knock-listener.cpp

bool listen(time: int) {
    // Blocking function that waits for `time` and then returns a bool representing 
    // if there was a hit within that time period
}

int main() {
    // Succeeded in hitting the correct knock
    int succeeded = false;
    
    // A knock is a sequence of durations. Each duration is
    // the time between physical hits on a detector
    int correctKnock [5] = [ 1000, 500, 500, 1000, 1500, 1000, 1000 ];

    // Human beings arent going to hit the detector
    // at precisely the expected duration so we 
    // need a margin of error within which a 
    // human being could hit the detector and still
    // count
    int marginOfError = 150;

    // The first knock has not yet occurred, so 
    // the index is -1 to indicate that we are not
    // yet within the sequence
    int indexInKnockSequence = -1;

    while (!succeeded) {
        if (indexInKnockSequence == -1) {
            // Just wait for the first hit
            while (!listen(1)) {
                // the first hit occurred, so
                // we are at the first position
                // in the knock sequence
                indexInKnockSequence = 0;                
            }
        }

        if (listen(correctKnock[indexInKnockSequence] - marginOfError)) {
            // They hit the detector, but too early, so 
            // reset the position so they can try again
            indexInKnockSequence = -1;
        }

        if (listen(marginOfError * 2)) {
            // They hit the detector within the time period, so
            // incremenet to listen for the next hit
            indexInKnockSequence++;
        } else {
            // They never hit the detector, so reset the position
            // in the sequence so we can go back to listening
            // for them to start the knock
            indexInKnockSequence = -1;
        }

        if (indexInKnockSequence == 6) {
            // They reached the end of the sequence
            // so they succeeded and we can exit this
            // while loop and hit the print() below
            succeeded = true;
        }
    }

    print("That is the correct knock!")
}