Source file src/encoding/ascii85/ascii85.go

Documentation: encoding/ascii85

     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  	buf     [1024]byte // leftover input
   253  	nbuf    int
   254  	out     []byte // leftover decoded output
   255  	outbuf  [1024]byte
   256  }
   257  
   258  func (d *decoder) Read(p []byte) (n int, err error) {
   259  	if len(p) == 0 {
   260  		return 0, nil
   261  	}
   262  	if d.err != nil {
   263  		return 0, d.err
   264  	}
   265  
   266  	for {
   267  		// Copy leftover output from last decode.
   268  		if len(d.out) > 0 {
   269  			n = copy(p, d.out)
   270  			d.out = d.out[n:]
   271  			return
   272  		}
   273  
   274  		// Decode leftover input from last read.
   275  		var nn, nsrc, ndst int
   276  		if d.nbuf > 0 {
   277  			ndst, nsrc, d.err = Decode(d.outbuf[0:], d.buf[0:d.nbuf], d.readErr != nil)
   278  			if ndst > 0 {
   279  				d.out = d.outbuf[0:ndst]
   280  				d.nbuf = copy(d.buf[0:], d.buf[nsrc:d.nbuf])
   281  				continue // copy out and return
   282  			}
   283  			if ndst == 0 && d.err == nil {
   284  				// Special case: input buffer is mostly filled with non-data bytes.
   285  				// Filter out such bytes to make room for more input.
   286  				off := 0
   287  				for i := 0; i < d.nbuf; i++ {
   288  					if d.buf[i] > ' ' {
   289  						d.buf[off] = d.buf[i]
   290  						off++
   291  					}
   292  				}
   293  				d.nbuf = off
   294  			}
   295  		}
   296  
   297  		// Out of input, out of decoded output. Check errors.
   298  		if d.err != nil {
   299  			return 0, d.err
   300  		}
   301  		if d.readErr != nil {
   302  			d.err = d.readErr
   303  			return 0, d.err
   304  		}
   305  
   306  		// Read more data.
   307  		nn, d.readErr = d.r.Read(d.buf[d.nbuf:])
   308  		d.nbuf += nn
   309  	}
   310  }
   311  

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