From 75b835d92925d0e80a12eaa7975f00c12c5ee767 Mon Sep 17 00:00:00 2001
From: Yury Yarashevich <yura.yaroshevich@gmail.com>
Date: Mon, 3 Feb 2025 23:28:42 +0100
Subject: [PATCH] Cache hash code of immutable values of type Tags & KeyValues

---
 .../java/io/micrometer/common/KeyValues.java   | 18 +++++++++++++++---
 .../io/micrometer/core/instrument/Tags.java    | 18 +++++++++++++++---
 2 files changed, 30 insertions(+), 6 deletions(-)

diff --git a/micrometer-commons/src/main/java/io/micrometer/common/KeyValues.java b/micrometer-commons/src/main/java/io/micrometer/common/KeyValues.java
index 6c32e8f878..ca2f0921bc 100644
--- a/micrometer-commons/src/main/java/io/micrometer/common/KeyValues.java
+++ b/micrometer-commons/src/main/java/io/micrometer/common/KeyValues.java
@@ -51,6 +51,11 @@ public final class KeyValues implements Iterable<KeyValue> {
      */
     private final int length;
 
+    /**
+     * Cache the hash code of the {@code KeyValues}, 0 value means hash was not computed.
+     */
+    private int hash;
+
     /**
      * A constructor that initializes a {@code KeyValues} object with a sorted set of
      * key-values and its length.
@@ -291,9 +296,16 @@ public Stream<KeyValue> stream() {
 
     @Override
     public int hashCode() {
-        int result = 1;
-        for (int i = 0; i < length; i++) {
-            result = 31 * result + sortedSet[i].hashCode();
+        int result = hash;
+        if (result == 0) {
+            result = 1;
+            for (int i = 0; i < length; i++) {
+                result = 31 * result + sortedSet[i].hashCode();
+            }
+            if (result == 0) { // Re-map 0 hash code
+                result = 1;
+            }
+            hash = result;
         }
         return result;
     }
diff --git a/micrometer-core/src/main/java/io/micrometer/core/instrument/Tags.java b/micrometer-core/src/main/java/io/micrometer/core/instrument/Tags.java
index 1c696066e8..014bbd1441 100644
--- a/micrometer-core/src/main/java/io/micrometer/core/instrument/Tags.java
+++ b/micrometer-core/src/main/java/io/micrometer/core/instrument/Tags.java
@@ -48,6 +48,11 @@ public final class Tags implements Iterable<Tag> {
      */
     private final int length;
 
+    /**
+     * Cache the hash code of the {@code Tags}, 0 value means hash was not computed.
+     */
+    private int hash;
+
     /**
      * A constructor that initializes a {@code Tags} object with a sorted set of tags and
      * its length.
@@ -267,9 +272,16 @@ public Stream<Tag> stream() {
 
     @Override
     public int hashCode() {
-        int result = 1;
-        for (int i = 0; i < length; i++) {
-            result = 31 * result + sortedSet[i].hashCode();
+        int result = hash;
+        if (result == 0) {
+            result = 1;
+            for (int i = 0; i < length; i++) {
+                result = 31 * result + sortedSet[i].hashCode();
+            }
+            if (result == 0) { // Re-map 0 hash code
+                result = 1;
+            }
+            hash = result;
         }
         return result;
     }