var ZEROS = '0000000000000000000'
var ZERO_OFFSET = '0'.charCodeAt(0)
var USTAR = 'ustar\x0000'
var MASK = parseInt('7777', 8)

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 alloc = function (size) {
  var buf = new Buffer(size)
  buf.fill(0)
  return buf
}

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)
  return ZEROS.slice(0, n - val.length) + val + ' '
}

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

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) {
  return decodeStr(buf, 0, buf.length)
}

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')
  return new Buffer(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)

  buf.write(USTAR, 257)
  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) {
  var typeflag = buf[156] === 0 ? 0 : buf[156] - ZERO_OFFSET

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

  if (buf[345]) name = decodeStr(buf, 345, 155) + '/' + name

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

  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)) throw new Error('Invalid tar header. Maybe the tar is corrupted or it needs to be gunzipped?')

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