src/compress/lzw/reader.go - go - Git at Google

 // 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 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.
 //
 // The TIFF file format uses a similar but incompatible version of the LZW
 // algorithm. See the golang.org/x/image/tiff/lzw package for an
 // implementation.
 package lzw

 // TODO(nigeltao): check that PDF uses LZW in the same way as GIF,
 // modulo LSB/MSB packing order.

 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. It always
 	// equals 1 << width.
 	//
 	// last is the most recently seen code, or decoderInvalidCode.
 	//
 	// An invariant is that hi < overflow.
 	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 >= d.overflow {
 			if d.hi > d.overflow {
 				panic("unreachable")
 			}
 			if d.width == maxWidth {
 				d.last = decoderInvalidCode
 				// Undo the d.hi++ a few lines above, so that (1) we maintain
 				// the invariant that d.hi < d.overflow, and (2) d.hi does not
 				// eventually overflow a uint16.
 				d.hi--
 			} else {
 				d.width++
 				d.overflow = 1 << d.width
 			}
 		}
 		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
 }
	// 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 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.
	//
	// The TIFF file format uses a similar but incompatible version of the LZW
	// algorithm. See the golang.org/x/image/tiff/lzw package for an
	// implementation.
	package lzw

	// TODO(nigeltao): check that PDF uses LZW in the same way as GIF,
	// modulo LSB/MSB packing order.

	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. It always
	// equals 1 << width.
	//
	// last is the most recently seen code, or decoderInvalidCode.
	//
	// An invariant is that hi < overflow.
	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 >= d.overflow {
	if d.hi > d.overflow {
	panic("unreachable")
	}
	if d.width == maxWidth {
	d.last = decoderInvalidCode
	// Undo the d.hi++ a few lines above, so that (1) we maintain
	// the invariant that d.hi < d.overflow, and (2) d.hi does not
	// eventually overflow a uint16.
	d.hi--
	} else {
	d.width++
	d.overflow = 1 << d.width
	}
	}
	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
	}