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bug#58158: 29.0.50; [overlay] Interval tree iteration considered harmful
From: |
Gerd Möllmann |
Subject: |
bug#58158: 29.0.50; [overlay] Interval tree iteration considered harmful |
Date: |
Sat, 01 Oct 2022 09:25:47 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/29.0.50 (darwin) |
Stefan Monnier <monnier@iro.umontreal.ca> writes:
>> Maybe you could also help me with the questions below?
>
> I'll try (BTW, the original author is Andreas Politz who we can still
> reach at <mail@andreas-politz.de>. He doesn't have much time to devote
> to it, tho, so best not to Cc him through all the conversations).
>
>> I'm assuming, from a comment somewhere, that an interval tree is an
>> rb-tree with keys being interval start positions, and allowing
>> duplicates.
>
> Yup.
>
>> That is, if N is a node, all nodes in the subtree N->left are strictly <
>> N, and nodes in N->right are >=.
>
> The following code in `interval_tree_insert`:
>
> while (child != ITREE_NULL)
> {
> parent = child;
> offset += child->offset;
> child->limit = max (child->limit, node->end - offset);
> /* This suggests that nodes in the right subtree are strictly
> greater. But this is not true due to later rotations. */
> child = node->begin <= child->begin ? child->left : child->right;
> }
>
> suggests that N->left are <= and N->right are > but my reading of the
> comment is that the only thing we can rely on is that N-<left is <= and
> N->right is >=
>
Yup. I've used overlay-tree a bit (compile with ITREE_DEBUG defined
after pulling), and used this:
(defun make-ivs ()
(with-current-buffer (get-buffer-create "*iv*")
(delete-all-overlays)
(erase-buffer)
(insert (make-string 50 ?x))
(let ((o1 (make-overlay 1 1))
(o2 (make-overlay 10 11))
(o3 (make-overlay 10 12))
(o4 (make-overlay 1 1))
)
(move-overlay o4 10 13)
(overlay-tree))))
(pp (make-ivs))
((:begin 10 :end 12 :limit 13 :offset 0 :rear-advance nil :front-advance nil)
((:begin 1 :end 1 :limit 13 :offset 0 :rear-advance nil :front-advance nil)
nil
((:begin 10 :end 13 :limit 13 :offset 0 :rear-advance nil :front-advance nil)
nil nil))
((:begin 10 :end 11 :limit 11 :offset 0 :rear-advance nil :front-advance nil)
nil nil))
That's
[10, 12]
/ \
[1, 1] [10, 11]
/ \ /\
[10, 13]
/ \
The 10 is found "all over the place".
I surmise no reasonable successor function can be written for such a
tree.
I have to look at std::multimap, they manage to do this somehow.