Class: Omnizip::Formats::XzImpl::StreamFooterParser

Inherits:
Object
  • Object
show all
Defined in:
lib/omnizip/formats/xz_impl/stream_footer_parser.rb

Overview

XZ Stream Footer parser

Stream Footer format (12 bytes):

  • CRC32: of backward_size + flags (4 bytes, little-endian)
  • Backward Size: size of Index in 4-byte units (4 bytes, little-endian)
  • Stream Flags: same as in Stream Header (2 bytes)
  • Magic: 0x59 0x5A (2 bytes)

Reference: /tmp/xz-source/src/liblzma/common/stream_footer_decoder.c

Constant Summary collapse 

[0x59, 0x5A].freeze
12

Class Method Summary collapse

Class Method Details

.parse(input) ⇒ Hash

Parse stream footer from input stream

Parameters:

  • input (IO) —

    Input stream positioned at footer start

Returns:

  • (Hash) —

    Parsed footer data with keys:

    • backward_size: Integer (size of Index in 4-byte units)
    • check_type: Integer (0=None, 1=CRC32, 4=CRC64, 10=SHA256)

Raises:

  • (RuntimeError) —

    If footer is invalid or CRC mismatch



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# File 'lib/omnizip/formats/xz_impl/stream_footer_parser.rb', line 51

def self.parse(input)
  # Read 12 bytes
  footer = input.read(FOOTER_SIZE)
  if footer.nil? || footer.bytesize < FOOTER_SIZE
    raise FormatError,
          "Unexpected end of file: incomplete stream footer"
  end

  # Verify magic bytes (last 2 bytes)
  magic = footer[-2..].bytes
  unless magic == FOOTER_MAGIC
    raise FormatError, "Invalid XZ footer magic: got #{magic.map do |b|
      b.to_s(16).upcase
    end.join(' ')}"
  end

  # Parse CRC32 (first 4 bytes)
  stored_crc = footer[0..3].unpack1("V")

  # Parse backward size (next 4 bytes, little-endian)
  backward_size = footer[4..7].unpack1("V")

  # Parse stream flags (next 2 bytes)
  flags = footer[8..9]

  # Byte 8: reserved (must be 0)
  if flags.getbyte(0) != 0
    raise FormatError,
          "Invalid stream footer: reserved byte is non-zero"
  end

  # Byte 9: check type (low 4 bits) + reserved (high 4 bits)
  check_flags = flags.getbyte(1)
  check_type = check_flags & 0x0F
  reserved = (check_flags >> 4) & 0x0F

  if reserved != 0
    raise FormatError,
          "Invalid stream footer: reserved bits are non-zero"
  end

  # Validate check type
  unless [0, 1, 4, 10].include?(check_type)
    raise FormatError, "Invalid check type in footer: #{check_type}"
  end

  # Verify CRC32
  # CRC is calculated over backward_size + flags (bytes 4-9)
  crc_data = footer[4..9]
  actual_crc = Zlib.crc32(crc_data)

  if actual_crc != stored_crc
    raise FormatError,
          "Stream footer CRC mismatch: expected #{stored_crc}, got #{actual_crc}"
  end

  {
    backward_size: backward_size,
    check_type: check_type,
  }
end