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// 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/flate"

        "errors"

        "hash"

        "hash/crc32"

        "io"

        "time"

)

// BUG(nigeltao): Comments and Names don't properly map UTF-8 character codes outside of

// the 0x00-0x7f range to ISO 8859-1 (Latin-1).

const (

        gzipID1     = 0x1f

        gzipID2     = 0x8b

        gzipDeflate = 8

        flagText    = 1 << 0

        flagHdrCrc  = 1 << 1

        flagExtra   = 1 << 2

        flagName    = 1 << 3

        flagComment = 1 << 4

)

func makeReader(r io.Reader) flate.Reader {

        if rr, ok := r.(flate.Reader); ok {

                return rr

        }

        return bufio.NewReader(r)

}

var ErrHeader = errors.New("invalid gzip header")

var ErrChecksum = errors.New("gzip checksum error")

// The gzip file stores a header giving metadata about the compressed file.

// That header is exposed as the fields of the Compressor and Decompressor structs.

type Header struct {

        Comment string    // comment

        Extra   []byte    // "extra data"

        ModTime time.Time // modification time

        Name    string    // file name

        OS      byte      // operating system type

}

// An Decompressor 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 Decompressor

// return the concatenation of the uncompressed data of each.

// Only the first header is recorded in the Decompressor fields.

//

// Gzip files store a length and checksum of the uncompressed data.

// The Decompressor 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 successful

// (zero length, nil error) Read marking the end of the data.

type Decompressor struct {

        Header

        r            flate.Reader

        decompressor io.ReadCloser

        digest       hash.Hash32

        size         uint32

        flg          byte

        buf          [512]byte

        err          error

}

// NewReader creates a new Decompressor reading the given reader.

// The implementation buffers input and may read more data than necessary from r.

// It is the caller's responsibility to call Close on the Decompressor when done.

func NewReader(r io.Reader) (*Decompressor, error) {

        z := new(Decompressor)

        z.r = makeReader(r)

        z.digest = crc32.NewIEEE()

        if err := z.readHeader(true); err != nil {

                z.err = err

                return nil, err

        }

        return z, nil

}

// GZIP (RFC 1952) is little-endian, unlike ZLIB (RFC 1950).

func get4(p []byte) uint32 {

        return uint32(p[0]) | uint32(p[1])<<8 | uint32(p[2])<<16 | uint32(p[3])<<24

}

func (z *Decompressor) 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 {

                        // GZIP (RFC 1952) specifies that strings are NUL-terminated ISO 8859-1 (Latin-1).

                        if needconv {

                                s := make([]rune, 0, i)

                                for _, v := range z.buf[0:i] {

                                        s = append(s, rune(v))

                                }

                                return string(s), nil

                        }

                        return string(z.buf[0:i]), nil

                }

        }

        panic("not reached")

}

func (z *Decompressor) read2() (uint32, error) {

        _, err := io.ReadFull(z.r, z.buf[0:2])

        if err != nil {

                return 0, err

        }

        return uint32(z.buf[0]) | uint32(z.buf[1])<<8, nil

}

func (z *Decompressor) readHeader(save bool) error {

        _, err := io.ReadFull(z.r, z.buf[0:10])

        if err != nil {

                return err

        }

        if z.buf[0] != gzipID1 || z.buf[1] != gzipID2 || z.buf[2] != gzipDeflate {

                return ErrHeader

        }

        z.flg = z.buf[3]

        if save {

                z.ModTime = time.Unix(int64(get4(z.buf[4:8])), 0)

                // z.buf[8] is xfl, ignored

                z.OS = z.buf[9]

        }

        z.digest.Reset()

        z.digest.Write(z.buf[0:10])

        if z.flg&flagExtra != 0 {

                n, err := z.read2()

                if err != nil {

                        return err

                }

                data := make([]byte, n)

                if _, err = io.ReadFull(z.r, data); err != nil {

                        return err

                }

                if save {

                        z.Extra = data

                }

        }

        var s string

        if z.flg&flagName != 0 {

                if s, err = z.readString(); err != nil {

                        return err

                }

                if save {

                        z.Name = s

                }

        }

        if z.flg&flagComment != 0 {

                if s, err = z.readString(); err != nil {

                        return err

                }

                if save {

                        z.Comment = s

                }

        }

        if z.flg&flagHdrCrc != 0 {

                n, err := z.read2()

                if err != nil {

                        return err

                }

                sum := z.digest.Sum32() & 0xFFFF

                if n != sum {

                        return ErrHeader

                }

        }

        z.digest.Reset()

        z.decompressor = flate.NewReader(z.r)

        return nil

}

func (z *Decompressor) Read(p []byte) (n int, err error) {

        if z.err != nil {

                return 0, z.err

        }

        if len(p) == 0 {

                return 0, nil

        }

        n, err = z.decompressor.Read(p)

        z.digest.Write(p[0:n])

        z.size += uint32(n)

        if n != 0 || err != io.EOF {

                z.err = err

                return

        }

        // Finished file; check checksum + size.

        if _, err := io.ReadFull(z.r, z.buf[0:8]); err != nil {

                z.err = err

                return 0, err

        }

        crc32, isize := get4(z.buf[0:4]), get4(z.buf[4:8])

        sum := z.digest.Sum32()

        if sum != crc32 || isize != z.size {

                z.err = ErrChecksum

                return 0, z.err

        }

        // File is ok; is there another?

        if err = z.readHeader(false); err != nil {

                z.err = err

                return

        }

        // Yes.  Reset and read from it.

        z.digest.Reset()

        z.size = 0

        return z.Read(p)

}

// Calling Close does not close the wrapped io.Reader originally passed to NewReader.

func (z *Decompressor) Close() error { return z.decompressor.Close() }