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, 3 deletions

diff --git a/src/encoding/base32/base32.go b/src/encoding/base32/base32.go
index 69ced9ca3c..7cccbd17be 100644
--- a/src/encoding/base32/base32.go
+++ b/src/encoding/base32/base32.go
@@ -7,6 +7,7 @@ package base32
 
 import (
 	"io"
+	"slices"
 	"strconv"
 )
 
@@ -176,6 +177,15 @@ func (enc *Encoding) Encode(dst, src []byte) {
 	}
 }
 
+// AppendEncode appends the base32 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 base32 encoding of src.
 func (enc *Encoding) EncodeToString(src []byte) string {
 	buf := make([]byte, enc.EncodedLen(len(src)))
@@ -378,6 +388,16 @@ func (enc *Encoding) Decode(dst, src []byte) (n int, err error) {
 	return
 }
 
+// AppendDecode appends the base32 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 base32 string s.
 func (enc *Encoding) DecodeString(s string) ([]byte, error) {
 	buf := []byte(s)
diff --git a/src/encoding/base32/base32_test.go b/src/encoding/base32/base32_test.go
index bdb9f0e61f..0132744507 100644
--- a/src/encoding/base32/base32_test.go
+++ b/src/encoding/base32/base32_test.go
@@ -57,6 +57,8 @@ func TestEncode(t *testing.T) {
 	for _, p := range pairs {
 		got := StdEncoding.EncodeToString([]byte(p.decoded))
 		testEqual(t, "Encode(%q) = %q, want %q", p.decoded, got, p.encoded)
+		dst := StdEncoding.AppendEncode([]byte("lead"), []byte(p.decoded))
+		testEqual(t, `AppendEncode("lead", %q) = %q, want %q`, p.decoded, string(dst), "lead"+p.encoded)
 	}
 }
 
@@ -99,13 +101,15 @@ func TestDecode(t *testing.T) {
 		if len(p.encoded) > 0 {
 			testEqual(t, "Decode(%q) = end %v, want %v", p.encoded, end, (p.encoded[len(p.encoded)-1] == '='))
 		}
-		testEqual(t, "Decode(%q) = %q, want %q", p.encoded,
-			string(dbuf[0:count]),
-			p.decoded)
+		testEqual(t, "Decode(%q) = %q, want %q", p.encoded, string(dbuf[0:count]), p.decoded)
 
 		dbuf, err = StdEncoding.DecodeString(p.encoded)
 		testEqual(t, "DecodeString(%q) = error %v, want %v", p.encoded, err, error(nil))
 		testEqual(t, "DecodeString(%q) = %q, want %q", p.encoded, string(dbuf), p.decoded)
+
+		dst, err := StdEncoding.AppendDecode([]byte("lead"), []byte(p.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)
 	}
 }