2022-08-10 14:05:14 +02:00
|
|
|
// 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,
|
2022-09-28 19:30:40 +02:00
|
|
|
// described in T. A. Welch, “A Technique for High-Performance Data
|
|
|
|
// Compression”, Computer, 17(6) (June 1984), pp 8-19.
|
2022-08-10 14:05:14 +02:00
|
|
|
//
|
|
|
|
// In particular, it implements LZW as used by the TIFF file format, including
|
|
|
|
// an "off by one" algorithmic difference when compared to standard LZW.
|
|
|
|
package lzw // import "golang.org/x/image/tiff/lzw"
|
|
|
|
|
|
|
|
/*
|
|
|
|
This file was branched from src/pkg/compress/lzw/reader.go in the
|
|
|
|
standard library. Differences from the original are marked with "NOTE".
|
|
|
|
|
|
|
|
The tif_lzw.c file in the libtiff C library has this comment:
|
|
|
|
|
|
|
|
----
|
|
|
|
The 5.0 spec describes a different algorithm than Aldus
|
|
|
|
implements. Specifically, Aldus does code length transitions
|
|
|
|
one code earlier than should be done (for real LZW).
|
|
|
|
Earlier versions of this library implemented the correct
|
|
|
|
LZW algorithm, but emitted codes in a bit order opposite
|
|
|
|
to the TIFF spec. Thus, to maintain compatibility w/ Aldus
|
|
|
|
we interpret MSB-LSB ordered codes to be images written w/
|
|
|
|
old versions of this library, but otherwise adhere to the
|
|
|
|
Aldus "off by one" algorithm.
|
|
|
|
----
|
|
|
|
|
|
|
|
The Go code doesn't read (invalid) TIFF files written by old versions of
|
|
|
|
libtiff, but the LZW algorithm in this package still differs from the one in
|
2022-09-28 19:30:40 +02:00
|
|
|
Go's standard package library to accommodate this "off by one" in valid TIFFs.
|
2022-08-10 14:05:14 +02:00
|
|
|
*/
|
|
|
|
|
|
|
|
import (
|
|
|
|
"bufio"
|
|
|
|
"errors"
|
|
|
|
"fmt"
|
|
|
|
"io"
|
|
|
|
)
|
|
|
|
|
|
|
|
// 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
|
|
|
|
)
|
|
|
|
|
|
|
|
const (
|
|
|
|
maxWidth = 12
|
|
|
|
decoderInvalidCode = 0xffff
|
|
|
|
flushBuffer = 1 << maxWidth
|
|
|
|
)
|
|
|
|
|
|
|
|
// 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
|
|
|
|
read func(*decoder) (uint16, error) // readLSB or readMSB
|
|
|
|
litWidth int // width in bits of literal codes
|
|
|
|
err error
|
|
|
|
|
|
|
|
// 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.
|
|
|
|
// overflow is the code at which hi overflows the code width. NOTE: TIFF's LZW is "off by one".
|
|
|
|
// last is the most recently seen code, or decoderInvalidCode.
|
|
|
|
clear, eof, hi, overflow, last uint16
|
|
|
|
|
|
|
|
// 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
|
|
|
|
|
|
|
|
// output is the temporary output buffer.
|
|
|
|
// Literal codes are accumulated from the start of the buffer.
|
|
|
|
// Non-literal codes decode to a sequence of suffixes that are first
|
|
|
|
// written right-to-left from the end of the buffer before being copied
|
|
|
|
// to the start of the buffer.
|
|
|
|
// It is flushed when it contains >= 1<<maxWidth bytes,
|
|
|
|
// so that there is always room to decode an entire code.
|
|
|
|
output [2 * 1 << maxWidth]byte
|
|
|
|
o int // write index into output
|
|
|
|
toRead []byte // bytes to return from Read
|
|
|
|
}
|
|
|
|
|
|
|
|
// readLSB returns the next code for "Least Significant Bits first" data.
|
|
|
|
func (d *decoder) readLSB() (uint16, 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, 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
|
|
|
|
}
|
|
|
|
|
|
|
|
func (d *decoder) Read(b []byte) (int, error) {
|
|
|
|
for {
|
|
|
|
if len(d.toRead) > 0 {
|
|
|
|
n := copy(b, d.toRead)
|
|
|
|
d.toRead = d.toRead[n:]
|
|
|
|
return n, nil
|
|
|
|
}
|
|
|
|
if d.err != nil {
|
|
|
|
return 0, d.err
|
|
|
|
}
|
|
|
|
d.decode()
|
|
|
|
}
|
|
|
|
}
|
|
|
|
|
|
|
|
// decode decompresses bytes from r and leaves them in d.toRead.
|
|
|
|
// read specifies how to decode bytes into codes.
|
|
|
|
// litWidth is the width in bits of literal codes.
|
|
|
|
func (d *decoder) decode() {
|
|
|
|
// Loop over the code stream, converting codes into decompressed bytes.
|
|
|
|
loop:
|
|
|
|
for {
|
|
|
|
code, err := d.read(d)
|
|
|
|
if err != nil {
|
|
|
|
if err == io.EOF {
|
|
|
|
err = io.ErrUnexpectedEOF
|
|
|
|
}
|
|
|
|
d.err = err
|
|
|
|
break
|
|
|
|
}
|
|
|
|
switch {
|
|
|
|
case code < d.clear:
|
|
|
|
// We have a literal code.
|
|
|
|
d.output[d.o] = uint8(code)
|
|
|
|
d.o++
|
|
|
|
if d.last != decoderInvalidCode {
|
|
|
|
// Save what the hi code expands to.
|
|
|
|
d.suffix[d.hi] = uint8(code)
|
|
|
|
d.prefix[d.hi] = d.last
|
|
|
|
}
|
|
|
|
case code == d.clear:
|
|
|
|
d.width = 1 + uint(d.litWidth)
|
|
|
|
d.hi = d.eof
|
|
|
|
d.overflow = 1 << d.width
|
|
|
|
d.last = decoderInvalidCode
|
|
|
|
continue
|
|
|
|
case code == d.eof:
|
|
|
|
d.err = io.EOF
|
|
|
|
break loop
|
|
|
|
case code <= d.hi:
|
|
|
|
c, i := code, len(d.output)-1
|
|
|
|
if code == d.hi && d.last != decoderInvalidCode {
|
|
|
|
// 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 = d.last
|
|
|
|
for c >= d.clear {
|
|
|
|
c = d.prefix[c]
|
|
|
|
}
|
|
|
|
d.output[i] = uint8(c)
|
|
|
|
i--
|
|
|
|
c = d.last
|
|
|
|
}
|
|
|
|
// Copy the suffix chain into output and then write that to w.
|
|
|
|
for c >= d.clear {
|
|
|
|
d.output[i] = d.suffix[c]
|
|
|
|
i--
|
|
|
|
c = d.prefix[c]
|
|
|
|
}
|
|
|
|
d.output[i] = uint8(c)
|
|
|
|
d.o += copy(d.output[d.o:], d.output[i:])
|
|
|
|
if d.last != decoderInvalidCode {
|
|
|
|
// Save what the hi code expands to.
|
|
|
|
d.suffix[d.hi] = uint8(c)
|
|
|
|
d.prefix[d.hi] = d.last
|
|
|
|
}
|
|
|
|
default:
|
|
|
|
d.err = errors.New("lzw: invalid code")
|
|
|
|
break loop
|
|
|
|
}
|
|
|
|
d.last, d.hi = code, d.hi+1
|
|
|
|
if d.hi+1 >= d.overflow { // NOTE: the "+1" is where TIFF's LZW differs from the standard algorithm.
|
|
|
|
if d.width == maxWidth {
|
|
|
|
d.last = decoderInvalidCode
|
|
|
|
} else {
|
|
|
|
d.width++
|
|
|
|
d.overflow <<= 1
|
|
|
|
}
|
|
|
|
}
|
|
|
|
if d.o >= flushBuffer {
|
|
|
|
break
|
|
|
|
}
|
|
|
|
}
|
|
|
|
// Flush pending output.
|
|
|
|
d.toRead = d.output[:d.o]
|
|
|
|
d.o = 0
|
|
|
|
}
|
|
|
|
|
|
|
|
var errClosed = errors.New("lzw: reader/writer is closed")
|
|
|
|
|
|
|
|
func (d *decoder) Close() error {
|
|
|
|
d.err = errClosed // in case any Reads come along
|
|
|
|
return nil
|
|
|
|
}
|
|
|
|
|
|
|
|
// NewReader creates a new io.ReadCloser.
|
|
|
|
// Reads from the returned io.ReadCloser read and decompress data from r.
|
|
|
|
// If r does not also implement io.ByteReader,
|
|
|
|
// the decompressor may read more data than necessary 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. It must equal the litWidth
|
|
|
|
// used during compression.
|
|
|
|
func NewReader(r io.Reader, order Order, litWidth int) io.ReadCloser {
|
|
|
|
d := new(decoder)
|
|
|
|
switch order {
|
|
|
|
case LSB:
|
|
|
|
d.read = (*decoder).readLSB
|
|
|
|
case MSB:
|
|
|
|
d.read = (*decoder).readMSB
|
|
|
|
default:
|
|
|
|
d.err = errors.New("lzw: unknown order")
|
|
|
|
return d
|
|
|
|
}
|
|
|
|
if litWidth < 2 || 8 < litWidth {
|
|
|
|
d.err = fmt.Errorf("lzw: litWidth %d out of range", litWidth)
|
|
|
|
return d
|
|
|
|
}
|
|
|
|
if br, ok := r.(io.ByteReader); ok {
|
|
|
|
d.r = br
|
|
|
|
} else {
|
|
|
|
d.r = bufio.NewReader(r)
|
|
|
|
}
|
|
|
|
d.litWidth = litWidth
|
|
|
|
d.width = 1 + uint(litWidth)
|
|
|
|
d.clear = uint16(1) << uint(litWidth)
|
|
|
|
d.eof, d.hi = d.clear+1, d.clear+1
|
|
|
|
d.overflow = uint16(1) << d.width
|
|
|
|
d.last = decoderInvalidCode
|
|
|
|
|
|
|
|
return d
|
|
|
|
}
|