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Source file src/compress/gzip/gzip.go

Documentation: compress/gzip

  // Copyright 2010 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 gzip
  
  import (
  	"compress/flate"
  	"errors"
  	"fmt"
  	"hash/crc32"
  	"io"
  	"time"
  )
  
  // These constants are copied from the flate package, so that code that imports
  // "compress/gzip" 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 is an io.WriteCloser.
  // Writes to a Writer are compressed and written to w.
  type Writer struct {
  	Header      // written at first call to Write, Flush, or Close
  	w           io.Writer
  	level       int
  	wroteHeader bool
  	compressor  *flate.Writer
  	digest      uint32 // CRC-32, IEEE polynomial (section 8)
  	size        uint32 // Uncompressed size (section 2.3.1)
  	closed      bool
  	buf         [10]byte
  	err         error
  }
  
  // NewWriter returns 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.
  //
  // Callers that wish to set the fields in Writer.Header must do so before
  // the first call to Write, Flush, or Close.
  func NewWriter(w io.Writer) *Writer {
  	z, _ := NewWriterLevel(w, DefaultCompression)
  	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) {
  	if level < HuffmanOnly || level > BestCompression {
  		return nil, fmt.Errorf("gzip: invalid compression level: %d", level)
  	}
  	z := new(Writer)
  	z.init(w, level)
  	return z, nil
  }
  
  func (z *Writer) init(w io.Writer, level int) {
  	compressor := z.compressor
  	if compressor != nil {
  		compressor.Reset(w)
  	}
  	*z = Writer{
  		Header: Header{
  			OS: 255, // unknown
  		},
  		w:          w,
  		level:      level,
  		compressor: compressor,
  	}
  }
  
  // Reset discards the Writer z's state and makes it equivalent to the
  // result of its original state from NewWriter or NewWriterLevel, but
  // writing to w instead. This permits reusing a Writer rather than
  // allocating a new one.
  func (z *Writer) Reset(w io.Writer) {
  	z.init(w, z.level)
  }
  
  // writeBytes writes a length-prefixed byte slice to z.w.
  func (z *Writer) writeBytes(b []byte) error {
  	if len(b) > 0xffff {
  		return errors.New("gzip.Write: Extra data is too large")
  	}
  	le.PutUint16(z.buf[:2], uint16(len(b)))
  	_, err := z.w.Write(z.buf[:2])
  	if err != nil {
  		return err
  	}
  	_, err = z.w.Write(b)
  	return err
  }
  
  // writeString writes a UTF-8 string s in GZIP's format to z.w.
  // GZIP (RFC 1952) specifies that strings are NUL-terminated ISO 8859-1 (Latin-1).
  func (z *Writer) writeString(s string) (err error) {
  	// GZIP stores Latin-1 strings; error if non-Latin-1; convert if non-ASCII.
  	needconv := false
  	for _, v := range s {
  		if v == 0 || v > 0xff {
  			return errors.New("gzip.Write: non-Latin-1 header string")
  		}
  		if v > 0x7f {
  			needconv = true
  		}
  	}
  	if needconv {
  		b := make([]byte, 0, len(s))
  		for _, v := range s {
  			b = append(b, byte(v))
  		}
  		_, err = z.w.Write(b)
  	} else {
  		_, err = io.WriteString(z.w, s)
  	}
  	if err != nil {
  		return err
  	}
  	// GZIP strings are NUL-terminated.
  	z.buf[0] = 0
  	_, err = z.w.Write(z.buf[:1])
  	return err
  }
  
  // Write writes a compressed form of p to the underlying io.Writer. The
  // compressed bytes are not necessarily flushed until the Writer is closed.
  func (z *Writer) Write(p []byte) (int, error) {
  	if z.err != nil {
  		return 0, z.err
  	}
  	var n int
  	// Write the GZIP header lazily.
  	if !z.wroteHeader {
  		z.wroteHeader = true
  		z.buf = [10]byte{0: gzipID1, 1: gzipID2, 2: gzipDeflate}
  		if z.Extra != nil {
  			z.buf[3] |= 0x04
  		}
  		if z.Name != "" {
  			z.buf[3] |= 0x08
  		}
  		if z.Comment != "" {
  			z.buf[3] |= 0x10
  		}
  		if z.ModTime.After(time.Unix(0, 0)) {
  			// Section 2.3.1, the zero value for MTIME means that the
  			// modified time is not set.
  			le.PutUint32(z.buf[4:8], uint32(z.ModTime.Unix()))
  		}
  		if z.level == BestCompression {
  			z.buf[8] = 2
  		} else if z.level == BestSpeed {
  			z.buf[8] = 4
  		}
  		z.buf[9] = z.OS
  		n, z.err = z.w.Write(z.buf[:10])
  		if z.err != nil {
  			return n, z.err
  		}
  		if z.Extra != nil {
  			z.err = z.writeBytes(z.Extra)
  			if z.err != nil {
  				return n, z.err
  			}
  		}
  		if z.Name != "" {
  			z.err = z.writeString(z.Name)
  			if z.err != nil {
  				return n, z.err
  			}
  		}
  		if z.Comment != "" {
  			z.err = z.writeString(z.Comment)
  			if z.err != nil {
  				return n, z.err
  			}
  		}
  		if z.compressor == nil {
  			z.compressor, _ = flate.NewWriter(z.w, z.level)
  		}
  	}
  	z.size += uint32(len(p))
  	z.digest = crc32.Update(z.digest, crc32.IEEETable, p)
  	n, z.err = z.compressor.Write(p)
  	return n, z.err
  }
  
  // Flush flushes any pending compressed data to the underlying writer.
  //
  // It is useful mainly in compressed network protocols, to ensure that
  // a remote reader has enough data to reconstruct a packet. Flush does
  // not return until the data has been written. If the underlying
  // writer returns an error, Flush returns that error.
  //
  // In the terminology of the zlib library, Flush is equivalent to Z_SYNC_FLUSH.
  func (z *Writer) Flush() error {
  	if z.err != nil {
  		return z.err
  	}
  	if z.closed {
  		return nil
  	}
  	if !z.wroteHeader {
  		z.Write(nil)
  		if z.err != nil {
  			return z.err
  		}
  	}
  	z.err = z.compressor.Flush()
  	return z.err
  }
  
  // Close closes the Writer by flushing any unwritten data to the underlying
  // io.Writer and writing the GZIP footer.
  // It does not close the underlying io.Writer.
  func (z *Writer) Close() error {
  	if z.err != nil {
  		return z.err
  	}
  	if z.closed {
  		return nil
  	}
  	z.closed = true
  	if !z.wroteHeader {
  		z.Write(nil)
  		if z.err != nil {
  			return z.err
  		}
  	}
  	z.err = z.compressor.Close()
  	if z.err != nil {
  		return z.err
  	}
  	le.PutUint32(z.buf[:4], z.digest)
  	le.PutUint32(z.buf[4:8], z.size)
  	_, z.err = z.w.Write(z.buf[:8])
  	return z.err
  }
  

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