source: code/trunk/vendor/github.com/klauspost/compress/gzip/gunzip.go@ 145

// 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 gzip implements reading and writing of gzip format compressed files,
// as specified in RFC 1952.
package gzip

import (
        "bufio"
        "compress/gzip"
        "encoding/binary"
        "hash/crc32"
        "io"
        "time"

        "github.com/klauspost/compress/flate"
)

const (
        gzipID1     = 0x1f
        gzipID2     = 0x8b
        gzipDeflate = 8
        flagText    = 1 << 0
        flagHdrCrc  = 1 << 1
        flagExtra   = 1 << 2
        flagName    = 1 << 3
        flagComment = 1 << 4
)

var (
        // ErrChecksum is returned when reading GZIP data that has an invalid checksum.
        ErrChecksum = gzip.ErrChecksum
        // ErrHeader is returned when reading GZIP data that has an invalid header.
        ErrHeader = gzip.ErrHeader
)

var le = binary.LittleEndian

// noEOF converts io.EOF to io.ErrUnexpectedEOF.
func noEOF(err error) error {
        if err == io.EOF {
                return io.ErrUnexpectedEOF
        }
        return err
}

// The gzip file stores a header giving metadata about the compressed file.
// That header is exposed as the fields of the Writer and Reader structs.
//
// Strings must be UTF-8 encoded and may only contain Unicode code points
// U+0001 through U+00FF, due to limitations of the GZIP file format.
type Header struct {
        Comment string    // comment
        Extra   []byte    // "extra data"
        ModTime time.Time // modification time
        Name    string    // file name
        OS      byte      // operating system type
}

// A Reader is an io.Reader that can be read to retrieve
// uncompressed data from a gzip-format compressed file.
//
// In general, a gzip file can be a concatenation of gzip files,
// each with its own header. Reads from the Reader
// return the concatenation of the uncompressed data of each.
// Only the first header is recorded in the Reader fields.
//
// Gzip files store a length and checksum of the uncompressed data.
// The Reader will return a ErrChecksum when Read
// reaches the end of the uncompressed data if it does not
// have the expected length or checksum. Clients should treat data
// returned by Read as tentative until they receive the io.EOF
// marking the end of the data.
type Reader struct {
        Header       // valid after NewReader or Reader.Reset
        r            flate.Reader
        br           *bufio.Reader
        decompressor io.ReadCloser
        digest       uint32 // CRC-32, IEEE polynomial (section 8)
        size         uint32 // Uncompressed size (section 2.3.1)
        buf          [512]byte
        err          error
        multistream  bool
}

// NewReader creates a new Reader reading the given reader.
// If r does not also implement io.ByteReader,
// the decompressor may read more data than necessary from r.
//
// It is the caller's responsibility to call Close on the Reader when done.
//
// The Reader.Header fields will be valid in the Reader returned.
func NewReader(r io.Reader) (*Reader, error) {
        z := new(Reader)
        if err := z.Reset(r); err != nil {
                return nil, err
        }
        return z, nil
}

// Reset discards the Reader z's state and makes it equivalent to the
// result of its original state from NewReader, but reading from r instead.
// This permits reusing a Reader rather than allocating a new one.
func (z *Reader) Reset(r io.Reader) error {
        *z = Reader{
                decompressor: z.decompressor,
                multistream:  true,
        }
        if rr, ok := r.(flate.Reader); ok {
                z.r = rr
        } else {
                // Reuse if we can.
                if z.br != nil {
                        z.br.Reset(r)
                } else {
                        z.br = bufio.NewReader(r)
                }
                z.r = z.br
        }
        z.Header, z.err = z.readHeader()
        return z.err
}

// Multistream controls whether the reader supports multistream files.
//
// If enabled (the default), the Reader expects the input to be a sequence
// of individually gzipped data streams, each with its own header and
// trailer, ending at EOF. The effect is that the concatenation of a sequence
// of gzipped files is treated as equivalent to the gzip of the concatenation
// of the sequence. This is standard behavior for gzip readers.
//
// Calling Multistream(false) disables this behavior; disabling the behavior
// can be useful when reading file formats that distinguish individual gzip
// data streams or mix gzip data streams with other data streams.
// In this mode, when the Reader reaches the end of the data stream,
// Read returns io.EOF. If the underlying reader implements io.ByteReader,
// it will be left positioned just after the gzip stream.
// To start the next stream, call z.Reset(r) followed by z.Multistream(false).
// If there is no next stream, z.Reset(r) will return io.EOF.
func (z *Reader) Multistream(ok bool) {
        z.multistream = ok
}

// readString reads a NUL-terminated string from z.r.
// It treats the bytes read as being encoded as ISO 8859-1 (Latin-1) and
// will output a string encoded using UTF-8.
// This method always updates z.digest with the data read.
func (z *Reader) readString() (string, error) {
        var err error
        needConv := false
        for i := 0; ; i++ {
                if i >= len(z.buf) {
                        return "", ErrHeader
                }
                z.buf[i], err = z.r.ReadByte()
                if err != nil {
                        return "", err
                }
                if z.buf[i] > 0x7f {
                        needConv = true
                }
                if z.buf[i] == 0 {
                        // Digest covers the NUL terminator.
                        z.digest = crc32.Update(z.digest, crc32.IEEETable, z.buf[:i+1])

                        // Strings are ISO 8859-1, Latin-1 (RFC 1952, section 2.3.1).
                        if needConv {
                                s := make([]rune, 0, i)
                                for _, v := range z.buf[:i] {
                                        s = append(s, rune(v))
                                }
                                return string(s), nil
                        }
                        return string(z.buf[:i]), nil
                }
        }
}

// readHeader reads the GZIP header according to section 2.3.1.
// This method does not set z.err.
func (z *Reader) readHeader() (hdr Header, err error) {
        if _, err = io.ReadFull(z.r, z.buf[:10]); err != nil {
                // RFC 1952, section 2.2, says the following:
                //      A gzip file consists of a series of "members" (compressed data sets).
                //
                // Other than this, the specification does not clarify whether a
                // "series" is defined as "one or more" or "zero or more". To err on the
                // side of caution, Go interprets this to mean "zero or more".
                // Thus, it is okay to return io.EOF here.
                return hdr, err
        }
        if z.buf[0] != gzipID1 || z.buf[1] != gzipID2 || z.buf[2] != gzipDeflate {
                return hdr, ErrHeader
        }
        flg := z.buf[3]
        hdr.ModTime = time.Unix(int64(le.Uint32(z.buf[4:8])), 0)
        // z.buf[8] is XFL and is currently ignored.
        hdr.OS = z.buf[9]
        z.digest = crc32.ChecksumIEEE(z.buf[:10])

        if flg&flagExtra != 0 {
                if _, err = io.ReadFull(z.r, z.buf[:2]); err != nil {
                        return hdr, noEOF(err)
                }
                z.digest = crc32.Update(z.digest, crc32.IEEETable, z.buf[:2])
                data := make([]byte, le.Uint16(z.buf[:2]))
                if _, err = io.ReadFull(z.r, data); err != nil {
                        return hdr, noEOF(err)
                }
                z.digest = crc32.Update(z.digest, crc32.IEEETable, data)
                hdr.Extra = data
        }

        var s string
        if flg&flagName != 0 {
                if s, err = z.readString(); err != nil {
                        return hdr, err
                }
                hdr.Name = s
        }

        if flg&flagComment != 0 {
                if s, err = z.readString(); err != nil {
                        return hdr, err
                }
                hdr.Comment = s
        }

        if flg&flagHdrCrc != 0 {
                if _, err = io.ReadFull(z.r, z.buf[:2]); err != nil {
                        return hdr, noEOF(err)
                }
                digest := le.Uint16(z.buf[:2])
                if digest != uint16(z.digest) {
                        return hdr, ErrHeader
                }
        }

        z.digest = 0
        if z.decompressor == nil {
                z.decompressor = flate.NewReader(z.r)
        } else {
                z.decompressor.(flate.Resetter).Reset(z.r, nil)
        }
        return hdr, nil
}

// Read implements io.Reader, reading uncompressed bytes from its underlying Reader.
func (z *Reader) Read(p []byte) (n int, err error) {
        if z.err != nil {
                return 0, z.err
        }

        n, z.err = z.decompressor.Read(p)
        z.digest = crc32.Update(z.digest, crc32.IEEETable, p[:n])
        z.size += uint32(n)
        if z.err != io.EOF {
                // In the normal case we return here.
                return n, z.err
        }

        // Finished file; check checksum and size.
        if _, err := io.ReadFull(z.r, z.buf[:8]); err != nil {
                z.err = noEOF(err)
                return n, z.err
        }
        digest := le.Uint32(z.buf[:4])
        size := le.Uint32(z.buf[4:8])
        if digest != z.digest || size != z.size {
                z.err = ErrChecksum
                return n, z.err
        }
        z.digest, z.size = 0, 0

        // File is ok; check if there is another.
        if !z.multistream {
                return n, io.EOF
        }
        z.err = nil // Remove io.EOF

        if _, z.err = z.readHeader(); z.err != nil {
                return n, z.err
        }

        // Read from next file, if necessary.
        if n > 0 {
                return n, nil
        }
        return z.Read(p)
}

// Support the io.WriteTo interface for io.Copy and friends.
func (z *Reader) WriteTo(w io.Writer) (int64, error) {
        total := int64(0)
        crcWriter := crc32.NewIEEE()
        for {
                if z.err != nil {
                        if z.err == io.EOF {
                                return total, nil
                        }
                        return total, z.err
                }

                // We write both to output and digest.
                mw := io.MultiWriter(w, crcWriter)
                n, err := z.decompressor.(io.WriterTo).WriteTo(mw)
                total += n
                z.size += uint32(n)
                if err != nil {
                        z.err = err
                        return total, z.err
                }

                // Finished file; check checksum + size.
                if _, err := io.ReadFull(z.r, z.buf[0:8]); err != nil {
                        if err == io.EOF {
                                err = io.ErrUnexpectedEOF
                        }
                        z.err = err
                        return total, err
                }
                z.digest = crcWriter.Sum32()
                digest := le.Uint32(z.buf[:4])
                size := le.Uint32(z.buf[4:8])
                if digest != z.digest || size != z.size {
                        z.err = ErrChecksum
                        return total, z.err
                }
                z.digest, z.size = 0, 0

                // File is ok; check if there is another.
                if !z.multistream {
                        return total, nil
                }
                crcWriter.Reset()
                z.err = nil // Remove io.EOF

                if _, z.err = z.readHeader(); z.err != nil {
                        if z.err == io.EOF {
                                return total, nil
                        }
                        return total, z.err
                }
        }
}

// Close closes the Reader. It does not close the underlying io.Reader.
// In order for the GZIP checksum to be verified, the reader must be
// fully consumed until the io.EOF.
func (z *Reader) Close() error { return z.decompressor.Close() }