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Source file src/encoding/ascii85/ascii85.go

Documentation: encoding/ascii85

  // 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:]
  			src = src[4:]
  			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
  	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
  			}
  			if ndst == 0 && d.err == nil {
  				// Special case: input buffer is mostly filled with non-data bytes.
  				// Filter out such bytes to make room for more input.
  				off := 0
  				for i := 0; i < d.nbuf; i++ {
  					if d.buf[i] > ' ' {
  						d.buf[off] = d.buf[i]
  						off++
  					}
  				}
  				d.nbuf = off
  			}
  		}
  
  		// 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
  	}
  }
  

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