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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 ascii85 implements the ascii85 data encoding
// as used in the btoa tool and Adobe's PostScript and PDF document formats.
package ascii85

import (
        "io"
        "strconv"
)

/*
 * Encoder
 */

// Encode encodes src into at most MaxEncodedLen(len(src))
// bytes of dst, returning the actual number of bytes written.
//
// The encoding handles 4-byte chunks, using a special encoding
// for the last fragment, so Encode is not appropriate for use on
// individual blocks of a large data stream.  Use NewEncoder() instead.
//
// Often, ascii85-encoded data is wrapped in <~ and ~> symbols.
// Encode does not add these.
func Encode(dst, src []byte) int {
        if len(src) == 0 {
                return 0
        }

        n := 0
        for len(src) > 0 {
                dst[0] = 0
                dst[1] = 0
                dst[2] = 0
                dst[3] = 0
                dst[4] = 0

                // Unpack 4 bytes into uint32 to repack into base 85 5-byte.
                var v uint32
                switch len(src) {
                default:
                        v |= uint32(src[3])
                        fallthrough
                case 3:
                        v |= uint32(src[2]) << 8
                        fallthrough
                case 2:
                        v |= uint32(src[1]) << 16
                        fallthrough
                case 1:
                        v |= uint32(src[0]) << 24
                }

                // Special case: zero (!!!!!) shortens to z.
                if v == 0 && len(src) >= 4 {
                        dst[0] = 'z'
                        dst = dst[1:]
                        n++
                        continue
                }

                // Otherwise, 5 base 85 digits starting at !.
                for i := 4; i >= 0; i-- {
                        dst[i] = '!' + byte(v%85)
                        v /= 85
                }

                // If src was short, discard the low destination bytes.
                m := 5
                if len(src) < 4 {
                        m -= 4 - len(src)
                        src = nil
                } else {
                        src = src[4:]
                }
                dst = dst[m:]
                n += m
        }
        return n
}

// MaxEncodedLen returns the maximum length of an encoding of n source bytes.
func MaxEncodedLen(n int) int { return (n + 3) / 4 * 5 }

// NewEncoder returns a new ascii85 stream encoder.  Data written to
// the returned writer will be encoded and then written to w.
// Ascii85 encodings operate in 32-bit blocks; when finished
// writing, the caller must Close the returned encoder to flush any
// trailing partial block.
func NewEncoder(w io.Writer) io.WriteCloser { return &encoder{w: w} }

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

        // Large interior chunks.
        for len(p) >= 4 {
                nn := len(e.out) / 5 * 4
                if nn > len(p) {
                        nn = len(p)
                }
                nn -= nn % 4
                if nn > 0 {
                        nout := Encode(e.out[0:], p[0:nn])
                        if _, e.err = e.w.Write(e.out[0:nout]); 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 {
                nout := Encode(e.out[0:], e.buf[0:e.nbuf])
                e.nbuf = 0
                _, e.err = e.w.Write(e.out[0:nout])
        }
        return e.err
}

/*
 * Decoder
 */

type CorruptInputError int64

func (e CorruptInputError) Error() string {
        return "illegal ascii85 data at input byte " + strconv.FormatInt(int64(e), 10)
}

// Decode decodes src into dst, returning both the number
// of bytes written to dst and the number consumed from src.
// If src contains invalid ascii85 data, Decode will return the
// number of bytes successfully written and a CorruptInputError.
// Decode ignores space and control characters in src.
// Often, ascii85-encoded data is wrapped in <~ and ~> symbols.
// Decode expects these to have been stripped by the caller.
//
// If flush is true, Decode assumes that src represents the
// end of the input stream and processes it completely rather
// than wait for the completion of another 32-bit block.
//
// NewDecoder wraps an io.Reader interface around Decode.
//
func Decode(dst, src []byte, flush bool) (ndst, nsrc int, err error) {
        var v uint32
        var nb int
        for i, b := range src {
                if len(dst)-ndst < 4 {
                        return
                }
                switch {
                case b <= ' ':
                        continue
                case b == 'z' && nb == 0:
                        nb = 5
                        v = 0
                case '!' <= b && b <= 'u':
                        v = v*85 + uint32(b-'!')
                        nb++
                default:
                        return 0, 0, CorruptInputError(i)
                }
                if nb == 5 {
                        nsrc = i + 1
                        dst[ndst] = byte(v >> 24)
                        dst[ndst+1] = byte(v >> 16)
                        dst[ndst+2] = byte(v >> 8)
                        dst[ndst+3] = byte(v)
                        ndst += 4
                        nb = 0
                        v = 0
                }
        }
        if flush {
                nsrc = len(src)
                if nb > 0 {
                        // The number of output bytes in the last fragment
                        // is the number of leftover input bytes - 1:
                        // the extra byte provides enough bits to cover
                        // the inefficiency of the encoding for the block.
                        if nb == 1 {
                                return 0, 0, CorruptInputError(len(src))
                        }
                        for i := nb; i < 5; i++ {
                                // The short encoding truncated the output value.
                                // We have to assume the worst case values (digit 84)
                                // in order to ensure that the top bits are correct.
                                v = v*85 + 84
                        }
                        for i := 0; i < nb-1; i++ {
                                dst[ndst] = byte(v >> 24)
                                v <<= 8
                                ndst++
                        }
                }
        }
        return
}

// NewDecoder constructs a new ascii85 stream decoder.
func NewDecoder(r io.Reader) io.Reader { return &decoder{r: r} }

type decoder struct {
        err     error
        readErr error
        r       io.Reader
        end     bool       // saw end of message
        buf     [1024]byte // leftover input
        nbuf    int
        out     []byte // leftover decoded output
        outbuf  [1024]byte
}

func (d *decoder) Read(p []byte) (n int, err error) {
        if len(p) == 0 {
                return 0, nil
        }
        if d.err != nil {
                return 0, d.err
        }

        for {
                // Copy leftover output from last decode.
                if len(d.out) > 0 {
                        n = copy(p, d.out)
                        d.out = d.out[n:]
                        return
                }

                // Decode leftover input from last read.
                var nn, nsrc, ndst int
                if d.nbuf > 0 {
                        ndst, nsrc, d.err = Decode(d.outbuf[0:], d.buf[0:d.nbuf], d.readErr != nil)
                        if ndst > 0 {
                                d.out = d.outbuf[0:ndst]
                                d.nbuf = copy(d.buf[0:], d.buf[nsrc:d.nbuf])
                                continue // copy out and return
                        }
                }

                // Out of input, out of decoded output.  Check errors.
                if d.err != nil {
                        return 0, d.err
                }
                if d.readErr != nil {
                        d.err = d.readErr
                        return 0, d.err
                }

                // Read more data.
                nn, d.readErr = d.r.Read(d.buf[d.nbuf:])
                d.nbuf += nn
        }
        panic("unreachable")
}