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// Copyright 2009 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 zlib
import (
"compress/flate"
"hash"
"hash/adler32"
"io"
"os"
)
// These constants are copied from the flate package, so that code that imports
// "compress/zlib" does not also have to import "compress/flate".
const (
NoCompression = flate.NoCompression
BestSpeed = flate.BestSpeed
BestCompression = flate.BestCompression
DefaultCompression = flate.DefaultCompression
)
// A Writer takes data written to it and writes the compressed
// form of that data to an underlying writer (see NewWriter).
type Writer struct {
w io.Writer
compressor *flate.Writer
digest hash.Hash32
err os.Error
scratch [4]byte
}
// NewWriter calls NewWriterLevel with the default compression level.
func NewWriter(w io.Writer) (*Writer, os.Error) {
return NewWriterLevel(w, DefaultCompression)
}
// NewWriterLevel calls NewWriterDict with no dictionary.
func NewWriterLevel(w io.Writer, level int) (*Writer, os.Error) {
return NewWriterDict(w, level, 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.NewWriter(w, level)
z.digest = adler32.New()
return z, nil
}
func (z *Writer) Write(p []byte) (n int, err os.Error) {
if z.err != nil {
return 0, z.err
}
if len(p) == 0 {
return 0, nil
}
n, err = z.compressor.Write(p)
if err != nil {
z.err = err
return
}
z.digest.Write(p)
return
}
// Flush flushes the underlying compressor.
func (z *Writer) Flush() os.Error {
if z.err != nil {
return z.err
}
z.err = z.compressor.Flush()
return z.err
}
// Calling Close does not close the wrapped io.Writer originally passed to NewWriter.
func (z *Writer) Close() os.Error {
if z.err != nil {
return z.err
}
z.err = z.compressor.Close()
if z.err != nil {
return z.err
}
checksum := z.digest.Sum32()
// ZLIB (RFC 1950) is big-endian, unlike GZIP (RFC 1952).
z.scratch[0] = uint8(checksum >> 24)
z.scratch[1] = uint8(checksum >> 16)
z.scratch[2] = uint8(checksum >> 8)
z.scratch[3] = uint8(checksum >> 0)
_, z.err = z.w.Write(z.scratch[0:4])
return z.err
}
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