Source file src/pkg/encoding/base64/base64.go

// Copyright 2009 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 base64 implements base64 encoding as specified by RFC 4648.
package base64

import (
    "io"
    "os"
    "strconv"
)

/*
 * Encodings
 */

// An Encoding is a radix 64 encoding/decoding scheme, defined by a
// 64-character alphabet.  The most common encoding is the "base64"
// encoding defined in RFC 4648 and used in MIME (RFC 2045) and PEM
// (RFC 1421).  RFC 4648 also defines an alternate encoding, which is
// the standard encoding with - and _ substituted for + and /.
type Encoding struct {
    encode    string
    decodeMap [256]byte
}

const encodeStd = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
const encodeURL = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_"

// NewEncoding returns a new Encoding defined by the given alphabet,
// which must be a 64-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 base64 encoding, as defined in
// RFC 4648.
var StdEncoding = NewEncoding(encodeStd)

// URLEncoding is the alternate base64 encoding defined in RFC 4648.
// It is typically used in URLs and file names.
var URLEncoding = NewEncoding(encodeURL)

/*
 * Encoder
 */

// Encode encodes src using the encoding enc, writing
// EncodedLen(len(src)) bytes to dst.
//
// The encoding pads the output to a multiple of 4 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

        // Unpack 4x 6-bit source blocks into a 4 byte
        // destination quantum
        switch len(src) {
        default:
            dst[3] |= src[2] & 0x3F
            dst[2] |= src[2] >> 6
            fallthrough
        case 2:
            dst[2] |= (src[1] << 2) & 0x3F
            dst[1] |= src[1] >> 4
            fallthrough
        case 1:
            dst[1] |= (src[0] << 4) & 0x3F
            dst[0] |= src[0] >> 2
        }

        // Encode 6-bit blocks using the base64 alphabet
        for j := 0; j < 4; j++ {
            dst[j] = enc.encode[dst[j]]
        }

        // Pad the final quantum
        if len(src) < 3 {
            dst[3] = '='
            if len(src) < 2 {
                dst[2] = '='
            }
            break
        }

        src = src[3:]
        dst = dst[4:]
    }
}

type encoder struct {
    err  os.Error
    enc  *Encoding
    w    io.Writer
    buf  [3]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 < 3; i++ {
            e.buf[e.nbuf] = p[i]
            e.nbuf++
        }
        n += i
        p = p[i:]
        if e.nbuf < 3 {
            return
        }
        e.enc.Encode(e.out[0:], e.buf[0:])
        if _, e.err = e.w.Write(e.out[0:4]); e.err != nil {
            return n, e.err
        }
        e.nbuf = 0
    }

    // Large interior chunks.
    for len(p) >= 3 {
        nn := len(e.out) / 4 * 3
        if nn > len(p) {
            nn = len(p)
        }
        nn -= nn % 3
        if nn > 0 {
            e.enc.Encode(e.out[0:], p[0:nn])
            if _, e.err = e.w.Write(e.out[0 : nn/3*4]); 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[0:4])
    }
    return e.err
}

// NewEncoder returns a new base64 stream encoder.  Data written to
// the returned writer will be encoded using enc and then written to w.
// Base64 encodings operate in 4-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 base64 encoding
// of an input buffer of length n.
func (enc *Encoding) EncodedLen(n int) int { return (n + 2) / 3 * 4 }

/*
 * Decoder
 */

type CorruptInputError int64

func (e CorruptInputError) String() string {
    return "illegal base64 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)%4==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)/4 && !end; i++ {
        // Decode quantum using the base64 alphabet
        var dbuf [4]byte
        dlen := 4

    dbufloop:
        for j := 0; j < 4; j++ {
            in := src[i*4+j]
            if in == '=' && j >= 2 && i == len(src)/4-1 {
                // We've reached the end and there's
                // padding
                if src[i*4+3] != '=' {
                    return n, false, CorruptInputError(i*4 + 2)
                }
                dlen = j
                end = true
                break dbufloop
            }
            dbuf[j] = enc.decodeMap[in]
            if dbuf[j] == 0xFF {
                return n, false, CorruptInputError(i*4 + j)
            }
        }

        // Pack 4x 6-bit source blocks into 3 byte destination
        // quantum
        switch dlen {
        case 4:
            dst[i*3+2] = dbuf[2]<<6 | dbuf[3]
            fallthrough
        case 3:
            dst[i*3+1] = dbuf[1]<<4 | dbuf[2]>>2
            fallthrough
        case 2:
            dst[i*3+0] = dbuf[0]<<2 | dbuf[1]>>4
        }
        n += dlen - 1
    }

    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 base64 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)%4 != 0 {
        return 0, CorruptInputError(len(src) / 4 * 4)
    }

    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 / 4 * 3]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) / 3 * 4
    if nn < 4 {
        nn = 4
    }
    if nn > len(d.buf) {
        nn = len(d.buf)
    }
    nn, d.err = io.ReadAtLeast(d.r, d.buf[d.nbuf:nn], 4-d.nbuf)
    d.nbuf += nn
    if d.nbuf < 4 {
        return 0, d.err
    }

    // Decode chunk into p, or d.out and then p if p is too small.
    nr := d.nbuf / 4 * 4
    nw := d.nbuf / 4 * 3
    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 base64 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 base64-encoded data.
func (enc *Encoding) DecodedLen(n int) int { return n / 4 * 3 }