CIS 542 Embedded Systems Programming – Summer 2013 Lecture ...

The EFSM allows us to verify some desirable properties of this system. For instance:
• it is always possible to sound the alarm (we know this is true because there is a path from each
state to the “alarm sounding” state)
• it is always possible to disarm the system (there are paths from all states to “disarmed” too,
though they require the input to be equal to the password)
These are referred to as liveness properties: desirable things that can always (eventually) happen.
Automata-Based Programming
How would you implement the above EFSM in code? If you don't think it through, this can easily turn
into an unwieldy mess of if/else statements (just look at the original specification!).
A simpler approach, known as automata-based programming, is to use enums to represent the
different possible states and signals, and then simply use switch statements to first figure out which
state you're in, and then which signal you've received, and then act accordingly.
For instance:
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enum states {disarmed, armed, alarm_sounding} state;
enum signals {arm, disarm, panic, motion, quiet};
void handle_signal (enum signals signal) {
switch (state) {
case disarmed:
switch (signal) {
case arm:
state = armed;
break;
case panic:
state = alarm_sounding;
break;
}
break;
case armed:
switch (signal) {
case panic:
state = alarm_sounding;
break;
case motion:
state = alarm_sounding;
break;
case disarm:
if (password_correct()) state = disarmed;
else state = alarm_sounding;
break;
}