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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() }