From 2bda5757173428ed8d2312e1c8f55336c18efb82 Mon Sep 17 00:00:00 2001 From: Nigel Tao Date: Mon, 2 Jun 2014 13:56:21 +1000 Subject: [PATCH] go.image/tiff/lzw: fork the standard library's compress/lzw package. This CL has no code changes. Future CLs will modify the code to implement TIFF's LZW instead of the standard LZW. LGTM=bradfitz R=djd, bradfitz CC=golang-codereviews https://golang.org/cl/105750044 --- tiff/lzw/reader.go | 253 +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 253 insertions(+) create mode 100644 tiff/lzw/reader.go diff --git a/tiff/lzw/reader.go b/tiff/lzw/reader.go new file mode 100644 index 0000000..ef59699 --- /dev/null +++ b/tiff/lzw/reader.go @@ -0,0 +1,253 @@ +// 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, 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. + +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<= 1<>= 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. + for { + code, err := d.read(d) + if err != nil { + if err == io.EOF { + err = io.ErrUnexpectedEOF + } + d.err = err + return + } + 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.flush() + d.err = io.EOF + return + 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") + return + } + 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 { + d.flush() + return + } + } +} + +func (d *decoder) flush() { + d.toRead = d.output[:d.o] + d.o = 0 +} + +var errClosed = errors.New("compress/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. +// 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. +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 +}