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>

6 files changed, 86 insertions, 5 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)
 	}
 }
 
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)
 		}
 	}
 }
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))
+		}
 	}
 }