encoding/base32: make NoPadding Encoding's DecodedLen return exact size

CL 47341 added support for decoding non-padded messages. But DecodedLen
still returned a multiple of 5 for messages without a padding, even
though it is possible to calculate the len exactly when using NoPadding.

This change makes DecodedLen return the exact number of bytes that
will be written. A change to the decoding logic is also made so that it
can handle this case.

DecodedLen now has the same behaviour as DecodedLen in encoding/base64.

Fixes #20854

Change-Id: I729e0b1c0946c866fb675c854f835f366dd4b5a4
Reviewed-on: https://go-review.googlesource.com/47710Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/base32/base32.go

@@ -341,7 +341,11 @@ func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
 		case 2:
 			dst[0] = dbuf[0]<<3 | dbuf[1]>>2
 		}
+
+		if !end {
 			dst = dst[5:]
+		}
+
 		switch dlen {
 		case 2:
 			n += 1
@@ -495,8 +499,7 @@ func NewDecoder(enc *Encoding, r io.Reader) io.Reader {
 // corresponding to n bytes of base32-encoded data.
 func (enc *Encoding) DecodedLen(n int) int {
 	if enc.padChar == NoPadding {
-		// +6 represents the missing padding
-		return (n + 6) / 8 * 5
+		return n * 5 / 8
 	}
 
 	return n / 8 * 5

src/encoding/base32/base32_test.go

@@ -551,13 +551,13 @@ func TestEncodedDecodedLen(t *testing.T) {
 		}},
 		{"NoPadding", StdEncoding.WithPadding(NoPadding), []test{
 			{0, 0, 0},
-			{1, 2, 5},
-			{2, 4, 5},
+			{1, 2, 1},
+			{2, 4, 2},
 			{5, 8, 5},
-			{6, 10, 10},
-			{7, 12, 10},
+			{6, 10, 6},
+			{7, 12, 7},
 			{10, 16, 10},
-			{11, 18, 15},
+			{11, 18, 11},
 		}},
 	} {
 		t.Run(test.name, func(t *testing.T) {