bug#48841: fido-mode is slower than ido-mode with similar settings

[João Távora]

Dmitry Gutov <dgutov@yandex.ru> writes:

> On 14.06.2021 03:16, João Távora wrote:
>>> Perhaps predicate it on the value of icomplete-hide-common-prefix instead?
>>>
>>> fido-mode sets it to nil, and this way we retain a better level of
>>> abstraction, and better backward compatibility for vanilla
>>> icomplete-mode users.
>> This is a good idea, the level of abstraction.  But what is this
>> "common prefix" anyway?  Is it the the same as the "determ"
>> thing,  or the "[mplete...] dance" as I called it earlier.  Shouldn't
>> fido-mode then_hide_  it?
>> I'm confused, but if you're not, go ahead and make that more
>> abstract change instead of relying on fido-mode.
>
> So... it's a bit more complex than that.

Yes, my batch broke the things you mentioned.

> It seems there are two ways to proceed from here:
>
> - Just alter the printing logic in the "single match" case to print
>   the match text in full is it's not equal to the input string. I
>   haven't puzzled out the logic doing that yet.
>   
> - Try to keep the current behavior while avoiding the duplicate work.

Both sound absolutely fine to me.

> About the latter option: the result of that most-try stuff is only
> useful when there is only one match, right?

No idea, but may be.

> Unless I'm missing something and the value does see some use in the
> multiple-matches situations, the patch below both keeps the current
> behavior and gives the same performance improvement:

That'd be fantastic, but I doubt you'd be keeping the exact same
behaviour.  I never understood it -- that's the thing here -- but I
think that completion-try-completion is doing more stuff when multiple
candidates matched by a pattern happen to share the same prefix or
suffix or something like that.  I might be completely wrong, tho.

But really if you make this patch conditional to fido-mode or that other
var that you think is more abstract, I think it's fine and it's a very
clear win. I really doubt that the tiny number of fido-mode users care
about that behaviour anyway, but I'm sure they'll appreciate the
considerable speedup.

João