all: implement AppendText alongside MarshalText (#9207)

This eventually allows encoding packages that may respect
the proposed encoding.TextAppender interface.
The performance gains from this is between 10-30%.

Updates tailscale/corp#14379

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

tka/aum.go

@@ -9,6 +9,7 @@ import (
 	"encoding/base32"
 	"errors"
 	"fmt"
+	"slices"
 
 	"github.com/fxamacker/cbor/v2"
 	"golang.org/x/crypto/blake2s"
@@ -37,11 +38,22 @@ func (h *AUMHash) UnmarshalText(text []byte) error {
 	return nil
 }
 
+// TODO(https://go.dev/issue/53693): Use base32.Encoding.AppendEncode instead.
+func base32AppendEncode(enc *base32.Encoding, 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]
+}
+
+// AppendText implements encoding.TextAppender.
+func (h AUMHash) AppendText(b []byte) ([]byte, error) {
+	return base32AppendEncode(base32StdNoPad, b, h[:]), nil
+}
+
 // MarshalText implements encoding.TextMarshaler.
 func (h AUMHash) MarshalText() ([]byte, error) {
-	b := make([]byte, base32StdNoPad.EncodedLen(len(h)))
-	base32StdNoPad.Encode(b, h[:])
-	return b, nil
+	return h.AppendText(nil)
 }
 
 // IsZero returns true if the hash is the empty value.