258 lines
6.9 KiB
Go
258 lines
6.9 KiB
Go
// Copyright 2011 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package lzw implements the Lempel-Ziv-Welch compressed data format,
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// described in T. A. Welch, ``A Technique for High-Performance Data
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// Compression'', Computer, 17(6) (June 1984), pp 8-19.
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//
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// In particular, it implements LZW as used by the GIF and PDF file
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// formats, which means variable-width codes up to 12 bits and the first
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// two non-literal codes are a clear code and an EOF code.
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//
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// The TIFF file format uses a similar but incompatible version of the LZW
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// algorithm. See the golang.org/x/image/tiff/lzw package for an
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// implementation.
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package lzw
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// TODO(nigeltao): check that PDF uses LZW in the same way as GIF,
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// modulo LSB/MSB packing order.
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import (
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"bufio"
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"errors"
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"fmt"
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"io"
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)
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// Order specifies the bit ordering in an LZW data stream.
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type Order int
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const (
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// LSB means Least Significant Bits first, as used in the GIF file format.
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LSB Order = iota
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// MSB means Most Significant Bits first, as used in the TIFF and PDF
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// file formats.
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MSB
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)
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const (
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maxWidth = 12
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decoderInvalidCode = 0xffff
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flushBuffer = 1 << maxWidth
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)
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// decoder is the state from which the readXxx method converts a byte
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// stream into a code stream.
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type decoder struct {
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r io.ByteReader
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bits uint32
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nBits uint
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width uint
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read func(*decoder) (uint16, error) // readLSB or readMSB
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litWidth int // width in bits of literal codes
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err error
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// The first 1<<litWidth codes are literal codes.
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// The next two codes mean clear and EOF.
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// Other valid codes are in the range [lo, hi] where lo := clear + 2,
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// with the upper bound incrementing on each code seen.
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// overflow is the code at which hi overflows the code width.
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// last is the most recently seen code, or decoderInvalidCode.
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clear, eof, hi, overflow, last uint16
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// Each code c in [lo, hi] expands to two or more bytes. For c != hi:
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// suffix[c] is the last of these bytes.
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// prefix[c] is the code for all but the last byte.
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// This code can either be a literal code or another code in [lo, c).
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// The c == hi case is a special case.
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suffix [1 << maxWidth]uint8
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prefix [1 << maxWidth]uint16
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// output is the temporary output buffer.
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// Literal codes are accumulated from the start of the buffer.
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// Non-literal codes decode to a sequence of suffixes that are first
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// written right-to-left from the end of the buffer before being copied
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// to the start of the buffer.
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// It is flushed when it contains >= 1<<maxWidth bytes,
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// so that there is always room to decode an entire code.
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output [2 * 1 << maxWidth]byte
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o int // write index into output
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toRead []byte // bytes to return from Read
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}
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// readLSB returns the next code for "Least Significant Bits first" data.
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func (d *decoder) readLSB() (uint16, error) {
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for d.nBits < d.width {
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x, err := d.r.ReadByte()
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if err != nil {
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return 0, err
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}
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d.bits |= uint32(x) << d.nBits
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d.nBits += 8
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}
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code := uint16(d.bits & (1<<d.width - 1))
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d.bits >>= d.width
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d.nBits -= d.width
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return code, nil
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}
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// readMSB returns the next code for "Most Significant Bits first" data.
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func (d *decoder) readMSB() (uint16, error) {
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for d.nBits < d.width {
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x, err := d.r.ReadByte()
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if err != nil {
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return 0, err
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}
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d.bits |= uint32(x) << (24 - d.nBits)
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d.nBits += 8
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}
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code := uint16(d.bits >> (32 - d.width))
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d.bits <<= d.width
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d.nBits -= d.width
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return code, nil
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}
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func (d *decoder) Read(b []byte) (int, error) {
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for {
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if len(d.toRead) > 0 {
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n := copy(b, d.toRead)
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d.toRead = d.toRead[n:]
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return n, nil
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}
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if d.err != nil {
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return 0, d.err
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}
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d.decode()
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}
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}
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// decode decompresses bytes from r and leaves them in d.toRead.
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// read specifies how to decode bytes into codes.
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// litWidth is the width in bits of literal codes.
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func (d *decoder) decode() {
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// Loop over the code stream, converting codes into decompressed bytes.
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loop:
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for {
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code, err := d.read(d)
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if err != nil {
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if err == io.EOF {
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err = io.ErrUnexpectedEOF
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}
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d.err = err
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break
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}
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switch {
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case code < d.clear:
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// We have a literal code.
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d.output[d.o] = uint8(code)
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d.o++
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if d.last != decoderInvalidCode {
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// Save what the hi code expands to.
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d.suffix[d.hi] = uint8(code)
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d.prefix[d.hi] = d.last
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}
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case code == d.clear:
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d.width = 1 + uint(d.litWidth)
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d.hi = d.eof
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d.overflow = 1 << d.width
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d.last = decoderInvalidCode
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continue
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case code == d.eof:
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d.err = io.EOF
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break loop
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case code <= d.hi:
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c, i := code, len(d.output)-1
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if code == d.hi {
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// code == hi is a special case which expands to the last expansion
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// followed by the head of the last expansion. To find the head, we walk
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// the prefix chain until we find a literal code.
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c = d.last
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for c >= d.clear {
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c = d.prefix[c]
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}
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d.output[i] = uint8(c)
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i--
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c = d.last
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}
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// Copy the suffix chain into output and then write that to w.
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for c >= d.clear {
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d.output[i] = d.suffix[c]
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i--
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c = d.prefix[c]
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}
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d.output[i] = uint8(c)
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d.o += copy(d.output[d.o:], d.output[i:])
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if d.last != decoderInvalidCode {
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// Save what the hi code expands to.
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d.suffix[d.hi] = uint8(c)
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d.prefix[d.hi] = d.last
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}
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default:
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d.err = errors.New("lzw: invalid code")
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break loop
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}
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d.last, d.hi = code, d.hi+1
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if d.hi >= d.overflow {
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if d.width == maxWidth {
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d.last = decoderInvalidCode
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} else {
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d.width++
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d.overflow <<= 1
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}
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}
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if d.o >= flushBuffer {
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break
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}
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}
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// Flush pending output.
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d.toRead = d.output[:d.o]
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d.o = 0
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}
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var errClosed = errors.New("lzw: reader/writer is closed")
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func (d *decoder) Close() error {
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d.err = errClosed // in case any Reads come along
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return nil
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}
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// NewReader creates a new io.ReadCloser.
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// Reads from the returned io.ReadCloser read and decompress data from r.
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// If r does not also implement io.ByteReader,
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// the decompressor may read more data than necessary from r.
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// It is the caller's responsibility to call Close on the ReadCloser when
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// finished reading.
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// The number of bits to use for literal codes, litWidth, must be in the
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// range [2,8] and is typically 8. It must equal the litWidth
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// used during compression.
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func NewReader(r io.Reader, order Order, litWidth int) io.ReadCloser {
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d := new(decoder)
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switch order {
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case LSB:
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d.read = (*decoder).readLSB
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case MSB:
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d.read = (*decoder).readMSB
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default:
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d.err = errors.New("lzw: unknown order")
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return d
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}
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if litWidth < 2 || 8 < litWidth {
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d.err = fmt.Errorf("lzw: litWidth %d out of range", litWidth)
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return d
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}
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if br, ok := r.(io.ByteReader); ok {
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d.r = br
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} else {
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d.r = bufio.NewReader(r)
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}
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d.litWidth = litWidth
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d.width = 1 + uint(litWidth)
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d.clear = uint16(1) << uint(litWidth)
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d.eof, d.hi = d.clear+1, d.clear+1
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d.overflow = uint16(1) << d.width
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d.last = decoderInvalidCode
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return d
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}
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