source: node_modules/tar-stream/headers.js@ 81bc7da

var alloc = Buffer.alloc

var ZEROS = '0000000000000000000'
var SEVENS = '7777777777777777777'
var ZERO_OFFSET = '0'.charCodeAt(0)
var USTAR_MAGIC = Buffer.from('ustar\x00', 'binary')
var USTAR_VER = Buffer.from('00', 'binary')
var GNU_MAGIC = Buffer.from('ustar\x20', 'binary')
var GNU_VER = Buffer.from('\x20\x00', 'binary')
var MASK = parseInt('7777', 8)
var MAGIC_OFFSET = 257
var VERSION_OFFSET = 263

var clamp = function (index, len, defaultValue) {
  if (typeof index !== 'number') return defaultValue
  index = ~~index // Coerce to integer.
  if (index >= len) return len
  if (index >= 0) return index
  index += len
  if (index >= 0) return index
  return 0
}

var toType = function (flag) {
  switch (flag) {
    case 0:
      return 'file'
    case 1:
      return 'link'
    case 2:
      return 'symlink'
    case 3:
      return 'character-device'
    case 4:
      return 'block-device'
    case 5:
      return 'directory'
    case 6:
      return 'fifo'
    case 7:
      return 'contiguous-file'
    case 72:
      return 'pax-header'
    case 55:
      return 'pax-global-header'
    case 27:
      return 'gnu-long-link-path'
    case 28:
    case 30:
      return 'gnu-long-path'
  }

  return null
}

var toTypeflag = function (flag) {
  switch (flag) {
    case 'file':
      return 0
    case 'link':
      return 1
    case 'symlink':
      return 2
    case 'character-device':
      return 3
    case 'block-device':
      return 4
    case 'directory':
      return 5
    case 'fifo':
      return 6
    case 'contiguous-file':
      return 7
    case 'pax-header':
      return 72
  }

  return 0
}

var indexOf = function (block, num, offset, end) {
  for (; offset < end; offset++) {
    if (block[offset] === num) return offset
  }
  return end
}

var cksum = function (block) {
  var sum = 8 * 32
  for (var i = 0; i < 148; i++) sum += block[i]
  for (var j = 156; j < 512; j++) sum += block[j]
  return sum
}

var encodeOct = function (val, n) {
  val = val.toString(8)
  if (val.length > n) return SEVENS.slice(0, n) + ' '
  else return ZEROS.slice(0, n - val.length) + val + ' '
}

/* Copied from the node-tar repo and modified to meet
 * tar-stream coding standard.
 *
 * Source: https://github.com/npm/node-tar/blob/51b6627a1f357d2eb433e7378e5f05e83b7aa6cd/lib/header.js#L349
 */
function parse256 (buf) {
  // first byte MUST be either 80 or FF
  // 80 for positive, FF for 2's comp
  var positive
  if (buf[0] === 0x80) positive = true
  else if (buf[0] === 0xFF) positive = false
  else return null

  // build up a base-256 tuple from the least sig to the highest
  var tuple = []
  for (var i = buf.length - 1; i > 0; i--) {
    var byte = buf[i]
    if (positive) tuple.push(byte)
    else tuple.push(0xFF - byte)
  }

  var sum = 0
  var l = tuple.length
  for (i = 0; i < l; i++) {
    sum += tuple[i] * Math.pow(256, i)
  }

  return positive ? sum : -1 * sum
}

var decodeOct = function (val, offset, length) {
  val = val.slice(offset, offset + length)
  offset = 0

  // If prefixed with 0x80 then parse as a base-256 integer
  if (val[offset] & 0x80) {
    return parse256(val)
  } else {
    // Older versions of tar can prefix with spaces
    while (offset < val.length && val[offset] === 32) offset++
    var end = clamp(indexOf(val, 32, offset, val.length), val.length, val.length)
    while (offset < end && val[offset] === 0) offset++
    if (end === offset) return 0
    return parseInt(val.slice(offset, end).toString(), 8)
  }
}

var decodeStr = function (val, offset, length, encoding) {
  return val.slice(offset, indexOf(val, 0, offset, offset + length)).toString(encoding)
}

var addLength = function (str) {
  var len = Buffer.byteLength(str)
  var digits = Math.floor(Math.log(len) / Math.log(10)) + 1
  if (len + digits >= Math.pow(10, digits)) digits++

  return (len + digits) + str
}

exports.decodeLongPath = function (buf, encoding) {
  return decodeStr(buf, 0, buf.length, encoding)
}

exports.encodePax = function (opts) { // TODO: encode more stuff in pax
  var result = ''
  if (opts.name) result += addLength(' path=' + opts.name + '\n')
  if (opts.linkname) result += addLength(' linkpath=' + opts.linkname + '\n')
  var pax = opts.pax
  if (pax) {
    for (var key in pax) {
      result += addLength(' ' + key + '=' + pax[key] + '\n')
    }
  }
  return Buffer.from(result)
}

exports.decodePax = function (buf) {
  var result = {}

  while (buf.length) {
    var i = 0
    while (i < buf.length && buf[i] !== 32) i++
    var len = parseInt(buf.slice(0, i).toString(), 10)
    if (!len) return result

    var b = buf.slice(i + 1, len - 1).toString()
    var keyIndex = b.indexOf('=')
    if (keyIndex === -1) return result
    result[b.slice(0, keyIndex)] = b.slice(keyIndex + 1)

    buf = buf.slice(len)
  }

  return result
}

exports.encode = function (opts) {
  var buf = alloc(512)
  var name = opts.name
  var prefix = ''

  if (opts.typeflag === 5 && name[name.length - 1] !== '/') name += '/'
  if (Buffer.byteLength(name) !== name.length) return null // utf-8

  while (Buffer.byteLength(name) > 100) {
    var i = name.indexOf('/')
    if (i === -1) return null
    prefix += prefix ? '/' + name.slice(0, i) : name.slice(0, i)
    name = name.slice(i + 1)
  }

  if (Buffer.byteLength(name) > 100 || Buffer.byteLength(prefix) > 155) return null
  if (opts.linkname && Buffer.byteLength(opts.linkname) > 100) return null

  buf.write(name)
  buf.write(encodeOct(opts.mode & MASK, 6), 100)
  buf.write(encodeOct(opts.uid, 6), 108)
  buf.write(encodeOct(opts.gid, 6), 116)
  buf.write(encodeOct(opts.size, 11), 124)
  buf.write(encodeOct((opts.mtime.getTime() / 1000) | 0, 11), 136)

  buf[156] = ZERO_OFFSET + toTypeflag(opts.type)

  if (opts.linkname) buf.write(opts.linkname, 157)

  USTAR_MAGIC.copy(buf, MAGIC_OFFSET)
  USTAR_VER.copy(buf, VERSION_OFFSET)
  if (opts.uname) buf.write(opts.uname, 265)
  if (opts.gname) buf.write(opts.gname, 297)
  buf.write(encodeOct(opts.devmajor || 0, 6), 329)
  buf.write(encodeOct(opts.devminor || 0, 6), 337)

  if (prefix) buf.write(prefix, 345)

  buf.write(encodeOct(cksum(buf), 6), 148)

  return buf
}

exports.decode = function (buf, filenameEncoding, allowUnknownFormat) {
  var typeflag = buf[156] === 0 ? 0 : buf[156] - ZERO_OFFSET

  var name = decodeStr(buf, 0, 100, filenameEncoding)
  var mode = decodeOct(buf, 100, 8)
  var uid = decodeOct(buf, 108, 8)
  var gid = decodeOct(buf, 116, 8)
  var size = decodeOct(buf, 124, 12)
  var mtime = decodeOct(buf, 136, 12)
  var type = toType(typeflag)
  var linkname = buf[157] === 0 ? null : decodeStr(buf, 157, 100, filenameEncoding)
  var uname = decodeStr(buf, 265, 32)
  var gname = decodeStr(buf, 297, 32)
  var devmajor = decodeOct(buf, 329, 8)
  var devminor = decodeOct(buf, 337, 8)

  var c = cksum(buf)

  // checksum is still initial value if header was null.
  if (c === 8 * 32) return null

  // valid checksum
  if (c !== decodeOct(buf, 148, 8)) throw new Error('Invalid tar header. Maybe the tar is corrupted or it needs to be gunzipped?')

  if (USTAR_MAGIC.compare(buf, MAGIC_OFFSET, MAGIC_OFFSET + 6) === 0) {
    // ustar (posix) format.
    // prepend prefix, if present.
    if (buf[345]) name = decodeStr(buf, 345, 155, filenameEncoding) + '/' + name
  } else if (GNU_MAGIC.compare(buf, MAGIC_OFFSET, MAGIC_OFFSET + 6) === 0 &&
             GNU_VER.compare(buf, VERSION_OFFSET, VERSION_OFFSET + 2) === 0) {
    // 'gnu'/'oldgnu' format. Similar to ustar, but has support for incremental and
    // multi-volume tarballs.
  } else {
    if (!allowUnknownFormat) {
      throw new Error('Invalid tar header: unknown format.')
    }
  }

  // to support old tar versions that use trailing / to indicate dirs
  if (typeflag === 0 && name && name[name.length - 1] === '/') typeflag = 5

  return {
    name,
    mode,
    uid,
    gid,
    size,
    mtime: new Date(1000 * mtime),
    type,
    linkname,
    uname,
    gname,
    devmajor,
    devminor
  }
}