encoding: add AppendEncode and AppendDecode

Implement append-like equivalent of Encode and Decode functions.

Fixes #53693

Change-Id: I79d8d834e3c8f77fad32be2fd391e33d4d1527ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/504884
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>

2 files changed, 32 insertions, 0 deletions

diff --git a/src/encoding/hex/hex.go b/src/encoding/hex/hex.go
index 375f583170..ccc395e2f7 100644
--- a/src/encoding/hex/hex.go
+++ b/src/encoding/hex/hex.go
@@ -9,6 +9,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"slices"
 	"strings"
 )
 
@@ -51,6 +52,15 @@ func Encode(dst, src []byte) int {
 	return len(src) * 2
 }
 
+// AppendEncode appends the hexadecimally encoded src to dst
+// and returns the extended buffer.
+func AppendEncode(dst, src []byte) []byte {
+	n := EncodedLen(len(src))
+	dst = slices.Grow(dst, n)
+	Encode(dst[len(dst):][:n], src)
+	return dst[:len(dst)+n]
+}
+
 // ErrLength reports an attempt to decode an odd-length input
 // using Decode or DecodeString.
 // The stream-based Decoder returns io.ErrUnexpectedEOF instead of ErrLength.
@@ -102,6 +112,16 @@ func Decode(dst, src []byte) (int, error) {
 	return i, nil
 }
 
+// AppendDecode appends the hexadecimally decoded src to dst
+// and returns the extended buffer.
+// If the input is malformed, it returns the partially decoded src and an error.
+func AppendDecode(dst, src []byte) ([]byte, error) {
+	n := DecodedLen(len(src))
+	dst = slices.Grow(dst, n)
+	n, err := Decode(dst[len(dst):][:n], src)
+	return dst[:len(dst)+n], err
+}
+
 // EncodeToString returns the hexadecimal encoding of src.
 func EncodeToString(src []byte) string {
 	dst := make([]byte, EncodedLen(len(src)))
diff --git a/src/encoding/hex/hex_test.go b/src/encoding/hex/hex_test.go
index a820fe7a15..03331eaae5 100644
--- a/src/encoding/hex/hex_test.go
+++ b/src/encoding/hex/hex_test.go
@@ -37,6 +37,11 @@ func TestEncode(t *testing.T) {
 		if string(dst) != test.enc {
 			t.Errorf("#%d: got: %#v want: %#v", i, dst, test.enc)
 		}
+		dst = []byte("lead")
+		dst = AppendEncode(dst, test.dec)
+		if string(dst) != "lead"+test.enc {
+			t.Errorf("#%d: got: %#v want: %#v", i, dst, "lead"+test.enc)
+		}
 	}
 }
 
@@ -52,6 +57,13 @@ func TestDecode(t *testing.T) {
 		} else if !bytes.Equal(dst, test.dec) {
 			t.Errorf("#%d: got: %#v want: %#v", i, dst, test.dec)
 		}
+		dst = []byte("lead")
+		dst, err = AppendDecode(dst, []byte(test.enc))
+		if err != nil {
+			t.Errorf("#%d: AppendDecode error: %v", i, err)
+		} else if string(dst) != "lead"+string(test.dec) {
+			t.Errorf("#%d: got: %#v want: %#v", i, dst, "lead"+string(test.dec))
+		}
 	}
 }