src/compress/lzw/reader.go

// 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.
	// 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 {
				// 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.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
}