encoding/base32: add Encoding.WithPadding, StdPadding, NoPadding

Fixes #19478

Change-Id: I9fc186610d79fd003e7b5d88c0955286ebe7d3cf
Reviewed-on: https://go-review.googlesource.com/38634
Reviewed-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

@@ -23,8 +23,14 @@ import (
 type Encoding struct {
 	encode    string
 	decodeMap [256]byte
+	padChar   rune
 }
 
+const (
+	StdPadding rune = '=' // Standard padding character
+	NoPadding  rune = -1  // No padding
+)
+
 const encodeStd = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"
 const encodeHex = "0123456789ABCDEFGHIJKLMNOPQRSTUV"
 
@@ -33,6 +39,8 @@ const encodeHex = "0123456789ABCDEFGHIJKLMNOPQRSTUV"
 func NewEncoding(encoder string) *Encoding {
 	e := new(Encoding)
 	e.encode = encoder
+	e.padChar = StdPadding
+
 	for i := 0; i < len(e.decodeMap); i++ {
 		e.decodeMap[i] = 0xFF
 	}
@@ -57,6 +65,26 @@ var removeNewlinesMapper = func(r rune) rune {
 	return r
 }
 
+// WithPadding creates a new encoding identical to enc except
+// with a specified padding character, or NoPadding to disable padding.
+// The padding character must not be '\r' or '\n', must not
+// be contained in the encoding's alphabet and must be a rune equal or
+// below '\xff'.
+func (enc Encoding) WithPadding(padding rune) *Encoding {
+	if padding == '\r' || padding == '\n' || padding > 0xff {
+		panic("invalid padding")
+	}
+
+	for i := 0; i < len(enc.encode); i++ {
+		if rune(enc.encode[i]) == padding {
+			panic("padding contained in alphabet")
+		}
+	}
+
+	enc.padChar = padding
+	return &enc
+}
+
 /*
  * Encoder
  */
@@ -73,60 +101,63 @@ func (enc *Encoding) Encode(dst, src []byte) {
 	}
 
 	for len(src) > 0 {
-		var b0, b1, b2, b3, b4, b5, b6, b7 byte
+		var b [8]byte
 
 		// Unpack 8x 5-bit source blocks into a 5 byte
 		// destination quantum
 		switch len(src) {
 		default:
-			b7 = src[4] & 0x1F
-			b6 = src[4] >> 5
+			b[7] = src[4] & 0x1F
+			b[6] = src[4] >> 5
 			fallthrough
 		case 4:
-			b6 |= (src[3] << 3) & 0x1F
-			b5 = (src[3] >> 2) & 0x1F
-			b4 = src[3] >> 7
+			b[6] |= (src[3] << 3) & 0x1F
+			b[5] = (src[3] >> 2) & 0x1F
+			b[4] = src[3] >> 7
 			fallthrough
 		case 3:
-			b4 |= (src[2] << 1) & 0x1F
-			b3 = (src[2] >> 4) & 0x1F
+			b[4] |= (src[2] << 1) & 0x1F
+			b[3] = (src[2] >> 4) & 0x1F
 			fallthrough
 		case 2:
-			b3 |= (src[1] << 4) & 0x1F
-			b2 = (src[1] >> 1) & 0x1F
-			b1 = (src[1] >> 6) & 0x1F
+			b[3] |= (src[1] << 4) & 0x1F
+			b[2] = (src[1] >> 1) & 0x1F
+			b[1] = (src[1] >> 6) & 0x1F
 			fallthrough
 		case 1:
-			b1 |= (src[0] << 2) & 0x1F
-			b0 = src[0] >> 3
+			b[1] |= (src[0] << 2) & 0x1F
+			b[0] = src[0] >> 3
 		}
 
 		// Encode 5-bit blocks using the base32 alphabet
-		dst[0] = enc.encode[b0]
-		dst[1] = enc.encode[b1]
-		dst[2] = enc.encode[b2]
-		dst[3] = enc.encode[b3]
-		dst[4] = enc.encode[b4]
-		dst[5] = enc.encode[b5]
-		dst[6] = enc.encode[b6]
-		dst[7] = enc.encode[b7]
+		for i := 0; i < 8; i++ {
+			if len(dst) > i {
+				dst[i] = enc.encode[b[i]]
+			}
+		}
 
 		// Pad the final quantum
 		if len(src) < 5 {
-			dst[7] = '='
+			if enc.padChar == NoPadding {
+				break
+			}
+
+			dst[7] = byte(enc.padChar)
 			if len(src) < 4 {
-				dst[6] = '='
-				dst[5] = '='
+				dst[6] = byte(enc.padChar)
+				dst[5] = byte(enc.padChar)
 				if len(src) < 3 {
-					dst[4] = '='
+					dst[4] = byte(enc.padChar)
 					if len(src) < 2 {
-						dst[3] = '='
-						dst[2] = '='
+						dst[3] = byte(enc.padChar)
+						dst[2] = byte(enc.padChar)
 					}
 				}
 			}
+
 			break
 		}
+
 		src = src[5:]
 		dst = dst[8:]
 	}
@@ -219,7 +250,12 @@ func NewEncoder(enc *Encoding, w io.Writer) io.WriteCloser {
 
 // EncodedLen returns the length in bytes of the base32 encoding
 // of an input buffer of length n.
-func (enc *Encoding) EncodedLen(n int) int { return (n + 4) / 5 * 8 }
+func (enc *Encoding) EncodedLen(n int) int {
+	if enc.padChar == NoPadding {
+		return (n*8 + 4) / 5
+	}
+	return (n + 4) / 5 * 8
+}
 
 /*
  * Decoder

src/encoding/base32/base32_test.go

@@ -455,3 +455,33 @@ func BenchmarkDecodeString(b *testing.B) {
 		StdEncoding.DecodeString(data)
 	}
 }
+
+func TestWithCustomPadding(t *testing.T) {
+	for _, testcase := range pairs {
+		defaultPadding := StdEncoding.EncodeToString([]byte(testcase.decoded))
+		customPadding := StdEncoding.WithPadding('@').EncodeToString([]byte(testcase.decoded))
+		expected := strings.Replace(defaultPadding, "=", "@", -1)
+
+		if expected != customPadding {
+			t.Errorf("Expected custom %s, got %s", expected, customPadding)
+		}
+		if testcase.encoded != defaultPadding {
+			t.Errorf("Expected %s, got %s", testcase.encoded, defaultPadding)
+		}
+	}
+}
+
+func TestWithoutPadding(t *testing.T) {
+	for _, testcase := range pairs {
+		defaultPadding := StdEncoding.EncodeToString([]byte(testcase.decoded))
+		customPadding := StdEncoding.WithPadding(NoPadding).EncodeToString([]byte(testcase.decoded))
+		expected := strings.TrimRight(defaultPadding, "=")
+
+		if expected != customPadding {
+			t.Errorf("Expected custom %s, got %s", expected, customPadding)
+		}
+		if testcase.encoded != defaultPadding {
+			t.Errorf("Expected %s, got %s", testcase.encoded, defaultPadding)
+		}
+	}
+}