Source file src/pkg/encoding/ascii85/ascii85.go

     1	// Copyright 2009 The Go Authors. All rights reserved.
     2	// Use of this source code is governed by a BSD-style
     3	// license that can be found in the LICENSE file.
     4	
     5	// Package ascii85 implements the ascii85 data encoding
     6	// as used in the btoa tool and Adobe's PostScript and PDF document formats.
     7	package ascii85
     8	
     9	import (
    10		"io"
    11		"strconv"
    12	)
    13	
    14	/*
    15	 * Encoder
    16	 */
    17	
    18	// Encode encodes src into at most MaxEncodedLen(len(src))
    19	// bytes of dst, returning the actual number of bytes written.
    20	//
    21	// The encoding handles 4-byte chunks, using a special encoding
    22	// for the last fragment, so Encode is not appropriate for use on
    23	// individual blocks of a large data stream.  Use NewEncoder() instead.
    24	//
    25	// Often, ascii85-encoded data is wrapped in <~ and ~> symbols.
    26	// Encode does not add these.
    27	func Encode(dst, src []byte) int {
    28		if len(src) == 0 {
    29			return 0
    30		}
    31	
    32		n := 0
    33		for len(src) > 0 {
    34			dst[0] = 0
    35			dst[1] = 0
    36			dst[2] = 0
    37			dst[3] = 0
    38			dst[4] = 0
    39	
    40			// Unpack 4 bytes into uint32 to repack into base 85 5-byte.
    41			var v uint32
    42			switch len(src) {
    43			default:
    44				v |= uint32(src[3])
    45				fallthrough
    46			case 3:
    47				v |= uint32(src[2]) << 8
    48				fallthrough
    49			case 2:
    50				v |= uint32(src[1]) << 16
    51				fallthrough
    52			case 1:
    53				v |= uint32(src[0]) << 24
    54			}
    55	
    56			// Special case: zero (!!!!!) shortens to z.
    57			if v == 0 && len(src) >= 4 {
    58				dst[0] = 'z'
    59				dst = dst[1:]
    60				src = src[4:]
    61				n++
    62				continue
    63			}
    64	
    65			// Otherwise, 5 base 85 digits starting at !.
    66			for i := 4; i >= 0; i-- {
    67				dst[i] = '!' + byte(v%85)
    68				v /= 85
    69			}
    70	
    71			// If src was short, discard the low destination bytes.
    72			m := 5
    73			if len(src) < 4 {
    74				m -= 4 - len(src)
    75				src = nil
    76			} else {
    77				src = src[4:]
    78			}
    79			dst = dst[m:]
    80			n += m
    81		}
    82		return n
    83	}
    84	
    85	// MaxEncodedLen returns the maximum length of an encoding of n source bytes.
    86	func MaxEncodedLen(n int) int { return (n + 3) / 4 * 5 }
    87	
    88	// NewEncoder returns a new ascii85 stream encoder.  Data written to
    89	// the returned writer will be encoded and then written to w.
    90	// Ascii85 encodings operate in 32-bit blocks; when finished
    91	// writing, the caller must Close the returned encoder to flush any
    92	// trailing partial block.
    93	func NewEncoder(w io.Writer) io.WriteCloser { return &encoder{w: w} }
    94	
    95	type encoder struct {
    96		err  error
    97		w    io.Writer
    98		buf  [4]byte    // buffered data waiting to be encoded
    99		nbuf int        // number of bytes in buf
   100		out  [1024]byte // output buffer
   101	}
   102	
   103	func (e *encoder) Write(p []byte) (n int, err error) {
   104		if e.err != nil {
   105			return 0, e.err
   106		}
   107	
   108		// Leading fringe.
   109		if e.nbuf > 0 {
   110			var i int
   111			for i = 0; i < len(p) && e.nbuf < 4; i++ {
   112				e.buf[e.nbuf] = p[i]
   113				e.nbuf++
   114			}
   115			n += i
   116			p = p[i:]
   117			if e.nbuf < 4 {
   118				return
   119			}
   120			nout := Encode(e.out[0:], e.buf[0:])
   121			if _, e.err = e.w.Write(e.out[0:nout]); e.err != nil {
   122				return n, e.err
   123			}
   124			e.nbuf = 0
   125		}
   126	
   127		// Large interior chunks.
   128		for len(p) >= 4 {
   129			nn := len(e.out) / 5 * 4
   130			if nn > len(p) {
   131				nn = len(p)
   132			}
   133			nn -= nn % 4
   134			if nn > 0 {
   135				nout := Encode(e.out[0:], p[0:nn])
   136				if _, e.err = e.w.Write(e.out[0:nout]); e.err != nil {
   137					return n, e.err
   138				}
   139			}
   140			n += nn
   141			p = p[nn:]
   142		}
   143	
   144		// Trailing fringe.
   145		for i := 0; i < len(p); i++ {
   146			e.buf[i] = p[i]
   147		}
   148		e.nbuf = len(p)
   149		n += len(p)
   150		return
   151	}
   152	
   153	// Close flushes any pending output from the encoder.
   154	// It is an error to call Write after calling Close.
   155	func (e *encoder) Close() error {
   156		// If there's anything left in the buffer, flush it out
   157		if e.err == nil && e.nbuf > 0 {
   158			nout := Encode(e.out[0:], e.buf[0:e.nbuf])
   159			e.nbuf = 0
   160			_, e.err = e.w.Write(e.out[0:nout])
   161		}
   162		return e.err
   163	}
   164	
   165	/*
   166	 * Decoder
   167	 */
   168	
   169	type CorruptInputError int64
   170	
   171	func (e CorruptInputError) Error() string {
   172		return "illegal ascii85 data at input byte " + strconv.FormatInt(int64(e), 10)
   173	}
   174	
   175	// Decode decodes src into dst, returning both the number
   176	// of bytes written to dst and the number consumed from src.
   177	// If src contains invalid ascii85 data, Decode will return the
   178	// number of bytes successfully written and a CorruptInputError.
   179	// Decode ignores space and control characters in src.
   180	// Often, ascii85-encoded data is wrapped in <~ and ~> symbols.
   181	// Decode expects these to have been stripped by the caller.
   182	//
   183	// If flush is true, Decode assumes that src represents the
   184	// end of the input stream and processes it completely rather
   185	// than wait for the completion of another 32-bit block.
   186	//
   187	// NewDecoder wraps an io.Reader interface around Decode.
   188	//
   189	func Decode(dst, src []byte, flush bool) (ndst, nsrc int, err error) {
   190		var v uint32
   191		var nb int
   192		for i, b := range src {
   193			if len(dst)-ndst < 4 {
   194				return
   195			}
   196			switch {
   197			case b <= ' ':
   198				continue
   199			case b == 'z' && nb == 0:
   200				nb = 5
   201				v = 0
   202			case '!' <= b && b <= 'u':
   203				v = v*85 + uint32(b-'!')
   204				nb++
   205			default:
   206				return 0, 0, CorruptInputError(i)
   207			}
   208			if nb == 5 {
   209				nsrc = i + 1
   210				dst[ndst] = byte(v >> 24)
   211				dst[ndst+1] = byte(v >> 16)
   212				dst[ndst+2] = byte(v >> 8)
   213				dst[ndst+3] = byte(v)
   214				ndst += 4
   215				nb = 0
   216				v = 0
   217			}
   218		}
   219		if flush {
   220			nsrc = len(src)
   221			if nb > 0 {
   222				// The number of output bytes in the last fragment
   223				// is the number of leftover input bytes - 1:
   224				// the extra byte provides enough bits to cover
   225				// the inefficiency of the encoding for the block.
   226				if nb == 1 {
   227					return 0, 0, CorruptInputError(len(src))
   228				}
   229				for i := nb; i < 5; i++ {
   230					// The short encoding truncated the output value.
   231					// We have to assume the worst case values (digit 84)
   232					// in order to ensure that the top bits are correct.
   233					v = v*85 + 84
   234				}
   235				for i := 0; i < nb-1; i++ {
   236					dst[ndst] = byte(v >> 24)
   237					v <<= 8
   238					ndst++
   239				}
   240			}
   241		}
   242		return
   243	}
   244	
   245	// NewDecoder constructs a new ascii85 stream decoder.
   246	func NewDecoder(r io.Reader) io.Reader { return &decoder{r: r} }
   247	
   248	type decoder struct {
   249		err     error
   250		readErr error
   251		r       io.Reader
   252		end     bool       // saw end of message
   253		buf     [1024]byte // leftover input
   254		nbuf    int
   255		out     []byte // leftover decoded output
   256		outbuf  [1024]byte
   257	}
   258	
   259	func (d *decoder) Read(p []byte) (n int, err error) {
   260		if len(p) == 0 {
   261			return 0, nil
   262		}
   263		if d.err != nil {
   264			return 0, d.err
   265		}
   266	
   267		for {
   268			// Copy leftover output from last decode.
   269			if len(d.out) > 0 {
   270				n = copy(p, d.out)
   271				d.out = d.out[n:]
   272				return
   273			}
   274	
   275			// Decode leftover input from last read.
   276			var nn, nsrc, ndst int
   277			if d.nbuf > 0 {
   278				ndst, nsrc, d.err = Decode(d.outbuf[0:], d.buf[0:d.nbuf], d.readErr != nil)
   279				if ndst > 0 {
   280					d.out = d.outbuf[0:ndst]
   281					d.nbuf = copy(d.buf[0:], d.buf[nsrc:d.nbuf])
   282					continue // copy out and return
   283				}
   284			}
   285	
   286			// Out of input, out of decoded output.  Check errors.
   287			if d.err != nil {
   288				return 0, d.err
   289			}
   290			if d.readErr != nil {
   291				d.err = d.readErr
   292				return 0, d.err
   293			}
   294	
   295			// Read more data.
   296			nn, d.readErr = d.r.Read(d.buf[d.nbuf:])
   297			d.nbuf += nn
   298		}
   299		panic("unreachable")
   300	}