compress/lzw: implement a decoder.

R=rsc
CC=bsiegert, golang-dev, mpl
https://golang.org/cl/4182081

src/pkg/Makefile

@@ -23,6 +23,7 @@ DIRS=\
 	cmath\
 	compress/flate\
 	compress/gzip\
+	compress/lzw \
 	compress/zlib\
 	container/heap\
 	container/list\

src/pkg/compress/lzw/Makefile

+# 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.
+
+include ../../../Make.inc
+
+TARG=compress/lzw
+GOFILES=\
+	reader.go\
+
+include ../../../Make.pkg

src/pkg/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.
+
+// The lzw package 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.
+
+// TODO(nigeltao): write an encoder.
+
+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 {
+		c, err := d.r.ReadByte()
+		if err != nil {
+			return 0, err
+		}
+		d.bits |= uint32(c) << 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 {
+		c, err := d.r.ReadByte()
+		if err != nil {
+			return 0, err
+		}
+		d.bits |= uint32(c) << (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(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
+	)
+
+	// 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:
+			// 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.
+			var buf [1 << maxWidth]byte
+			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 returns a new ReadCloser that can be used to read the uncompressed
+// version of r. It is the caller's responsibility to call Close on the
+// ReadCloser when finished reading.
+// order is either LSB or MSB for Least or Most Significant Bits first packing
+// order. GIF uses LSB. TIFF and PDF use MSB.
+// litWidth is the width in bits for literal codes. Valid values range from
+// 2 to 8 inclusive.
+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 func() {
+		br, ok := r.(io.ByteReader)
+		if !ok {
+			br = bufio.NewReader(r)
+		}
+		pw.CloseWithError(decode(pw, br, read, uint(litWidth)))
+	}()
+	return pr
+}

src/pkg/compress/lzw/reader_test.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
+
+import (
+	"bytes"
+	"io"
+	"os"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+type lzwTest struct {
+	desc       string
+	raw        string
+	compressed string
+	err        os.Error
+}
+
+var lzwTests = []lzwTest{
+	{
+		"empty;LSB;8",
+		"",
+		"\x01\x01",
+		nil,
+	},
+	{
+		"empty;MSB;8",
+		"",
+		"\x80\x80",
+		nil,
+	},
+	{
+		"tobe;LSB;7",
+		"TOBEORNOTTOBEORTOBEORNOT",
+		"\x54\x4f\x42\x45\x4f\x52\x4e\x4f\x54\x82\x84\x86\x8b\x85\x87\x89\x81",
+		nil,
+	},
+	{
+		"tobe;LSB;8",
+		"TOBEORNOTTOBEORTOBEORNOT",
+		"\x54\x9e\x08\x29\xf2\x44\x8a\x93\x27\x54\x04\x12\x34\xb8\xb0\xe0\xc1\x84\x01\x01",
+		nil,
+	},
+	{
+		"tobe;MSB;7",
+		"TOBEORNOTTOBEORTOBEORNOT",
+		"\x54\x4f\x42\x45\x4f\x52\x4e\x4f\x54\x82\x84\x86\x8b\x85\x87\x89\x81",
+		nil,
+	},
+	{
+		"tobe;MSB;8",
+		"TOBEORNOTTOBEORTOBEORNOT",
+		"\x2a\x13\xc8\x44\x52\x79\x48\x9c\x4f\x2a\x40\xa0\x90\x68\x5c\x16\x0f\x09\x80\x80",
+		nil,
+	},
+	{
+		"tobe-truncated;LSB;8",
+		"TOBEORNOTTOBEORTOBEORNOT",
+		"\x54\x9e\x08\x29\xf2\x44\x8a\x93\x27\x54\x04",
+		io.ErrUnexpectedEOF,
+	},
+	// This example comes from http://en.wikipedia.org/wiki/Graphics_Interchange_Format.
+	{
+		"gif;LSB;8",
+		"\x28\xff\xff\xff\x28\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff",
+		"\x00\x51\xfc\x1b\x28\x70\xa0\xc1\x83\x01\x01",
+		nil,
+	},
+	// This example comes from http://compgroups.net/comp.lang.ruby/Decompressing-LZW-compression-from-PDF-file
+	{
+		"pdf;MSB;8",
+		"-----A---B",
+		"\x80\x0b\x60\x50\x22\x0c\x0c\x85\x01",
+		nil,
+	},
+}
+
+func TestReader(t *testing.T) {
+	b := bytes.NewBuffer(nil)
+	for _, tt := range lzwTests {
+		d := strings.Split(tt.desc, ";", -1)
+		var order Order
+		switch d[1] {
+		case "LSB":
+			order = LSB
+		case "MSB":
+			order = MSB
+		default:
+			t.Errorf("%s: bad order %q", tt.desc, d[1])
+		}
+		litWidth, _ := strconv.Atoi(d[2])
+		rc := NewReader(strings.NewReader(tt.compressed), order, litWidth)
+		defer rc.Close()
+		b.Reset()
+		n, err := io.Copy(b, rc)
+		if err != nil {
+			if err != tt.err {
+				t.Errorf("%s: io.Copy: %v want %v", tt.desc, err, tt.err)
+			}
+			continue
+		}
+		s := b.String()
+		if s != tt.raw {
+			t.Errorf("%s: got %d-byte %q want %d-byte %q", tt.desc, n, s, len(tt.raw), tt.raw)
+		}
+	}
+}