140 lines
3.9 KiB
Go
140 lines
3.9 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package zlib
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import (
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"compress/flate"
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"hash"
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"hash/adler32"
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"io"
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"os"
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)
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// These constants are copied from the flate package, so that code that imports
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// "compress/zlib" does not also have to import "compress/flate".
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const (
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NoCompression = flate.NoCompression
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BestSpeed = flate.BestSpeed
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BestCompression = flate.BestCompression
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DefaultCompression = flate.DefaultCompression
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)
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// A Writer takes data written to it and writes the compressed
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// form of that data to an underlying writer (see NewWriter).
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type Writer struct {
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w io.Writer
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compressor *flate.Writer
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digest hash.Hash32
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err os.Error
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scratch [4]byte
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}
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// NewWriter calls NewWriterLevel with the default compression level.
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func NewWriter(w io.Writer) (*Writer, os.Error) {
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return NewWriterLevel(w, DefaultCompression)
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}
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// NewWriterLevel calls NewWriterDict with no dictionary.
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func NewWriterLevel(w io.Writer, level int) (*Writer, os.Error) {
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return NewWriterDict(w, level, nil)
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}
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// NewWriterDict creates a new io.WriteCloser that satisfies writes by compressing data written to w.
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// It is the caller's responsibility to call Close on the WriteCloser when done.
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// level is the compression level, which can be DefaultCompression, NoCompression,
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// or any integer value between BestSpeed and BestCompression (inclusive).
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// dict is the preset dictionary to compress with, or nil to use no dictionary.
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func NewWriterDict(w io.Writer, level int, dict []byte) (*Writer, os.Error) {
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z := new(Writer)
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// ZLIB has a two-byte header (as documented in RFC 1950).
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// The first four bits is the CINFO (compression info), which is 7 for the default deflate window size.
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// The next four bits is the CM (compression method), which is 8 for deflate.
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z.scratch[0] = 0x78
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// The next two bits is the FLEVEL (compression level). The four values are:
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// 0=fastest, 1=fast, 2=default, 3=best.
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// The next bit, FDICT, is set if a dictionary is given.
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// The final five FCHECK bits form a mod-31 checksum.
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switch level {
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case 0, 1:
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z.scratch[1] = 0 << 6
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case 2, 3, 4, 5:
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z.scratch[1] = 1 << 6
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case 6, -1:
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z.scratch[1] = 2 << 6
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case 7, 8, 9:
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z.scratch[1] = 3 << 6
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default:
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return nil, os.NewError("level out of range")
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}
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if dict != nil {
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z.scratch[1] |= 1 << 5
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}
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z.scratch[1] += uint8(31 - (uint16(z.scratch[0])<<8+uint16(z.scratch[1]))%31)
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_, err := w.Write(z.scratch[0:2])
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if err != nil {
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return nil, err
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}
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if dict != nil {
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// The next four bytes are the Adler-32 checksum of the dictionary.
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checksum := adler32.Checksum(dict)
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z.scratch[0] = uint8(checksum >> 24)
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z.scratch[1] = uint8(checksum >> 16)
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z.scratch[2] = uint8(checksum >> 8)
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z.scratch[3] = uint8(checksum >> 0)
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_, err = w.Write(z.scratch[0:4])
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if err != nil {
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return nil, err
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}
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}
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z.w = w
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z.compressor = flate.NewWriterDict(w, level, dict)
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z.digest = adler32.New()
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return z, nil
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}
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func (z *Writer) Write(p []byte) (n int, err os.Error) {
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if z.err != nil {
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return 0, z.err
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}
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if len(p) == 0 {
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return 0, nil
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}
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n, err = z.compressor.Write(p)
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if err != nil {
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z.err = err
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return
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}
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z.digest.Write(p)
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return
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}
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// Flush flushes the underlying compressor.
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func (z *Writer) Flush() os.Error {
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if z.err != nil {
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return z.err
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}
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z.err = z.compressor.Flush()
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return z.err
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}
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// Calling Close does not close the wrapped io.Writer originally passed to NewWriter.
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func (z *Writer) Close() os.Error {
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if z.err != nil {
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return z.err
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}
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z.err = z.compressor.Close()
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if z.err != nil {
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return z.err
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}
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checksum := z.digest.Sum32()
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// ZLIB (RFC 1950) is big-endian, unlike GZIP (RFC 1952).
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z.scratch[0] = uint8(checksum >> 24)
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z.scratch[1] = uint8(checksum >> 16)
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z.scratch[2] = uint8(checksum >> 8)
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z.scratch[3] = uint8(checksum >> 0)
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_, z.err = z.w.Write(z.scratch[0:4])
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return z.err
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}
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