encoding: show the alphabet for base32 and base64

There is not a great reason to hide the alphabet used
for StdEncoding, HexEncoding, and URLEncoding.

Although this is specified in RFC 4748,
showing it in GoDoc saves an extra click from going
to the RFC itself to see the alphabet being used.

Also, split exported and unexported constants apart
so that GoDoc renders more cleanly.

Fixes #55126

Change-Id: I03bfa607fb6c3df7f757e33fc0f4ec2b233de1a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/525296
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>

src/encoding/base32/base32.go

@@ -26,9 +26,12 @@ type Encoding struct {
 }
 
 const (
-	StdPadding          rune = '=' // Standard padding character
-	NoPadding           rune = -1  // No padding
-	decodeMapInitialize      = "" +
+	StdPadding rune = '=' // Standard padding character
+	NoPadding  rune = -1  // No padding
+)
+
+const (
+	decodeMapInitialize = "" +
 		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
 		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
 		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
@@ -48,9 +51,6 @@ const (
 	invalidIndex = '\xff'
 )
 
-const encodeStd = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"
-const encodeHex = "0123456789ABCDEFGHIJKLMNOPQRSTUV"
-
 // NewEncoding returns a new padded Encoding defined by the given alphabet,
 // which must be a 32-byte string that contains unique byte values and
 // does not contain the padding character or CR / LF ('\r', '\n').
@@ -83,13 +83,12 @@ func NewEncoding(encoder string) *Encoding {
 	return e
 }
 
-// StdEncoding is the standard base32 encoding, as defined in
-// RFC 4648.
-var StdEncoding = NewEncoding(encodeStd)
+// StdEncoding is the standard base32 encoding, as defined in RFC 4648.
+var StdEncoding = NewEncoding("ABCDEFGHIJKLMNOPQRSTUVWXYZ234567")
 
 // HexEncoding is the “Extended Hex Alphabet” defined in RFC 4648.
 // It is typically used in DNS.
-var HexEncoding = NewEncoding(encodeHex)
+var HexEncoding = NewEncoding("0123456789ABCDEFGHIJKLMNOPQRSTUV")
 
 // WithPadding creates a new encoding identical to enc except
 // with a specified padding character, or NoPadding to disable padding.

src/encoding/base64/base64.go

@@ -29,9 +29,12 @@ type Encoding struct {
 }
 
 const (
-	StdPadding          rune = '=' // Standard padding character
-	NoPadding           rune = -1  // No padding
-	decodeMapInitialize      = "" +
+	StdPadding rune = '=' // Standard padding character
+	NoPadding  rune = -1  // No padding
+)
+
+const (
+	decodeMapInitialize = "" +
 		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
 		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
 		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
@@ -51,9 +54,6 @@ const (
 	invalidIndex = '\xff'
 )
 
-const encodeStd = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
-const encodeURL = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_"
-
 // NewEncoding returns a new padded Encoding defined by the given alphabet,
 // which must be a 64-byte string that contains unique byte values and
 // does not contain the padding character or CR / LF ('\r', '\n').
@@ -115,13 +115,12 @@ func (enc Encoding) Strict() *Encoding {
 	return &enc
 }
 
-// StdEncoding is the standard base64 encoding, as defined in
-// RFC 4648.
-var StdEncoding = NewEncoding(encodeStd)
+// StdEncoding is the standard base64 encoding, as defined in RFC 4648.
+var StdEncoding = NewEncoding("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/")
 
 // URLEncoding is the alternate base64 encoding defined in RFC 4648.
 // It is typically used in URLs and file names.
-var URLEncoding = NewEncoding(encodeURL)
+var URLEncoding = NewEncoding("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_")
 
 // RawStdEncoding is the standard raw, unpadded base64 encoding,
 // as defined in RFC 4648 section 3.2.

src/encoding/base64/base64_test.go

@@ -70,6 +70,8 @@ func rawURLRef(ref string) string {
 	return rawRef(urlRef(ref))
 }
 
+const encodeStd = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
+
 // A nonstandard encoding with a funny padding character, for testing
 var funnyEncoding = NewEncoding(encodeStd).WithPadding(rune('@'))