code review 3926041: A base32 impl. from RFC4648. It is modeled after the

src/pkg/encoding/base32/base32.go

+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package base32 implements base32 encoding as specified by RFC 4648.
+package base32
+
+import (
+        "io"
+        "os"
+        "strconv"
+)
+
+/*
+ * Encodings
+ */
+
+// An Encoding is a radix 32 encoding/decoding scheme, defined by a
+// 32-character alphabet.  The most common is the "base32" encoding
+// introduced for SASL GSSAPI and standardized in RFC 4648.
+// The alternate "base32hex" encoding is used in DNSSEC.
+type Encoding struct {
+        encode    string
+        decodeMap [256]byte
+}
+
+const encodeStd = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"
+const encodeHex = "0123456789ABCDEFGHIJKLMNOPQRSTUV"
+
+// NewEncoding returns a new Encoding defined by the given alphabet,
+// which must be a 32-byte string.
+func NewEncoding(encoder string) *Encoding {
+        e := new(Encoding)
+        e.encode = encoder
+        for i := 0; i < len(e.decodeMap); i++ {
+                e.decodeMap[i] = 0xFF
+        }
+        for i := 0; i < len(encoder); i++ {
+                e.decodeMap[encoder[i]] = byte(i)
+        }
+        return e
+}
+
+// StdEncoding is the standard base32 encoding, as defined in
+// RFC 4648.
+var StdEncoding = NewEncoding(encodeStd)
+
+// HexEncoding is the ``Extended Hex Alphabet'' defined in RFC 4648.
+// It is typically used in DNS.
+var HexEncoding = NewEncoding(encodeHex)
+
+/*
+ * Encoder
+ */
+
+// Encode encodes src using the encoding enc, writing
+// EncodedLen(len(src)) bytes to dst.
+//
+// The encoding pads the output to a multiple of 8 bytes,
+// so Encode is not appropriate for use on individual blocks
+// of a large data stream.  Use NewEncoder() instead.
+func (enc *Encoding) Encode(dst, src []byte) {
+        if len(src) == 0 {
+                return
+        }
+
+        for len(src) > 0 {
+                dst[0] = 0
+                dst[1] = 0
+                dst[2] = 0
+                dst[3] = 0
+                dst[4] = 0
+                dst[5] = 0
+                dst[6] = 0
+                dst[7] = 0
+
+                // Unpack 8x 5-bit source blocks into a 5 byte
+                // destination quantum
+                switch len(src) {
+                default:
+                        dst[7] |= src[4] & 0x1F
+                        dst[6] |= src[4] >> 5
+                        fallthrough
+                case 4:
+                        dst[6] |= (src[3] << 3) & 0x1F
+                        dst[5] |= (src[3] >> 2) & 0x1F
+                        dst[4] |= src[3] >> 7
+                        fallthrough
+                case 3:
+                        dst[4] |= (src[2] << 1) & 0x1F
+                        dst[3] |= (src[2] >> 4) & 0x1F
+                        fallthrough
+                case 2:
+                        dst[3] |= (src[1] << 4) & 0x1F
+                        dst[2] |= (src[1] >> 1) & 0x1F
+                        dst[1] |= (src[1] >> 6) & 0x1F
+                        fallthrough
+                case 1:
+                        dst[1] |= (src[0] << 2) & 0x1F
+                        dst[0] |= src[0] >> 3
+                }
+
+                // Encode 5-bit blocks using the base32 alphabet
+                for j := 0; j < 8; j++ {
+                        dst[j] = enc.encode[dst[j]]
+                }
+
+                // Pad the final quantum
+                if len(src) < 5 {
+                        dst[7] = '='
+                        if len(src) < 4 {
+                                dst[6] = '='
+                                dst[5] = '='
+                                if len(src) < 3 {
+                                        dst[4] = '='
+                                        if len(src) < 2 {
+                                                dst[3] = '='
+                                                dst[2] = '='
+                                        }
+                                }
+                        }
+                        break
+                }
+                src = src[5:]
+                dst = dst[8:]
+        }
+}
+
+type encoder struct {
+        err  os.Error
+        enc  *Encoding
+        w    io.Writer
+        buf  [5]byte    // buffered data waiting to be encoded
+        nbuf int        // number of bytes in buf
+        out  [1024]byte // output buffer
+}
+
+func (e *encoder) Write(p []byte) (n int, err os.Error) {
+        if e.err != nil {
+                return 0, e.err
+        }
+
+        // Leading fringe.
+        if e.nbuf > 0 {
+                var i int
+                for i = 0; i < len(p) && e.nbuf < 5; i++ {
+                        e.buf[e.nbuf] = p[i]
+                        e.nbuf++
+                }
+                n += i
+                p = p[i:]
+                if e.nbuf < 5 {
+                        return
+                }
+                e.enc.Encode(e.out[0:], e.buf[0:])
+                if _, e.err = e.w.Write(e.out[0:8]); e.err != nil {
+                        return n, e.err
+                }
+                e.nbuf = 0
+        }
+
+        // Large interior chunks.
+        for len(p) >= 5 {
+                nn := len(e.out) / 8 * 5
+                if nn > len(p) {
+                        nn = len(p)
+                }
+                nn -= nn % 5
+                if nn > 0 {
+                        e.enc.Encode(e.out[0:], p[0:nn])
+                        if _, e.err = e.w.Write(e.out[0 : nn/5*8]); e.err != nil {
+                                return n, e.err
+                        }
+                }
+                n += nn
+                p = p[nn:]
+        }
+
+        // Trailing fringe.
+        for i := 0; i < len(p); i++ {
+                e.buf[i] = p[i]
+        }
+        e.nbuf = len(p)
+        n += len(p)
+        return
+}
+
+// Close flushes any pending output from the encoder.
+// It is an error to call Write after calling Close.
+func (e *encoder) Close() os.Error {
+        // If there's anything left in the buffer, flush it out
+        if e.err == nil && e.nbuf > 0 {
+                e.enc.Encode(e.out[0:], e.buf[0:e.nbuf])
+                e.nbuf = 0
+                _, e.err = e.w.Write(e.out[0:8])
+        }
+        return e.err
+}
+
+// NewEncoder returns a new base32 stream encoder.  Data written to
+// the returned writer will be encoded using enc and then written to w.
+// Base32 encodings operate in 5-byte blocks; when finished
+// writing, the caller must Close the returned encoder to flush any
+// partially written blocks.
+func NewEncoder(enc *Encoding, w io.Writer) io.WriteCloser {
+        return &encoder{enc: enc, w: w}
+}
+
+// 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 }
+
+/*
+ * Decoder
+ */
+
+type CorruptInputError int64
+
+func (e CorruptInputError) String() string {
+        return "illegal base32 data at input byte " + strconv.Itoa64(int64(e))
+}
+
+// decode is like Decode but returns an additional 'end' value, which
+// indicates if end-of-message padding was encountered and thus any
+// additional data is an error.  decode also assumes len(src)%8==0,
+// since it is meant for internal use.
+func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err os.Error) {
+        for i := 0; i < len(src)/8 && !end; i++ {
+                // Decode quantum using the base32 alphabet
+                var dbuf [8]byte
+                dlen := 8
+
+                // do the top bytes contain any data?
+        dbufloop:
+                for j := 0; j < 8; j++ {
+                        in := src[i*8+j]
+                        if in == '=' && j >= 2 && i == len(src)/8-1 {
+                                // We've reached the end and there's
+                                // padding, the rest should be padded
+                                for k := j; k < 8; k++ {
+                                        if src[i*8+k] != '=' {
+                                                return n, false, CorruptInputError(i*8 + j)
+                                        }
+                                }
+                                dlen = j
+                                end = true
+                                break dbufloop
+                        }
+                        dbuf[j] = enc.decodeMap[in]
+                        if dbuf[j] == 0xFF {
+                                return n, false, CorruptInputError(i*8 + j)
+                        }
+                }
+
+                // Pack 8x 5-bit source blocks into 5 byte destination
+                // quantum
+                switch dlen {
+                case 7, 8:
+                        dst[i*5+4] = dbuf[6]<<5 | dbuf[7]
+                        fallthrough
+                case 6, 5:
+                        dst[i*5+3] = dbuf[4]<<7 | dbuf[5]<<2 | dbuf[6]>>3
+                        fallthrough
+                case 4:
+                        dst[i*5+2] = dbuf[3]<<4 | dbuf[4]>>1
+                        fallthrough
+                case 3:
+                        dst[i*5+1] = dbuf[1]<<6 | dbuf[2]<<1 | dbuf[3]>>4
+                        fallthrough
+                case 2:
+                        dst[i*5+0] = dbuf[0]<<3 | dbuf[1]>>2
+                }
+                switch dlen {
+                case 2:
+                        n += 1
+                case 3, 4:
+                        n += 2
+                case 5:
+                        n += 3
+                case 6, 7:
+                        n += 4
+                case 8:
+                        n += 5
+                }
+        }
+        return n, end, nil
+}
+
+// Decode decodes src using the encoding enc.  It writes at most
+// DecodedLen(len(src)) bytes to dst and returns the number of bytes
+// written.  If src contains invalid base32 data, it will return the
+// number of bytes successfully written and CorruptInputError.
+func (enc *Encoding) Decode(dst, src []byte) (n int, err os.Error) {
+        if len(src)%8 != 0 {
+                return 0, CorruptInputError(len(src) / 8 * 8)
+        }
+
+        n, _, err = enc.decode(dst, src)
+        return
+}
+
+type decoder struct {
+        err    os.Error
+        enc    *Encoding
+        r      io.Reader
+        end    bool       // saw end of message
+        buf    [1024]byte // leftover input
+        nbuf   int
+        out    []byte // leftover decoded output
+        outbuf [1024 / 8 * 5]byte
+}
+
+func (d *decoder) Read(p []byte) (n int, err os.Error) {
+        if d.err != nil {
+                return 0, d.err
+        }
+
+        // Use leftover decoded output from last read.
+        if len(d.out) > 0 {
+                n = copy(p, d.out)
+                d.out = d.out[n:]
+                return n, nil
+        }
+
+        // Read a chunk.
+        nn := len(p) / 5 * 8
+        if nn < 8 {
+                nn = 8
+        }
+        if nn > len(d.buf) {
+                nn = len(d.buf)
+        }
+        nn, d.err = io.ReadAtLeast(d.r, d.buf[d.nbuf:nn], 8-d.nbuf)
+        d.nbuf += nn
+        if d.nbuf < 8 {
+                return 0, d.err
+        }
+
+        // Decode chunk into p, or d.out and then p if p is too small.
+        nr := d.nbuf / 8 * 8
+        nw := d.nbuf / 8 * 5
+        if nw > len(p) {
+                nw, d.end, d.err = d.enc.decode(d.outbuf[0:], d.buf[0:nr])
+                d.out = d.outbuf[0:nw]
+                n = copy(p, d.out)
+                d.out = d.out[n:]
+        } else {
+                n, d.end, d.err = d.enc.decode(p, d.buf[0:nr])
+        }
+        d.nbuf -= nr
+        for i := 0; i < d.nbuf; i++ {
+                d.buf[i] = d.buf[i+nr]
+        }
+
+        if d.err == nil {
+                d.err = err
+        }
+        return n, d.err
+}
+
+// NewDecoder constructs a new base32 stream decoder.
+func NewDecoder(enc *Encoding, r io.Reader) io.Reader {
+        return &decoder{enc: enc, r: r}
+}
+
+// DecodedLen returns the maximum length in bytes of the decoded data
+// corresponding to n bytes of base32-encoded data.
+func (enc *Encoding) DecodedLen(n int) int { return n / 8 * 5 }