From 2ab0573ee2c0940f6a3d3035cda8abcd00cfd25e Mon Sep 17 00:00:00 2001
From: Mert Can Altin <mertgold60@gmail.com>
Date: Sat, 10 May 2025 23:13:05 +0300
Subject: [PATCH 1/3] buffer: avoid unnecessary copy in Buffer.concat for
 single FastBuffer

---
 lib/buffer.js                       | 9 +++++++++
 test/parallel/test-buffer-concat.js | 9 +++++++++
 2 files changed, 18 insertions(+)

diff --git a/lib/buffer.js b/lib/buffer.js
index 22bbb95eaf79e6..4b755e1e3f0d22 100644
--- a/lib/buffer.js
+++ b/lib/buffer.js
@@ -593,6 +593,15 @@ Buffer.concat = function concat(list, length) {
     validateOffset(length, 'length');
   }
 
+  // If there is only one FastBuffer, rotate without copying
+  if (
+    list.length === 1 &&
+    list[0] instanceof FastBuffer &&
+    list[0].length === length
+  ) {
+    return Buffer.from(list[0]);
+  }
+
   const buffer = Buffer.allocUnsafe(length);
   let pos = 0;
   for (let i = 0; i < list.length; i++) {
diff --git a/test/parallel/test-buffer-concat.js b/test/parallel/test-buffer-concat.js
index 2e7541fb58b063..444644e03bab1d 100644
--- a/test/parallel/test-buffer-concat.js
+++ b/test/parallel/test-buffer-concat.js
@@ -44,6 +44,15 @@ assert.notStrictEqual(flatOne, one[0]);
 assert.strictEqual(flatLong.toString(), check);
 assert.strictEqual(flatLongLen.toString(), check);
 
+{
+  const original = Buffer.from('fast');
+  const result = Buffer.concat([original], original.length);
+  assert.notStrictEqual(result, original,
+    'Buffer.concat should return a new Buffer instance even with single FastBuffer');
+  assert.deepStrictEqual(result, original,
+    'Buffer.concat output should equal original content');
+}
+
 [undefined, null, Buffer.from('hello')].forEach((value) => {
   assert.throws(() => {
     Buffer.concat(value);

From 44d56f1439e7b1370b5e3606ca78229d2d9c3740 Mon Sep 17 00:00:00 2001
From: Mert Can Altin <mertgold60@gmail.com>
Date: Sat, 10 May 2025 23:19:26 +0300
Subject: [PATCH 2/3] lint

---
 test/parallel/test-buffer-concat.js | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/test/parallel/test-buffer-concat.js b/test/parallel/test-buffer-concat.js
index 444644e03bab1d..3b928c52a221b8 100644
--- a/test/parallel/test-buffer-concat.js
+++ b/test/parallel/test-buffer-concat.js
@@ -47,10 +47,16 @@ assert.strictEqual(flatLongLen.toString(), check);
 {
   const original = Buffer.from('fast');
   const result = Buffer.concat([original], original.length);
-  assert.notStrictEqual(result, original,
-    'Buffer.concat should return a new Buffer instance even with single FastBuffer');
-  assert.deepStrictEqual(result, original,
-    'Buffer.concat output should equal original content');
+  assert.notStrictEqual(
+    result,
+    original,
+    'Buffer.concat should return a new Buffer instance even with single FastBuffer'
+  );
+  assert.deepStrictEqual(
+    result,
+    original,
+    'Buffer.concat output should equal original content'
+  );
 }
 
 [undefined, null, Buffer.from('hello')].forEach((value) => {

From 62de85d0ba5f45af689aabc7ad466021204a177f Mon Sep 17 00:00:00 2001
From: Mert Can Altin <mertgold60@gmail.com>
Date: Sat, 10 May 2025 23:26:01 +0300
Subject: [PATCH 3/3] lint

---
 test/parallel/test-buffer-concat.js | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/test/parallel/test-buffer-concat.js b/test/parallel/test-buffer-concat.js
index 3b928c52a221b8..3531f98de8059d 100644
--- a/test/parallel/test-buffer-concat.js
+++ b/test/parallel/test-buffer-concat.js
@@ -47,16 +47,13 @@ assert.strictEqual(flatLongLen.toString(), check);
 {
   const original = Buffer.from('fast');
   const result = Buffer.concat([original], original.length);
-  assert.notStrictEqual(
-    result,
-    original,
-    'Buffer.concat should return a new Buffer instance even with single FastBuffer'
-  );
-  assert.deepStrictEqual(
-    result,
-    original,
-    'Buffer.concat output should equal original content'
-  );
+
+  const msgRef =
+    'Buffer.concat should return a new Buffer instance even with single FastBuffer';
+  const msgEq = 'Buffer.concat output should equal original content';
+
+  assert.notStrictEqual(result, original, msgRef);
+  assert.deepStrictEqual(result, original, msgEq);
 }
 
 [undefined, null, Buffer.from('hello')].forEach((value) => {