module BBS::Zmodem::Frames

Headers read out of the receiver’s bytes: hex headers and binary ones with a CRC-16 or a CRC-32, ZDLE escapes undone, the CRC checked.

Public Instance Methods

crc_ok? (bytes, crc_size)

Source

# File lib/bbs/zmodem/frames.rb, line 72
def crc_ok?(bytes, crc_size)
  frame = bytes[0, HEADER_SIZE].pack('C*')
  crc   = bytes[HEADER_SIZE, crc_size]
  return Zlib.crc32(frame) == crc.pack('C*').unpack1('V') if crc_size == CRC32_SIZE

  Zmodem.crc16(frame) == ((crc[0] << 8) | crc[1])
end

Whether the CRC of crc_size bytes after the header in bytes is right.

header_of (bytes)

Source

# File lib/bbs/zmodem/frames.rb, line 81
def header_of(bytes) = Header.new(bytes[0], bytes[1, 4])

The Header of the first HEADER_SIZE bytes.

parse (buf)

Source

# File lib/bbs/zmodem/frames.rb, line 14
def parse(buf)
  i = 0
  i += 1 while buf[i] == ZPAD
  return [nil, nil] if i == buf.size
  return [nil, i] unless buf[i] == ZDLE
  return [nil, nil] if i + 1 >= buf.size

  parse_frame(buf, buf[i + 1], i + 2)
end

Parses the header at the start of buf (an Array of byte Integers beginning with ZPAD). Returns [header, used]: the Header (nil for a broken one) and the bytes it took, or [nil, nil] when buf ends before the header does.

parse_bin (buf, from, crc_size)

Source

# File lib/bbs/zmodem/frames.rb, line 49
def parse_bin(buf, from, crc_size)
  bytes, used = unescape(buf, from, HEADER_SIZE + crc_size)
  return [nil, nil] unless bytes

  [(header_of(bytes) if crc_ok?(bytes, crc_size)), used]
end

A binary header from from with a CRC of crc_size bytes; see parse.

parse_frame (buf, kind, from)

Source

# File lib/bbs/zmodem/frames.rb, line 26
def parse_frame(buf, kind, from)
  case kind
  when ZHEX   then parse_hex(buf, from)
  when ZBIN   then parse_bin(buf, from, CRC16_SIZE)
  when ZBIN32 then parse_bin(buf, from, CRC32_SIZE)
  else [nil, from - 1]
  end
end

The header of kind kind (ZHEX, ZBIN, ZBIN32) whose bytes start at from; see parse.

parse_hex (buf, from)

Source

# File lib/bbs/zmodem/frames.rb, line 36
def parse_hex(buf, from)
  to = from + HEX_DIGITS
  return [nil, nil] if buf.size < to

  text = buf[from, HEX_DIGITS].pack('C*')
  return [nil, from] unless text.match?(HEX_HEADER_RE)

  bytes = [text].pack('H*').bytes
  [(header_of(bytes) if crc_ok?(bytes, CRC16_SIZE)), to]
end

A hex header from from; see parse.

unescape (buf, from, count)

Source

# File lib/bbs/zmodem/frames.rb, line 58
def unescape(buf, from, count)
  bytes = []
  while bytes.size < count
    step = buf[from] == ZDLE ? 2 : 1
    return if from + step > buf.size

    bytes << (step == 2 ? ZDLE_VALUES.fetch(buf[from + 1]) { |c| c ^ ZDLE_XOR } : buf[from])
    from += step
  end
  [bytes, from]
end

The count bytes from from with the ZDLE escapes undone, and where they end; nil when buf ends first.