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bug#20365: 24.5; all-completions returns duplicates for Info-read-node-n

From: Stefan Monnier
Subject: bug#20365: 24.5; all-completions returns duplicates for Info-read-node-name-1
Date: Sun, 19 Apr 2015 22:29:00 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.0.50 (gnu/linux)

> That would be fine. It just makes sense for any element of what
> `all-completions' returns to be a valid answer.

What's a valid answer?

all-completions will happily return "share/" when completing on
"/usr/s", even if you're looking for a .tex file rather than a directory
and even if there's no "share/" in the current directory either.

Yet, we don't want all-completions to return elements of the form
"/home/monnier/private/package/emacs/trunk/lisp/" since that would
result in a lot of redundancy that then needs to be found&removed.

So, no, all-completions just doesn't always return "valid answers".
Instead, it returns chunks of text that can added to some prefix (which
you can find via `completion-boundaries'), the result of which should be
a prefix of a valid answer.

> About the duplicate entries, I think it should be the responsibility
> of the caller to remove the duplicates.

That's the case currently.  The completion-table is called and the
caller is the UI, and currently it's the UI's responsibility to remove
the duplicates.

> Here's my line of thought: a completion function is expected to have
> an O(N) complexity, where N is the amount of candidates.  Removing
> duplicates is O(N^2) at worst, and O(NlogN) at best.

Actually, with a hash-table it's pretty much down to O(N).


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