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Re: noverlay branch
Re: noverlay branch
Mon, 26 Sep 2022 22:12:44 -0700
Stefan Monnier <email@example.com> writes:
> I just updated the noverlay branch to the code in `master` (most of it
> was mechanical except for the fact that since the last commit on that
> branch we have gotten rid of Lisp_Misc and we moved to pdumper).
This is pretty exciting!
I've been looking at the noverlay branch as well in some detail, but
only as a background task, over the past year.
I haven't considered the branch ready for merge because I saw some areas
where I think the implementation should improve. But, working code is a
compelling thing. If noverlay is ready to merge I'd suggest doing it.
Code can improve later, as and if needed.
I was planning to takle these problems before proposing a merge:
1) Improve the worst case run time of `previous-overlay-change' from
O(n) to O(log N). The noverlay branch uses an O(N) algorithm,
though it is difficult to spot. Since the point of using a tree is
O(log N) algorithms, and O(n) algorithms can easily become
exponential algorithms when composed in higher level loops (the
problem overlays sees today), this strikes me as important.
2) Look at reducing the number of malloc'd overlay blocks in half by
expressing the tree intrusively (the same way the overlay list is
today). I don't see a lot of value in itree.h/c abstracting away
the interval logic from the overlay object itself.
3) Improve quality of comments in the new code. Personally, I find the
algorithms quite subtle and quite a bit more complex than what you
find on, say, https://en.wikipedia.org/wiki/Interval_tree or the
Cormen et al. Introduction to Algorithms Book. I think I pieced
most of it together but it took a lot of effort. At top of mind is
looking at the interval_node.visited flag and figure out how that
flag is used, then describe the algorithm in detail. It isn't clear
to me how that flag gets set/cleared. Best case: doing so proves me
wrong on point (1).
And lower priority:
4) The overlay `front-advance' and `rear-advance' booleans are
conceptually part of the overlay's BEG and END positions, except
that this is ignored everhwhere except insertion. Upon insertion at
any given POS the overlays are according to *both* their BEG or END
positions and the *-advance booleans. Yet, this is not used when
ordering overlays in the tree. Doing that may bring an opportunity
to simplify code or make it more efficient. (Side note: there *may*
also be a way to encode the *-advance flags implicitly in the
beg/end fields positions if a way can be found to "steal" the free
bit in the currently signed ptrdiff_t fields, effectively causing
the *-advance flags to count as this extra "half" position for the
purpose of insertion).
5) Look at using an augmented B-tree of overlays instead of a binary
tree. B-trees are quite often faster than binary trees (at least on
hardware made over the past few decades), so there is that enticing
proposition. They're also typically not harder to implement,
either, requiring no "rotations" nor convluted deletion logic. An
*augmented* B-tree may also allow for simplification of the relative
vs. absolute offsets in the tree. The noverlay branch currently
handles this by treating the absolute offsets as cached values,
"dirtying" them whenever any mutation occurs in the tree, and
recomputing absolute offsets on demand. If augmented in the right
way a B-tree might be shallow enough in practice that recomputation
on demand is always fast enough, so the the invalidation/caching
approach (as well as the memory used to track it) can go away.
Re: noverlay branch, Gerd Möllmann, 2022/09/27