[fpc-pascal] How to implement a circular buffer object in pascal?

[Martin Frb]

On 03/09/2020 16:36, Bo Berglund via fpc-pascal wrote:
> A "simplest case" usage example would be nice to have. 
True. Once you got enough info from this thread, please write one.

> I still when looking at the sources have no clue as to how it would be
> used...
>
> It seems to me like at the very least one should define what type of
> data will be stored in the buffer...
> Is this how you use it?
>
> var
>    MyFifoBuffer: TLazThreadedQueue;  77Whgat do I do about the <T>???

Ok, I see. It's a generic, so this question is not directly related to 
the TLazThreadedQueue

type
   TMyData = class
       FData: array of byte;
    end;
   TMyThreadedQueue = specialize TLazThreadedQueue<TMyData>;

var
   a: TMyData
   q: TMyThreadedQueue

q := TMyThreadedQueue.create($8000);
a := TMyData.Create;
q.PushItem(a);


Of course you can do
   TMyData = array of byte;  // or record .... end if you want
   TMyThreadedQueue = specialize TLazThreadedQueue<TMyData>;

or even
   TMyThreadedQueue = specialize TLazThreadedQueue<Byte>;

Pushing arrays (dyn ar stat) is fine.

Pushing individual bytes => works. And if you get a byte once or twice 
in a second then it is ok.

If you get bytes every few milliseconds or faster, then pushing bytes 
may be too expensive (I don't know, but I would expect at some point....).
In order to push (or pop) the queue has to obtain a lock. You have to 
make sure your computer can handle that fast enough. (maybe it can....)



>
> And how do you use ShutDown, DoShutDown, QueSize and Grow?
>
> Very confused.

Grow gives a positive/negative delta. It is added to the current size.
But as I said, for this to work you need the patches.
Also "grow" on a live queue, requires to get the lock yourself. This is 
not automated. (Might be an idea to add that....)

- If the queue is full then PushItem will wait until timeout, or until 
something is popped.
- if the queue is empty PopItem will wait in that manner.
If threads are waiting, and ShutDown is called, then Push/PopItem return 
wrAbondoned. (So you need to check for this)