Re: Case mapping of sharp s

[grischka <grishka@gmx.de>]

Stefan Monnier wrote:
>> Actually I think there is something simply wrong with the simple
>> search, as it's much slower even for single chars (where bm doesn't
>> have any advantage) and additionally in some weird random fashion
>> it's again slower for backwards search, such as 14, 37, 66 ... 94
>> secs, where the bm takes 0.5 secs and simple forward constantly
>> ~3.7 secs, all for isearch'ing one character in a 100Mb file.
> 
> I can guess why it's much slower going backward: the simple search
> operates on chars rather than bytes.  The internal encoding we use
> (currently based on utf-8) is designed to be easy to parse going forward
> but not so easy going backward (IIRC our encoding is actually even a bit
> more painful in this case than pure utf-8).  BM on the other hand works
> on bytes, so there's no such slowdown.

Okay, it's utf-8 (aka multibyte I suppose) however I was using
the same buffer in both cases (just a different search string).
So whatever makes it more difficult to scan backwards the same
situation exists for bm too.  Also how can it happen that a C
function varies between 4 and 90 seconds for the same action.

> As for the general slowdown, it may also be due to having to parse the
> char (encoded in utf-8) and then look it up in the corresponding char table
> (a tree of arrays).

 From what I saw the simple_search() calls out to lisp for every
single buffer-byte whereas boyer_moore() just translates it
with a prefabricated C table.

However if simple_search just would prepare the first char of
the pattern in both lower and uppercase version readily for
comparison it could be as fast as bm I guess (for a pattern
length of less than 4 that is)

--- grischka

> 
> But maybe we're doing something silly somewhere.
> 
> 
>         Stefan
>