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// Copyright 2011 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 lzw implements the Lempel-Ziv-Welch compressed data format,
// described in T. A. Welch, ``A Technique for High-Performance Data
// Compression'', Computer, 17(6) (June 1984), pp 8-19.
//
// In particular, it implements LZW as used by the GIF, TIFF and PDF file
// formats, which means variable-width codes up to 12 bits and the first
// two non-literal codes are a clear code and an EOF code.
package lzw
// TODO(nigeltao): check that TIFF and PDF use LZW in the same way as GIF,
// modulo LSB/MSB packing order.
import (
"bufio"
"fmt"
"io"
"os"
)
// Order specifies the bit ordering in an LZW data stream.
type Order int
const (
// LSB means Least Significant Bits first, as used in the GIF file format.
LSB Order = iota
// MSB means Most Significant Bits first, as used in the TIFF and PDF
// file formats.
MSB
)
// decoder is the state from which the readXxx method converts a byte
// stream into a code stream.
type decoder struct {
r io.ByteReader
bits uint32
nBits uint
width uint
}
// readLSB returns the next code for "Least Significant Bits first" data.
func (d *decoder) readLSB() (uint16, os.Error) {
for d.nBits < d.width {
x, err := d.r.ReadByte()
if err != nil {
return 0, err
}
d.bits |= uint32(x) << d.nBits
d.nBits += 8
}
code := uint16(d.bits & (1<<d.width - 1))
d.bits >>= d.width
d.nBits -= d.width
return code, nil
}
// readMSB returns the next code for "Most Significant Bits first" data.
func (d *decoder) readMSB() (uint16, os.Error) {
for d.nBits < d.width {
x, err := d.r.ReadByte()
if err != nil {
return 0, err
}
d.bits |= uint32(x) << (24 - d.nBits)
d.nBits += 8
}
code := uint16(d.bits >> (32 - d.width))
d.bits <<= d.width
d.nBits -= d.width
return code, nil
}
// decode decompresses bytes from r and writes them to pw.
// read specifies how to decode bytes into codes.
// litWidth is the width in bits of literal codes.
func decode(r io.Reader, read func(*decoder) (uint16, os.Error), litWidth int, pw *io.PipeWriter) {
br, ok := r.(io.ByteReader)
if !ok {
br = bufio.NewReader(r)
}
pw.CloseWithError(decode1(pw, br, read, uint(litWidth)))
}
func decode1(pw *io.PipeWriter, r io.ByteReader, read func(*decoder) (uint16, os.Error), litWidth uint) os.Error {
const (
maxWidth = 12
invalidCode = 0xffff
)
d := decoder{r, 0, 0, 1 + litWidth}
w := bufio.NewWriter(pw)
// The first 1<<litWidth codes are literal codes.
// The next two codes mean clear and EOF.
// Other valid codes are in the range [lo, hi] where lo := clear + 2,
// with the upper bound incrementing on each code seen.
clear := uint16(1) << litWidth
eof, hi := clear+1, clear+1
// overflow is the code at which hi overflows the code width.
overflow := uint16(1) << d.width
var (
// Each code c in [lo, hi] expands to two or more bytes. For c != hi:
// suffix[c] is the last of these bytes.
// prefix[c] is the code for all but the last byte.
// This code can either be a literal code or another code in [lo, c).
// The c == hi case is a special case.
suffix [1 << maxWidth]uint8
prefix [1 << maxWidth]uint16
// buf is a scratch buffer for reconstituting the bytes that a code expands to.
// Code suffixes are written right-to-left from the end of the buffer.
buf [1 << maxWidth]byte
)
// Loop over the code stream, converting codes into decompressed bytes.
last := uint16(invalidCode)
for {
code, err := read(&d)
if err != nil {
if err == os.EOF {
err = io.ErrUnexpectedEOF
}
return err
}
switch {
case code < clear:
// We have a literal code.
if err := w.WriteByte(uint8(code)); err != nil {
return err
}
if last != invalidCode {
// Save what the hi code expands to.
suffix[hi] = uint8(code)
prefix[hi] = last
}
case code == clear:
d.width = 1 + litWidth
hi = eof
overflow = 1 << d.width
last = invalidCode
continue
case code == eof:
return w.Flush()
case code <= hi:
c, i := code, len(buf)-1
if code == hi {
// code == hi is a special case which expands to the last expansion
// followed by the head of the last expansion. To find the head, we walk
// the prefix chain until we find a literal code.
c = last
for c >= clear {
c = prefix[c]
}
buf[i] = uint8(c)
i--
c = last
}
// Copy the suffix chain into buf and then write that to w.
for c >= clear {
buf[i] = suffix[c]
i--
c = prefix[c]
}
buf[i] = uint8(c)
if _, err := w.Write(buf[i:]); err != nil {
return err
}
// Save what the hi code expands to.
suffix[hi] = uint8(c)
prefix[hi] = last
default:
return os.NewError("lzw: invalid code")
}
last, hi = code, hi+1
if hi == overflow {
if d.width == maxWidth {
return os.NewError("lzw: missing clear code")
}
d.width++
overflow <<= 1
}
}
panic("unreachable")
}
// NewReader creates a new io.ReadCloser that satisfies reads by decompressing
// the data read from r.
// It is the caller's responsibility to call Close on the ReadCloser when
// finished reading.
// The number of bits to use for literal codes, litWidth, must be in the
// range [2,8] and is typically 8.
func NewReader(r io.Reader, order Order, litWidth int) io.ReadCloser {
pr, pw := io.Pipe()
var read func(*decoder) (uint16, os.Error)
switch order {
case LSB:
read = (*decoder).readLSB
case MSB:
read = (*decoder).readMSB
default:
pw.CloseWithError(os.NewError("lzw: unknown order"))
return pr
}
if litWidth < 2 || 8 < litWidth {
pw.CloseWithError(fmt.Errorf("lzw: litWidth %d out of range", litWidth))
return pr
}
go decode(r, read, litWidth, pw)
return pr
}
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