require 'pty'

SOH = 0x01
EOT = 0x04
ACK = 0x06
NAK = 0x15
SUB = 0x1A

BLOCKSIZE = 128

infile, outfile = ARGV

File.open(infile, 'rb') do |f|
  PTY.spawn('rx', '-b', outfile) do |stdout, stdin, _pid|
    loop do
      break if stdout.readbyte == NAK
    end

    block = ''
    blocknum = 1

    loop do
      block = f.read(BLOCKSIZE)
      break unless block

      block += SUB.chr * (BLOCKSIZE - block.size) if block.size < BLOCKSIZE

      loop do
        inv_blocknum = 0xFF - blocknum
        stdin.putc(SOH)
        stdin.putc(blocknum)
        stdin.putc(inv_blocknum)

        checksum = block.bytes.reduce(0, &:+) & 0xFF
        stdin.print(block)
        stdin.putc(checksum)

        control = stdout.readbyte
        break if control == ACK
        raise('unexpected byte') unless control == NAK
      end

      blocknum = (blocknum + 1) & 0xFF
    end

    stdin.putc(EOT)
    control = stdout.readbyte
    control == ACK || raise('transmission error')
  end
end