Re: [container] Contributing 'stationary_vector' to Boost

From: Mehrdad Niknami (mniknami_at_[hidden])
Date: 2021-01-05 19:52:04

I don't think it's possible without hurting the efficiency.

Each block is represented by an (index, pointer) pair. Pushing to the front
would require re-adjusting the indices in every bucket on every push to the
front, and that would be very slow.
On the other hand if I change the representation to, say, a (begin, end)
pair, it would have repercussions. For example, subtracting iterators would
take logarithmic time instead of constant time, as they no longer have
global bucket indices to subtract.

There are also possibly less significant but still nontrivial trade-offs I
can think of. For example, worst-case space usage would become O(infinity)
(like vector) instead of the current ~2n space bound (which is even
slightly worse than a typical deque's n + n/16 or similar in the worst
case). Furthermore, random-indexing could no longer be done in O(log log n)
time. (Note: this last part is something I haven't yet implemented for
stationary_vector, but it's trivial to implement right now due to the
global index maintained for each block. This would no longer be possible
without that information.)

I think there are likely more trade-offs as well, but for ones I could
think of were more than sufficient to rule it out as a drop-in replacement
for any existing container in STL or Boost; that's why I ended up
implementing a new container.

Mehrdad

On Tue, Jan 5, 2021 at 11:12 AM Jeff Hajewski via Boost <
boost_at_[hidden]> wrote:

> On Tue, Jan 5, 2021 at 12:53 PM Mehrdad Niknami via Boost <
> boost_at_[hidden]> wrote:
>
> > Not quite - my container is single-ended.
> >
> I assume this is because you grow your block size as you extend the vector.
> Without looking at the implementation of deque, could you not use your
> approach when appending on the left side as well?
> Both sides of the deque would allocate geometrically increasing block sizes
> independently, which would maintain algorithmic complexity.
>
> Jeff
>
> Mehrdad
> >
> > On Tue, Jan 5, 2021 at 9:23 AM Hans Dembinski <hans.dembinski_at_[hidden]>
> > wrote:
> >
> > >
> > > > On 4. Jan 2021, at 19:45, Mehrdad Niknami <mniknami_at_[hidden]>
> > wrote:
> > > >
> > > > It's similar, but not quite. std::deque uses fixed-size blocks and
> > tends
> > > to be slow in my experience (at least in some implementations).
> > > > stationary_vector on the other hand uses variably-sized blocks (with
> > > geometrically increasing sizes).
> > > > Its capacity at least doubles every round; in fact, it reduces to a
> > > single array when the entire size is reserved beforehand.
> > > > This allows it to perform much more competitively with (and similarly
> > > to) std::vector.
> > > > It may be what std::deque should have been, but isn't currently.
> > >
> > > Ok, that sounds like your container offers the same basic guarantees
> as a
> > > std::deque with a more efficient implementation. If it is indeed more
> > > efficient due to the use of variable-sized blocks than
> > > boost::container::deque, then could you include your improvements into
> > > boost::container::deque?
> > >
> > >
> > >
> >
> https://github.com/boostorg/container/blob/develop/include/boost/container/deque.hpp
> >
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