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// 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"

        "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:]

        }

}

// EncodeToString returns the base64 encoding of src.

func (enc *Encoding) EncodeToString(src []byte) string {

        buf := make([]byte, enc.EncodedLen(len(src)))

        enc.Encode(buf, src)

        return string(buf)

}

type encoder struct {

        err  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 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() 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) Error() string {

        return "illegal base64 data at input byte " + strconv.FormatInt(int64(e), 10)

}

// 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.

func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {

        osrc := src

        for len(src) > 0 && !end {

                // Decode quantum using the base64 alphabet

                var dbuf [4]byte

                dlen := 4

        dbufloop:

                for j := 0; j < 4; {

                        if len(src) == 0 {

                                return n, false, CorruptInputError(len(osrc) - len(src) - j)

                        }

                        in := src[0]

                        src = src[1:]

                        if in == '\r' || in == '\n' {

                                // Ignore this character.

                                continue

                        }

                        if in == '=' && j >= 2 && len(src) < 4 {

                                // We've reached the end and there's

                                // padding

                                if len(src) > 0 && src[0] != '=' {

                                        return n, false, CorruptInputError(len(osrc) - len(src) - 1)

                                }

                                dlen = j

                                end = true

                                break dbufloop

                        }

                        dbuf[j] = enc.decodeMap[in]

                        if dbuf[j] == 0xFF {

                                return n, false, CorruptInputError(len(osrc) - len(src) - 1)

                        }

                        j++

                }

                // Pack 4x 6-bit source blocks into 3 byte destination

                // quantum

                switch dlen {

                case 4:

                        dst[2] = dbuf[2]<<6 | dbuf[3]

                        fallthrough

                case 3:

                        dst[1] = dbuf[1]<<4 | dbuf[2]>>2

                        fallthrough

                case 2:

                        dst[0] = dbuf[0]<<2 | dbuf[1]>>4

                }

                dst = dst[3:]

                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.

// New line characters (\r and \n) are ignored.

func (enc *Encoding) Decode(dst, src []byte) (n int, err error) {

        n, _, err = enc.decode(dst, src)

        return

}

// DecodeString returns the bytes represented by the base64 string s.

func (enc *Encoding) DecodeString(s string) ([]byte, error) {

        dbuf := make([]byte, enc.DecodedLen(len(s)))

        n, err := enc.Decode(dbuf, []byte(s))

        return dbuf[:n], err

}

type decoder struct {

        err    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 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 }