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Filename : tar_header.rb
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# frozen_string_literal: true

# rubocop:disable Style/AsciiComments

# Copyright (C) 2004 Mauricio Julio Fernández Pradier
# See LICENSE.txt for additional licensing information.

# rubocop:enable Style/AsciiComments

##
#--
# struct tarfile_entry_posix {
#   char name[100];     # ASCII + (Z unless filled)
#   char mode[8];       # 0 padded, octal, null
#   char uid[8];        # ditto
#   char gid[8];        # ditto
#   char size[12];      # 0 padded, octal, null
#   char mtime[12];     # 0 padded, octal, null
#   char checksum[8];   # 0 padded, octal, null, space
#   char typeflag[1];   # file: "0"  dir: "5"
#   char linkname[100]; # ASCII + (Z unless filled)
#   char magic[6];      # "ustar\0"
#   char version[2];    # "00"
#   char uname[32];     # ASCIIZ
#   char gname[32];     # ASCIIZ
#   char devmajor[8];   # 0 padded, octal, null
#   char devminor[8];   # o padded, octal, null
#   char prefix[155];   # ASCII + (Z unless filled)
# };
#++
# A header for a tar file

class Gem::Package::TarHeader
  ##
  # Fields in the tar header

  FIELDS = [
    :checksum,
    :devmajor,
    :devminor,
    :gid,
    :gname,
    :linkname,
    :magic,
    :mode,
    :mtime,
    :name,
    :prefix,
    :size,
    :typeflag,
    :uid,
    :uname,
    :version,
  ].freeze

  ##
  # Pack format for a tar header

  PACK_FORMAT = "a100" + # name
                "a8"   + # mode
                "a8"   + # uid
                "a8"   + # gid
                "a12"  + # size
                "a12"  + # mtime
                "a7a"  + # chksum
                "a"    + # typeflag
                "a100" + # linkname
                "a6"   + # magic
                "a2"   + # version
                "a32"  + # uname
                "a32"  + # gname
                "a8"   + # devmajor
                "a8"   + # devminor
                "a155"   # prefix

  ##
  # Unpack format for a tar header

  UNPACK_FORMAT = "A100" + # name
                  "A8"   + # mode
                  "A8"   + # uid
                  "A8"   + # gid
                  "A12"  + # size
                  "A12"  + # mtime
                  "A8"   + # checksum
                  "A"    + # typeflag
                  "A100" + # linkname
                  "A6"   + # magic
                  "A2"   + # version
                  "A32"  + # uname
                  "A32"  + # gname
                  "A8"   + # devmajor
                  "A8"   + # devminor
                  "A155"   # prefix

  attr_reader(*FIELDS)

  EMPTY_HEADER = ("\0" * 512).b.freeze # :nodoc:

  ##
  # Creates a tar header from IO +stream+

  def self.from(stream)
    header = stream.read 512
    return EMPTY if header == EMPTY_HEADER

    fields = header.unpack UNPACK_FORMAT

    new name: fields.shift,
        mode: strict_oct(fields.shift),
        uid: oct_or_256based(fields.shift),
        gid: oct_or_256based(fields.shift),
        size: strict_oct(fields.shift),
        mtime: strict_oct(fields.shift),
        checksum: strict_oct(fields.shift),
        typeflag: fields.shift,
        linkname: fields.shift,
        magic: fields.shift,
        version: strict_oct(fields.shift),
        uname: fields.shift,
        gname: fields.shift,
        devmajor: strict_oct(fields.shift),
        devminor: strict_oct(fields.shift),
        prefix: fields.shift,

        empty: false
  end

  def self.strict_oct(str)
    str.strip!
    return str.oct if /\A[0-7]*\z/.match?(str)

    raise ArgumentError, "#{str.inspect} is not an octal string"
  end

  def self.oct_or_256based(str)
    # \x80 flags a positive 256-based number
    # \ff flags a negative 256-based number
    # In case we have a match, parse it as a signed binary value
    # in big-endian order, except that the high-order bit is ignored.

    return str.unpack1("@4N") if /\A[\x80\xff]/n.match?(str)
    strict_oct(str)
  end

  ##
  # Creates a new TarHeader using +vals+

  def initialize(vals)
    unless vals[:name] && vals[:size] && vals[:prefix] && vals[:mode]
      raise ArgumentError, ":name, :size, :prefix and :mode required"
    end

    @checksum = vals[:checksum] || ""
    @devmajor = vals[:devmajor] || 0
    @devminor = vals[:devminor] || 0
    @gid = vals[:gid] || 0
    @gname = vals[:gname] || "wheel"
    @linkname = vals[:linkname]
    @magic = vals[:magic] || "ustar"
    @mode = vals[:mode]
    @mtime = vals[:mtime] || 0
    @name = vals[:name]
    @prefix = vals[:prefix]
    @size = vals[:size]
    @typeflag = vals[:typeflag]
    @typeflag = "0" if @typeflag.nil? || @typeflag.empty?
    @uid = vals[:uid] || 0
    @uname = vals[:uname] || "wheel"
    @version = vals[:version] || "00"

    @empty = vals[:empty]
  end

  EMPTY = new({ # :nodoc:
    checksum: 0,
    gname: "",
    linkname: "",
    magic: "",
    mode: 0,
    name: "",
    prefix: "",
    size: 0,
    uname: "",
    version: 0,

    empty: true,
  }).freeze
  private_constant :EMPTY

  ##
  # Is the tar entry empty?

  def empty?
    @empty
  end

  def ==(other) # :nodoc:
    self.class === other &&
      @checksum == other.checksum &&
      @devmajor == other.devmajor &&
      @devminor == other.devminor &&
      @gid      == other.gid      &&
      @gname    == other.gname    &&
      @linkname == other.linkname &&
      @magic    == other.magic    &&
      @mode     == other.mode     &&
      @mtime    == other.mtime    &&
      @name     == other.name     &&
      @prefix   == other.prefix   &&
      @size     == other.size     &&
      @typeflag == other.typeflag &&
      @uid      == other.uid      &&
      @uname    == other.uname    &&
      @version  == other.version
  end

  def to_s # :nodoc:
    update_checksum
    header
  end

  ##
  # Updates the TarHeader's checksum

  def update_checksum
    header = header " " * 8
    @checksum = oct calculate_checksum(header), 6
  end

  private

  def calculate_checksum(header)
    header.sum(0)
  end

  def header(checksum = @checksum)
    header = [
      name,
      oct(mode, 7),
      oct(uid, 7),
      oct(gid, 7),
      oct(size, 11),
      oct(mtime, 11),
      checksum,
      " ",
      typeflag,
      linkname,
      magic,
      oct(version, 2),
      uname,
      gname,
      oct(devmajor, 7),
      oct(devminor, 7),
      prefix,
    ]

    header = header.pack PACK_FORMAT

    header.ljust 512, "\0"
  end

  def oct(num, len)
    format("%0#{len}o", num)
  end
end