[Cst-1b] CST1-b Question: Java 98/4/3 (solution)

[Martin Harper]

"M.Y.W.Y.B." wrote:

> CST 98/4/3
> http://www.cl.cam.ac.uk/tripos/y1998p4q3.pdf
>
> Hi!
>
> Has anyone solved the last part of the question in 98/4/3?
> "Show how a synchronised buffer holding a single value could be implemented
> using this new scheme".
>
> I assume that you are not allowed to use "synchronized", "wait()",
> "notify()" etc but only TicketMachine and Scheduler methods. But how?
>
> Thanks
>
> Moritz

Previous response was sent to personal email by accident - but the gist of it
was
1) you can create critical regions (from the question)
2) hence you can create semaphores (from notes)
3) hence you can build a shared 1-slot buffer (from notes)

This is something of a clumsy answer, however - and I'm not sure if it would
lose marks for that. Something better can be done.

class Buffer {
    // value of the buffer
    int val;

    // queue of people who want to read from the buffer
    TicketMachine rm = ..
    Scheduler rs = ...

    // queue of people who want to write to the buffer
    TicketMachine wm = ...
    Scheduler ws = ...

    void store( int i ) {
        int writernum = wm.turn();        // we have to assume that this is an
atomic operation...
        ws.queue(writernum);            // queue up to write.
        val = i;
        rs.next();            // please would the next reader move along to be
served...
    }

    int read() {
        int readernum = rm.turn();
        rs.queue(readernum);
        int temp = val;
        rs.next();
        return temp;
    }
}

Martin.