monitor Smokers {
    enum { PAPER, TOBACCO, MATCHES } materials[3];
    enum { SMOKING, WAITING } state[3];
    condition self[3];

    void smoke(int i) {
        state[i] = WAITING;
        test(i);
        if(state[i] != SMOKING) self[i].wait();
    }

    void stopSmoking(int i) {
        state[i] = WAITING;
        test((i + 2) % 3);
        test((i + 1) % 3);
    }

    void test(int i) {
        //not smoking & has different materials from those on the table
        if(state[i] == WAITING &&
        materials[i] != materials[(i + 2) % 3] &&
        materials[i] != materials[(i + 1) % 3])
        {
            state[i] = SMOKING;
            self[i].signal();
        }
    }

    initialization_code() {
        for(int i = 0; i < 3; i++) {
            state[i] = WAITING;
        }
    }
}