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, 27 insertions, 2 deletions

diff --git a/src/encoding/base64/base64.go b/src/encoding/base64/base64.go
index 87f6897062..5db72b91e2 100644
--- a/src/encoding/base64/base64.go
+++ b/src/encoding/base64/base64.go
@@ -8,6 +8,7 @@ package base64
 import (
 	"encoding/binary"
 	"io"
+	"slices"
 	"strconv"
 )
 
@@ -191,6 +192,15 @@ func (enc *Encoding) Encode(dst, src []byte) {
 	}
 }
 
+// AppendEncode appends the base64 encoded src to dst
+// and returns the extended buffer.
+func (enc *Encoding) AppendEncode(dst, src []byte) []byte {
+	n := enc.EncodedLen(len(src))
+	dst = slices.Grow(dst, n)
+	enc.Encode(dst[len(dst):][:n], src)
+	return dst[:len(dst)+n]
+}
+
 // EncodeToString returns the base64 encoding of src.
 func (enc *Encoding) EncodeToString(src []byte) string {
 	buf := make([]byte, enc.EncodedLen(len(src)))
@@ -395,6 +405,16 @@ func (enc *Encoding) decodeQuantum(dst, src []byte, si int) (nsi, n int, err err
 	return si, dlen - 1, err
 }
 
+// AppendDecode appends the base64 decoded src to dst
+// and returns the extended buffer.
+// If the input is malformed, it returns the partially decoded src and an error.
+func (enc *Encoding) AppendDecode(dst, src []byte) ([]byte, error) {
+	n := enc.DecodedLen(len(src))
+	dst = slices.Grow(dst, n)
+	n, err := enc.Decode(dst[len(dst):][:n], src)
+	return dst[:len(dst)+n], err
+}
+
 // DecodeString returns the bytes represented by the base64 string s.
 func (enc *Encoding) DecodeString(s string) ([]byte, error) {
 	dbuf := make([]byte, enc.DecodedLen(len(s)))
diff --git a/src/encoding/base64/base64_test.go b/src/encoding/base64/base64_test.go
index 97aea845ae..4d7437b919 100644
--- a/src/encoding/base64/base64_test.go
+++ b/src/encoding/base64/base64_test.go
@@ -113,8 +113,9 @@ func TestEncode(t *testing.T) {
 	for _, p := range pairs {
 		for _, tt := range encodingTests {
 			got := tt.enc.EncodeToString([]byte(p.decoded))
-			testEqual(t, "Encode(%q) = %q, want %q", p.decoded,
-				got, tt.conv(p.encoded))
+			testEqual(t, "Encode(%q) = %q, want %q", p.decoded, got, tt.conv(p.encoded))
+			dst := tt.enc.AppendEncode([]byte("lead"), []byte(p.decoded))
+			testEqual(t, `AppendEncode("lead", %q) = %q, want %q`, p.decoded, string(dst), "lead"+tt.conv(p.encoded))
 		}
 	}
 }
@@ -162,6 +163,10 @@ func TestDecode(t *testing.T) {
 			dbuf, err = tt.enc.DecodeString(encoded)
 			testEqual(t, "DecodeString(%q) = error %v, want %v", encoded, err, error(nil))
 			testEqual(t, "DecodeString(%q) = %q, want %q", encoded, string(dbuf), p.decoded)
+
+			dst, err := tt.enc.AppendDecode([]byte("lead"), []byte(encoded))
+			testEqual(t, "AppendDecode(%q) = error %v, want %v", p.encoded, err, error(nil))
+			testEqual(t, `AppendDecode("lead", %q) = %q, want %q`, p.encoded, string(dst), "lead"+p.decoded)
 		}
 	}
 }