ func TestLabels_Has(t *testing.T) {
 
 func TestLabels_Get(t *testing.T) {
 	require.Equal(t, "", FromStrings("aaa", "111", "bbb", "222").Get("foo"))
-	require.Equal(t, "111", FromStrings("aaa", "111", "bbb", "222").Get("aaa"))
+	require.Equal(t, "111", FromStrings("aaaa", "111", "bbb", "222").Get("aaaa"))
+	require.Equal(t, "222", FromStrings("aaaa", "111", "bbb", "222").Get("bbb"))
 }
 
 // BenchmarkLabels_Get was written to check whether a binary search can improve the performance vs the linear search implementation
@@ -463,7 +464,7 @@ func BenchmarkLabels_Get(b *testing.B) {
 	maxLabels := 30
 	allLabels := make([]Label, maxLabels)
 	for i := 0; i < maxLabels; i++ {
-		allLabels[i] = Label{Name: strings.Repeat(string('a'+byte(i)), 5)}
+		allLabels[i] = Label{Name: strings.Repeat(string('a'+byte(i)), 5+(i%5))}
 	}
 	for _, size := range []int{5, 10, maxLabels} {
 		b.Run(fmt.Sprintf("with %d labels", size), func(b *testing.B) {
@@ -474,6 +475,7 @@ func BenchmarkLabels_Get(b *testing.B) {
 				{"get first label", allLabels[0].Name},
 				{"get middle label", allLabels[size/2].Name},
 				{"get last label", allLabels[size-1].Name},
+				{"get not-found label", "benchmark"},
 			} {
 				b.Run(scenario.desc, func(b *testing.B) {
 					b.ResetTimer()