Re: Character group folding in searches

[Juri Linkov <juri@linkov.net>]

> My question is:
>
> Do any of these options seem good enough? Which would you all like to explore?

This feature, as I see it, has several levels of complexity:

* 1-to-1 char-folding

  ?a <=> ?á

  This is already supported by char-tables, so there is no problem.

* 1-to-1 char-folding in combination with case-folding

  ?a <=> ?Á   (in this example one of them is in lower case
               and another in upper case with acute)

  I'm not sure how your patch handles this case.  We have to consult the
  information about case-folding from the case-table.  Otherwise, we would
  need two new tables instead of one: where character mappings are
  with and without case-folding.  In any case we have to take care of
  the correct interaction with case-fold-search.

* 1-to-1 char-folding plus a combining character

  ?a <=> "á"

  The simplest solution is just to ignore all combining characters in search.
  This should be easy to implement in the search engine by introducing
  a new list of ignorable characters to skip during the search.

* multi-character translation such as ligatures, etc.

  ?ffl <=> "ffl"

  This is the hardest case.  Maybe the existing translation tables
  from ucs-normalize.el could help.  Then configuring would be like

  (set-char-table-extra-slot case-table 3 (get 'ucs-normalize-nfd-table 'translation-table))

  But this requires a significant modification of the search engine to use
  the same logic in the search as is used in `translate-region-internal'
  to support multi-character translation in the search.

  Also it might require adding a new mode such as "lax-decomposition"
  that like lax-word mode will match partially, e.g. "f" will match "ffl".

  Or maybe better to use some external libraries like
  http://userguide.icu-project.org/collation/icu-string-search-service

I agree with your attempts to have something instead of having nothing
(as in an all-or-nothing attitude).  So to me it seems that the first
3 items would comprise the useful minimum, and the hardest last case
could be implemented afterwards.