Re: [RFC, 2.6] a simple FIFO implementation

[Andrew Morton <akpm@osdl.org>]

Stelian Pop <stelian@popies.net> wrote:
>
> > Implementation-wise, the head and tail indices should *not* be constrained
>  > to be less than the size of the buffer.  They should be allowed to wrap all
>  > the way back to zero.  This allows you to distinguish between the
>  > completely-empty and completely-full states while using 100% of the storage.
> 
>  Do you mean 'size' (the size of alloc'ed buffer) is redundant or 'len' 
>  (the amount of data in the FIFO) is redundant ? I see how 'len' could
>  be removed (and didn't do it in the first place because I choosed
>  code simplification over a 4 bytes gain in storage), but I hardly
>  see how 'size' could be removed...

Well I'm not sure what the semantic difference is between `size' and `len'.
They're not very well-chosen identifiers, really.

My point is that there is no need to store the "number of bytes currently
in the buffer", because that is always equal to `head - tail' if you allow
those indices to freely wrap.

All the struct needs is `head', `tail' and `number_of_bytes_at_buf', all
unsigned.

add(char c)
{
	p->buf[p->head++ % p->number_of_bytes_at_buf] = c;
}

get()
{
	return p->buf[p->tail++ %  p->number_of_bytes_at_buf];
}

free_space()
{
	return p->head - p->tail;
}

pretty simple...