Re: array index and modulo

["Wesley Smith" <wesley.hoke@...>]

The data is some expensive to create userdata.  Its an array of
textures in OpenGL which I'm using as a delay line.  Thus the ring
buffer.  If I'm not mistaken, this is the queue model from PIL which I
have used in other code but doesn't quite work for thids situation.

best,
wes

On 3/29/07, Ralph Hempel <rhempel@hempeldesigngroup.com> wrote:
> Wesley Smith wrote:
> > This is a very simple problem, but I'm failing to find an elegant
> > solution.  I have an array of data that I'm treating as a circular
> > buffer.  Typically I use modulo to wrap the index to the range of the
> > buffer, but since Lua arrays start from 1 (and I want to follow this
> > convention), the modulo operator is not really useful.
>
> Can you turn the problem on its head? You're probably using
> the circular buffer because that's one way to deal with
> the problem in a language that does not do garbage collection.
>
> What about keeping head and tail "pointers" and creating each new
> item as needed and setting the used ones to nil...
>
> -- this needs to be atomic
> buffer[head] = "Some new data"
> head = head + 1
>
> -- this needs to be atomic
> data = buffer[tail]
> buffer[tail] = nil
> tail = tail + 1
>
> Is there a severe performance penalty for this? What happens
> when the index wraps around?
>
> Cheers, Ralph