golang reader 代码
文件路径：/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
)
/ Reader is an io.Reader which can be used to read compressed data in the
/ LZW format.
type Reader struct {
r io.ByteReader
bits uint32
nBits uint
width uint
read func(*Reader) (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 (r *Reader) readLSB() (uint16, error) {
for r.nBits < r.width {
x, err := r.r.ReadByte()
if err != nil {
return 0, err
}
r.bits |= uint32(x) << r.nBits
r.nBits += 8
}
code := uint16(r.bits & (1<<r.width - 1))
r.bits >>= r.width
r.nBits -= r.width
return code, nil
}
/ readMSB returns the next code for "Most Significant Bits first" data.
func (r *Reader) readMSB() (uint16, error) {
for r.nBits < r.width {
x, err := r.r.ReadByte()
if err != nil {
return 0, err
}
r.bits |= uint32(x) << (24 - r.nBits)
r.nBits += 8
}
code := uint16(r.bits >> (32 - r.width))
r.bits <<= r.width
r.nBits -= r.width
return code, nil
}
/ Read implements io.Reader, reading uncompressed bytes from its underlying Reader.
func (r *Reader) Read(b []byte) (int, error) {
for {
if len(r.toRead) > 0 {
n := copy(b, r.toRead)
r.toRead = r.toRead[n:]
return n, nil
}
if r.err != nil {
return 0, r.err
}
r.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 (r *Reader) decode() {
/ Loop over the code stream, converting codes into decompressed bytes.
loop:
for {
code, err := r.read(r)
if err != nil {
if err == io.EOF {
err = io.ErrUnexpectedEOF
}
r.err = err
break
}
switch {
case code < r.clear:
/ We have a literal code.
r.output[r.o] = uint8(code)
r.o++
if r.last != decoderInvalidCode {
/ Save what the hi code expands to.
r.suffix[r.hi] = uint8(code)
r.prefix[r.hi] = r.last
}
case code == r.clear:
r.width = 1 + uint(r.litWidth)
r.hi = r.eof
r.overflow = 1 << r.width
r.last = decoderInvalidCode
continue
case code == r.eof:
r.err = io.EOF
break loop
case code <= r.hi:
c, i := code, len(r.output)-1
if code == r.hi && r.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 = r.last
for c >= r.clear {
c = r.prefix[c]
}
r.output[i] = uint8(c)
i--
c = r.last
}
/ Copy the suffix chain into output and then write that to w.
for c >= r.clear {
r.output[i] = r.suffix[c]
i--
c = r.prefix[c]
}
r.output[i] = uint8(c)
r.o += copy(r.output[r.o:], r.output[i:])
if r.last != decoderInvalidCode {
/ Save what the hi code expands to.
r.suffix[r.hi] = uint8(c)
r.prefix[r.hi] = r.last
}
default:
r.err = errors.New("lzw: invalid code")
break loop
}
r.last, r.hi = code, r.hi+1
if r.hi >= r.overflow {
if r.hi > r.overflow {
panic("unreachable")
}
if r.width == maxWidth {
r.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.
r.hi--
} else {
r.width++
r.overflow = 1 << r.width
}
}
if r.o >= flushBuffer {
break
}
}
/ Flush pending output.
r.toRead = r.output[:r.o]
r.o = 0
}
var errClosed = errors.New("lzw: reader/writer is closed")
/ Close closes the Reader and returns an error for any future read operation.
/ It does not close the underlying io.Reader.
func (r *Reader) Close() error {
r.err = errClosed / in case any Reads come along
return nil
}
/ Reset clears the Reader's state and allows it to be reused again
/ as a new Reader.
func (r *Reader) Reset(src io.Reader, order Order, litWidth int) {
*r = Reader{}
r.init(src, order, litWidth)
}
/ 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.
/
/ It is guaranteed that the underlying type of the returned io.ReadCloser
/ is a *Reader.
func NewReader(r io.Reader, order Order, litWidth int) io.ReadCloser {
return newReader(r, order, litWidth)
}
func newReader(src io.Reader, order Order, litWidth int) *Reader {
r := new(Reader)
r.init(src, order, litWidth)
return r
}
func (r *Reader) init(src io.Reader, order Order, litWidth int) {
switch order {
case LSB:
r.read = (*Reader).readLSB
case MSB:
r.read = (*Reader).readMSB
default:
r.err = errors.New("lzw: unknown order")
return
}
if litWidth < 2 || 8 < litWidth {
r.err = fmt.Errorf("lzw: litWidth %d out of range", litWidth)
return
}
br, ok := src.(io.ByteReader)
if !ok && src != nil {
br = bufio.NewReader(src)
}
r.r = br
r.litWidth = litWidth
r.width = 1 + uint(litWidth)
r.clear = uint16(1) << uint(litWidth)
r.eof, r.hi = r.clear+1, r.clear+1
r.overflow = uint16(1) << r.width
r.last = decoderInvalidCode
}