require 'pty'

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

BLOCKSIZE = 128

infile, outfile = ARGV

File.open(outfile, 'wb') do |f|
  PTY.spawn('sx', '-b', infile) do |stdout, stdin, _pid|
    stdout.gets # "Give your local XMODEM receive command now."
    stdin.putc(NAK)

    loop do
      control = stdout.readbyte
      break if control == EOT

      control == SOH || raise('expected start of header')
      blocknum = stdout.readbyte
      inv_blocknum = stdout.readbyte
      blocknum + inv_blocknum == 0xFF || raise("block numbers don't match")

      block = stdout.read(BLOCKSIZE)
      expected = block.bytes.reduce(0, &:+) & 0xFF
      actual = stdout.readbyte
      stdin.putc(expected == actual ? ACK : NAK)
      f.write(block) if expected == actual
    end

    stdin.putc(ACK)
  end
end