bug#26629: 25.1.50; Feature-request: Structure-preserving copying of sequences

[Lars Ingebrigtsen]

Tobias Zawada <i@tn-home.de> writes:

> The function copy-tree copies recursively but not structure preserving.
> If two cars or cdrs link to the same sub-list in the original data
> they point to two separate sub-lists in the copy.


[...]

> About the code: Traversing of the original graph and construction of the new 
> graph are
> stack-based as it is standard. There is an additional hash-map that maps
> the already traversed old nodes to the newly created ones. If some old
> node is already in the hash-map we do not create a new one in the copied
> graph but use the mapped one from the hash map.

(I'm going through old bug reports that unfortunately weren't resolved
at the time.)

I haven't looked closely at the code, but this is functionality that I
vaguely remember having been requested before, and I don't think Emacs
has it yet?  (But I may be wrong about that.)

I've added Stefan to the CCs; perhaps he has some comments.

This change is large enough to require a copyright assignment of the
code to the FSF -- if we decide to add the code, would you be willing to
sign such paperwork?

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no