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Source file src/compress/zlib/writer.go

Documentation: compress/zlib

  // 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"
  	"fmt"
  	"hash"
  	"hash/adler32"
  	"io"
  )
  
  // 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
  	HuffmanOnly        = flate.HuffmanOnly
  )
  
  // 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
  	level       int
  	dict        []byte
  	compressor  *flate.Writer
  	digest      hash.Hash32
  	err         error
  	scratch     [4]byte
  	wroteHeader bool
  }
  
  // NewWriter creates a new Writer.
  // Writes to the returned Writer are compressed and written to w.
  //
  // It is the caller's responsibility to call Close on the WriteCloser when done.
  // Writes may be buffered and not flushed until Close.
  func NewWriter(w io.Writer) *Writer {
  	z, _ := NewWriterLevelDict(w, DefaultCompression, nil)
  	return z
  }
  
  // NewWriterLevel is like NewWriter but specifies the compression level instead
  // of assuming DefaultCompression.
  //
  // The compression level can be DefaultCompression, NoCompression, HuffmanOnly
  // or any integer value between BestSpeed and BestCompression inclusive.
  // The error returned will be nil if the level is valid.
  func NewWriterLevel(w io.Writer, level int) (*Writer, error) {
  	return NewWriterLevelDict(w, level, nil)
  }
  
  // NewWriterLevelDict is like NewWriterLevel but specifies a dictionary to
  // compress with.
  //
  // The dictionary may be nil. If not, its contents should not be modified until
  // the Writer is closed.
  func NewWriterLevelDict(w io.Writer, level int, dict []byte) (*Writer, error) {
  	if level < HuffmanOnly || level > BestCompression {
  		return nil, fmt.Errorf("zlib: invalid compression level: %d", level)
  	}
  	return &Writer{
  		w:     w,
  		level: level,
  		dict:  dict,
  	}, nil
  }
  
  // Reset clears the state of the Writer z such that it is equivalent to its
  // initial state from NewWriterLevel or NewWriterLevelDict, but instead writing
  // to w.
  func (z *Writer) Reset(w io.Writer) {
  	z.w = w
  	// z.level and z.dict left unchanged.
  	if z.compressor != nil {
  		z.compressor.Reset(w)
  	}
  	if z.digest != nil {
  		z.digest.Reset()
  	}
  	z.err = nil
  	z.scratch = [4]byte{}
  	z.wroteHeader = false
  }
  
  // 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 -2, 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
  }
  
  // Write writes a compressed form of p to the underlying io.Writer. The
  // compressed bytes are not necessarily flushed until the Writer is closed or
  // explicitly flushed.
  func (z *Writer) Write(p []byte) (n int, err error) {
  	if !z.wroteHeader {
  		z.err = z.writeHeader()
  	}
  	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 Writer to its underlying io.Writer.
  func (z *Writer) Flush() error {
  	if !z.wroteHeader {
  		z.err = z.writeHeader()
  	}
  	if z.err != nil {
  		return z.err
  	}
  	z.err = z.compressor.Flush()
  	return z.err
  }
  
  // Close closes the Writer, flushing any unwritten data to the underlying
  // io.Writer, but does not close the underlying io.Writer.
  func (z *Writer) Close() error {
  	if !z.wroteHeader {
  		z.err = z.writeHeader()
  	}
  	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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