import time

# create a dictionary 
state = {
        'start': time.time(),
        'countdown': 5,
        'clock': 5,
        'ok': True,
        'boosters': 'off',
        }


def poll(state):
    '''
    this would probably just check the clock on the audrino
    but for now we mock it up using time.time
    '''

    running = time.time() - state['start']
    state['clock'] = int(state['countdown'] - running)

# now define a bunch of functions to be called at various times on the countdown
# each function gets passed the state dictionary and can modify that dictionary
# (sicne dictionaries are mutable) to pass information to other parts of the
# code

def do_systems_check(state):
    if state['ok']:
        print('all systems go')

def do_boosters(state):
    if state['ok']:
        state['boosters'] = 'on'
        print('boosters on')

def do_lift_off(state):
    if state['ok'] and state['boosters'] == 'on':
        print('lift off')
        
# now we put all of those do_ functions into a dictionary, with keys that
# identify when they should be called, thus:  at T-4 we do a systems check, at 


EVENTS = {
        4:do_systems_check,
        2:do_boosters,
        0:do_lift_off,
        }

last_clock = None # to prevent duplicates
while state['clock']>0:
    poll(state)
    clock = state['clock']
    if clock == last_clock:
        # the clock has not meaningfully changed, so just sleep and wait for
        # poll to return something interesting
        # the logic will need to be extended to allow for other events to
        # wake-up the logic, other than just the clock rolling over
        # also, consider doing away with the sleep, as it will cause a bit of a
        # lag

        time.sleep(.1)
        continue
    last_clock = clock

    print('T minus {}'.format(state['clock']))

    # now check the events dictionary to see if we trigger anything

    if clock in EVENTS:
        func = EVENTS[clock]
        func(state)