From e661c6546a6245d1729e8131a932341fe969650a Mon Sep 17 00:00:00 2001
From: 0xZens <zens@ldclabs.org>
Date: Sat, 7 May 2022 19:45:34 +0800
Subject: [PATCH] Reuse underlying array if RawMessage has sufficient capacity

This is a performance optimization for implementing cbor-patch.

Signed-off-by: Yan Qing <admin@zensh.com>
---
 stream.go      |  3 +--
 stream_test.go | 15 +++++++++++++++
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/stream.go b/stream.go
index 29172b92..05606e89 100644
--- a/stream.go
+++ b/stream.go
@@ -193,7 +193,6 @@ func (m *RawMessage) UnmarshalCBOR(data []byte) error {
 	if m == nil {
 		return errors.New("cbor.RawMessage: UnmarshalCBOR on nil pointer")
 	}
-	*m = make([]byte, len(data))
-	copy(*m, data)
+	*m = append((*m)[0:0], data...)
 	return nil
 }
diff --git a/stream_test.go b/stream_test.go
index 6469628e..8d4282e7 100644
--- a/stream_test.go
+++ b/stream_test.go
@@ -5,6 +5,7 @@ package cbor
 
 import (
 	"bytes"
+	"fmt"
 	"io"
 	"reflect"
 	"testing"
@@ -387,6 +388,20 @@ func TestRawMessage(t *testing.T) {
 	if !bytes.Equal(b, cborData) {
 		t.Errorf("Marshal(%+v) = 0x%x, want 0x%x", v, b, cborData)
 	}
+
+	address := fmt.Sprintf("%p", *v.B)
+	if err := Unmarshal(v.A, v.B); err != nil {
+		t.Fatalf("Unmarshal(0x%x) returned error %v", v.A, err)
+	}
+	if address != fmt.Sprintf("%p", *v.B) {
+		t.Fatalf("Unmarshal RawMessage should reuse underlying array if it has sufficient capacity")
+	}
+	if err := Unmarshal(cborData, v.B); err != nil {
+		t.Fatalf("Unmarshal(0x%x) returned error %v", cborData, err)
+	}
+	if address == fmt.Sprintf("%p", *v.B) {
+		t.Fatalf("Unmarshal RawMessage should allocate a new underlying array if it does not have sufficient capacity")
+	}
 }
 
 func TestNullRawMessage(t *testing.T) {