Source: gzip.go in package compress/gzip


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 Writer 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).

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
	}

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
		}

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
		_, z.err = z.w.Write(z.buf[:10])
		if z.err != nil {
			return 0, z.err
		}
		if z.Extra != nil {
			z.err = z.writeBytes(z.Extra)
			if z.err != nil {
				return 0, z.err
			}
		}
		if z.Name != "" {
			z.err = z.writeString(z.Name)
			if z.err != nil {
				return 0, z.err
			}
		}
		if z.Comment != "" {
			z.err = z.writeString(z.Comment)
			if z.err != nil {
				return 0, 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