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// 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"
        "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:]
        }
}

// EncodeToString returns the base32 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  [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 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() 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) Error() string {
        return "illegal base32 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 base32 alphabet
                var dbuf [8]byte
                dlen := 8

                // do the top bytes contain any data?
        dbufloop:
                for j := 0; j < 8; {
                        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) < 8 {
                                // We've reached the end and there's
                                // padding, the rest should be padded
                                for k := 0; k < 8-j-1; k++ {
                                        if len(src) > k && src[k] != '=' {
                                                return n, false, CorruptInputError(len(osrc) - len(src) + k - 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 8x 5-bit source blocks into 5 byte destination
                // quantum
                switch dlen {
                case 7, 8:
                        dst[4] = dbuf[6]<<5 | dbuf[7]
                        fallthrough
                case 6, 5:
                        dst[3] = dbuf[4]<<7 | dbuf[5]<<2 | dbuf[6]>>3
                        fallthrough
                case 4:
                        dst[2] = dbuf[3]<<4 | dbuf[4]>>1
                        fallthrough
                case 3:
                        dst[1] = dbuf[1]<<6 | dbuf[2]<<1 | dbuf[3]>>4
                        fallthrough
                case 2:
                        dst[0] = dbuf[0]<<3 | dbuf[1]>>2
                }
                dst = dst[5:]
                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.
// 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 base32 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 / 8 * 5]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) / 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 }