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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