Three-Way Handshake Computer Sync

local modemSide = 'left'
local modem = peripheral.wrap(modemSide)

function sync_master(channel, alarmTime)
  local timeout = os.startTimer(0.1)
  print("Doing three-way handshake")
  while true do
    modem.transmit(channel, channel, 'SYN')
    local e, p1, _, _, p4, _ = os.pullEvent()
    if e == 'modem_message' then
      if p4 == 'SYN-ACK' then
        modem.transmit(channel, channel, 'ACK')
        modem.transmit(channel, channel, tostring(alarmTime))
      elseif p4 == 'ACK' then
        break
      end
    elseif e == 'timer' and p1 == timeout then
      modem.transmit(channel, channel, 'SYN')
      timeout = os.startTimer(0.1)
      alarmTime = os.time() + 0.025
    end
  end

  return alarmTime
end

function sync_drone()
  local gotAck = false
  local syn = false
  print("Listening for three-way handshake")
  while true do
    local _, _, p3, _, p4, _ = os.pullEvent('modem_message')
    if p4 == 'SYN' then
      modem.transmit(p3, p3, 'SYN-ACK')
      syn = true
    elseif p4 == 'ACK' and syn and not gotAck then
      gotAck = true
      syn = false
    elseif tonumber(p4) and gotAck then
      modem.transmit(p3, p3, 'ACK')
      return tonumber(p4)
    end
  end
end

function do_timer(dur, alarm)
  os.setAlarm(alarm)
  os.pullEvent('alarm')

  local curr = 0
  local timer = os.startTimer(1)

  while true do
    print('Launch in '..(dur - curr)..' seconds...')

    local _, p = os.pullEvent('timer')

    if p == timer then
      timer = os.startTimer(1)
      curr = curr + 1
      if curr == dur then
        return false
      end
    end
  end
end

-- call this with the communication channel and how long you want the countdown to be.
function startSyncedTimer(channel, dur)
  -- if the channel is not open, open it...
  if not modem.isOpen(channel) then
    modem.open(channel)
  end

  -- figure out a time that both computers should start the timer, based off the Minecraft time
  local alarmTime = os.time() + 0.1

  -- send this time out to the world
  alarmTime = sync_master(channel, alarmTime)

  -- call the timer function.
  -- Param 1: The duration of the timer, i.e. the countdown start time
  -- Param 2: when to start the timer
  local ok = do_timer(dur, alarmTime)

  -- if the timer finished with a boom, say boom (when you have the override make sure that you return true from the do_timer function so as to not make things go boom, then in this function make sure that the missile is launched)
  if not ok then
    print "BOOM!"
  end
end

-- call this with the communication channel and how long you want the countdown to be.
-- the security in this function could be much better
function listenForSync(channel, dur)
  -- if the channel is not open, open it...
  if not modem.isOpen(channel) then
    modem.open(channel)
  end

  -- create our alarm time variable and wait for its contents
  local alarmTime = sync_drone()

  -- call the timer function.
  -- Param 1: The duration of the timer, i.e. the countdown start time
  -- Param 2: when to start the timer
  local ok = do_timer(dur, alarmTime)

  -- if the timer finished with a boom, say boom (when you have the override make sure that you return true from the do_timer function so as to not make things go boom, then in this function make sure that the missile is launched)
  if not ok then
    print "LAUNCH!"
  end
end