// writeHeader writes the ZLIB header.
func (z *Writer) writeHeader() (err error) {
z.wroteHeader = true
// ZLIB has a two-byte header (as documented in RFC 1950).
// The first four bits is the CINFO (compression info), which is 7 for the default deflate window size.
// The next four bits is the CM (compression method), which is 8 for deflate.
z.scratch[0] = 0x78
// The next two bits is the FLEVEL (compression level). The four values are:
// 0=fastest, 1=fast, 2=default, 3=best.
// The next bit, FDICT, is set if a dictionary is given.
// The final five FCHECK bits form a mod-31 checksum.
switch z.level {
case 0, 1:
z.scratch[1] = 0 << 6
case 2, 3, 4, 5:
z.scratch[1] = 1 << 6
case 6, -1:
z.scratch[1] = 2 << 6
case 7, 8, 9:
z.scratch[1] = 3 << 6
default:
panic("unreachable")
}
if z.dict != nil {
z.scratch[1] |= 1 << 5
}
z.scratch[1] += uint8(31 - (uint16(z.scratch[0])<<8+uint16(z.scratch[1]))%31)
if _, err = z.w.Write(z.scratch[0:2]); err != nil {
return err
}
if z.dict != nil {
// The next four bytes are the Adler-32 checksum of the dictionary.
checksum := adler32.Checksum(z.dict)
z.scratch[0] = uint8(checksum >> 24)
z.scratch[1] = uint8(checksum >> 16)
z.scratch[2] = uint8(checksum >> 8)
z.scratch[3] = uint8(checksum >> 0)
if _, err = z.w.Write(z.scratch[0:4]); err != nil {
return err
}
}
if z.compressor == nil {
// Initialize deflater unless the Writer is being reused
// after a Reset call.
z.compressor, err = flate.NewWriterDict(z.w, z.level, z.dict)
if err != nil {
return err
}
z.digest = adler32.New()
}
return nil
}
// NewWriterDict creates a new io.WriteCloser that satisfies writes by compressing data written to w.
// It is the caller's responsibility to call Close on the WriteCloser when done.
// level is the compression level, which can be DefaultCompression, NoCompression,
// or any integer value between BestSpeed and BestCompression (inclusive).
// dict is the preset dictionary to compress with, or nil to use no dictionary.
func NewWriterDict(w io.Writer, level int, dict []byte) (*Writer, os.Error) {
z := new(Writer)
// ZLIB has a two-byte header (as documented in RFC 1950).
// The first four bits is the CINFO (compression info), which is 7 for the default deflate window size.
// The next four bits is the CM (compression method), which is 8 for deflate.
z.scratch[0] = 0x78
// The next two bits is the FLEVEL (compression level). The four values are:
// 0=fastest, 1=fast, 2=default, 3=best.
// The next bit, FDICT, is set if a dictionary is given.
// The final five FCHECK bits form a mod-31 checksum.
switch level {
case 0, 1:
z.scratch[1] = 0 << 6
case 2, 3, 4, 5:
z.scratch[1] = 1 << 6
case 6, -1:
z.scratch[1] = 2 << 6
case 7, 8, 9:
z.scratch[1] = 3 << 6
default:
return nil, os.NewError("level out of range")
}
if dict != nil {
z.scratch[1] |= 1 << 5
}
z.scratch[1] += uint8(31 - (uint16(z.scratch[0])<<8+uint16(z.scratch[1]))%31)
_, err := w.Write(z.scratch[0:2])
if err != nil {
return nil, err
}
if dict != nil {
// The next four bytes are the Adler-32 checksum of the dictionary.
checksum := adler32.Checksum(dict)
z.scratch[0] = uint8(checksum >> 24)
z.scratch[1] = uint8(checksum >> 16)
z.scratch[2] = uint8(checksum >> 8)
z.scratch[3] = uint8(checksum >> 0)
_, err = w.Write(z.scratch[0:4])
if err != nil {
return nil, err
}
}
z.w = w
z.compressor = flate.NewWriterDict(w, level, dict)
z.digest = adler32.New()
return z, nil
}
func (self *FlateCompressor) Compress(src []byte) ([]byte, error) {
var err error
var compressor *flate.Writer
cdest := bytes.NewBuffer(make([]byte, 0, len(src)))
if self.dict == nil {
compressor, err = flate.NewWriter(cdest, self.level)
} else {
compressor, err = flate.NewWriterDict(cdest, self.level, self.dict)
}
//compressor.Reset(cdest)
compressor.Write(src)
err = compressor.Close()
if err != nil {
fmt.Println("Compress Close err:%s", err.Error())
}
return cdest.Bytes(), err
}
func marshalPacket(local *localNode) (data []byte, err error) {
var buf bytes.Buffer
buf.WriteByte(byte(local.mode.Id))
inflater, err := flate.NewWriterDict(&buf, flate.DefaultCompression, PacketCompressionDict)
if err != nil {
return
}
if err = local.encodeForPacket(inflater); err != nil {
return
}
inflater.Close()
mac := hmac.New(sha1.New, local.mode.Secret)
mac.Write(buf.Bytes())
buf.Write(mac.Sum(nil))
data = buf.Bytes()
return
}
func compressdict(src []byte, dest io.Writer, level int, dict []byte) {
compressor, _ := flate.NewWriterDict(dest, level, dict)
compressor.Write(src)
compressor.Close()
}
// A preset dictionary can be used to improve the compression ratio.
// The downside to using a dictionary is that the compressor and decompressor
// must agree in advance what dictionary to use.
func Example_dictionary() {
// The dictionary is a string of bytes. When compressing some input data,
// the compressor will attempt to substitute substrings with matches found
// in the dictionary. As such, the dictionary should only contain substrings
// that are expected to be found in the actual data stream.
const dict = `<?xml version="1.0"?>` + `<book>` + `<data>` + `<meta name="` + `" content="`
// The data to compress should (but is not required to) contain frequent
// substrings that match those in the dictionary.
const data = `<?xml version="1.0"?>
<book>
<meta name="title" content="The Go Programming Language"/>
<meta name="authors" content="Alan Donovan and Brian Kernighan"/>
<meta name="published" content="2015-10-26"/>
<meta name="isbn" content="978-0134190440"/>
<data>...</data>
</book>
`
var b bytes.Buffer
// Compress the data using the specially crafted dictionary.
zw, err := flate.NewWriterDict(&b, flate.DefaultCompression, []byte(dict))
if err != nil {
log.Fatal(err)
}
if _, err := io.Copy(zw, strings.NewReader(data)); err != nil {
log.Fatal(err)
}
if err := zw.Close(); err != nil {
log.Fatal(err)
}
// The decompressor must use the same dictionary as the compressor.
// Otherwise, the input may appear as corrupted.
fmt.Println("Decompressed output using the dictionary:")
zr := flate.NewReaderDict(bytes.NewReader(b.Bytes()), []byte(dict))
if _, err := io.Copy(os.Stdout, zr); err != nil {
log.Fatal(err)
}
if err := zr.Close(); err != nil {
log.Fatal(err)
}
fmt.Println()
// Substitute all of the bytes in the dictionary with a '#' to visually
// demonstrate the approximate effectiveness of using a preset dictionary.
fmt.Println("Substrings matched by the dictionary are marked with #:")
hashDict := []byte(dict)
for i := range hashDict {
hashDict[i] = '#'
}
zr = flate.NewReaderDict(&b, hashDict)
if _, err := io.Copy(os.Stdout, zr); err != nil {
log.Fatal(err)
}
if err := zr.Close(); err != nil {
log.Fatal(err)
}
// Output:
// Decompressed output using the dictionary:
// <?xml version="1.0"?>
// <book>
// <meta name="title" content="The Go Programming Language"/>
// <meta name="authors" content="Alan Donovan and Brian Kernighan"/>
// <meta name="published" content="2015-10-26"/>
// <meta name="isbn" content="978-0134190440"/>
// <data>...</data>
// </book>
//
// Substrings matched by the dictionary are marked with #:
// #####################
// ######
// ############title###########The Go Programming Language"/#
// ############authors###########Alan Donovan and Brian Kernighan"/#
// ############published###########2015-10-26"/#
// ############isbn###########978-0134190440"/#
// ######...</#####
// </#####
}