This is a distributed semaphore implementation for my Concurrency course at
UNO.

Basic Algorithm:
type kind = enum(reqP, reqV, VOP, POP, ACK);
chan semop[n](int sender; kind k; int timestamp);
chan go[n](int timestamp);

process User[i = 0 to n-1] {
    int lc = 0, ts;
    ...
    # ask my helper to do V(s)
    send semop[i](i, reqV, lc); lc = lc+1;
    ...
    # ask my helper to do P(s), then wait for permission
    send semop[i](i, reqP, lc); lc = lc+1;
    receive go[i](ts); lc = max(lc, ts+1); lc = lc+1;
}


process Helper[i = 0 to n-1] {
    queue mq = new queue(int, kind, int); # message queue
    int lc = 0, s = 0; # logical clock and semaphore
    int sender, ts; kind k; # values in received messages
    while (true) { # loop invariant DSEM
        receive semop[i](sender, k, ts);
        lc = max(lc, ts+1); lc = lc+1;
        if (k == reqP) 
            { broadcast semop(i, POP, lc); lc = lc+1; } 
        else if (k == reqV) 
            { broadcast semop(i, VOP, lc); lc = lc+1; }
        else if (k == POP or k == VOP) {
            insert (sender, k, ts) at appropriate place in mq;
            broadcast semop(i, ACK, lc); lc = lc+1; 
        }
        else { # k == ACK
            record that another ACK has been seen;
            for (all fully acknowledged VOP messages in mq)
                { remove the message from mq; s = s+1; }
            for (all fully acknowledged POP messages in mq st s > 0) { 
                remove the message from mq; s = s-1;
                if (sender == i) # my user’s P request 
                    { send go[i](lc); lc = lc+1; }
            }
        }
    }
}