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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 tar implements access to tar archives.
// It aims to cover most of the variations, including those produced
// by GNU and BSD tars.
//
// References:
//   http://www.freebsd.org/cgi/man.cgi?query=tar&sektion=5
//   http://www.gnu.org/software/tar/manual/html_node/Standard.html
package tar

import "time"

const (
        blockSize = 512

        // Types
        TypeReg           = '0'    // regular file
        TypeRegA          = '\x00' // regular file
        TypeLink          = '1'    // hard link
        TypeSymlink       = '2'    // symbolic link
        TypeChar          = '3'    // character device node
        TypeBlock         = '4'    // block device node
        TypeDir           = '5'    // directory
        TypeFifo          = '6'    // fifo node
        TypeCont          = '7'    // reserved
        TypeXHeader       = 'x'    // extended header
        TypeXGlobalHeader = 'g'    // global extended header
)

// A Header represents a single header in a tar archive.
// Some fields may not be populated.
type Header struct {
        Name       string    // name of header file entry
        Mode       int64     // permission and mode bits
        Uid        int       // user id of owner
        Gid        int       // group id of owner
        Size       int64     // length in bytes
        ModTime    time.Time // modified time
        Typeflag   byte      // type of header entry
        Linkname   string    // target name of link
        Uname      string    // user name of owner
        Gname      string    // group name of owner
        Devmajor   int64     // major number of character or block device
        Devminor   int64     // minor number of character or block device
        AccessTime time.Time // access time
        ChangeTime time.Time // status change time
}

var zeroBlock = make([]byte, blockSize)

// POSIX specifies a sum of the unsigned byte values, but the Sun tar uses signed byte values.
// We compute and return both.
func checksum(header []byte) (unsigned int64, signed int64) {
        for i := 0; i < len(header); i++ {
                if i == 148 {
                        // The chksum field (header[148:156]) is special: it should be treated as space bytes.
                        unsigned += ' ' * 8
                        signed += ' ' * 8
                        i += 7
                        continue
                }
                unsigned += int64(header[i])
                signed += int64(int8(header[i]))
        }
        return
}

type slicer []byte

func (sp *slicer) next(n int) (b []byte) {
        s := *sp
        b, *sp = s[0:n], s[n:]
        return
}