bug#33729: 27.0.50; Partial glyphs not rendered for Gujarati with Harfbuzz enabled (renders fine using m17n)

[Khaled Hosny]

On Sat, Dec 22, 2018 at 10:54:48AM +0200, Khaled Hosny wrote:
> On Mon, Dec 17, 2018 at 05:55:52PM +0200, Eli Zaretskii wrote:
> > > From: Glenn Morris <rgm@gnu.org>
> > > Cc: far.nasiri.m@gmail.com,  dr.khaled.hosny@gmail.com,  
> > > behdad@behdad.org,  33729@debbugs.gnu.org,  kaushal.modi@gmail.com
> > > Date: Sun, 16 Dec 2018 19:30:00 -0500
> > > 
> > > > After some thinking, my conclusion is that we should import the
> > > > ISO 15924 database from https://unicode.org/iso15924/, use a script
> > > > similar to admin/unidata/blocks.awk to generate an alist from it that
> > > > maps Emacs script names to ISO 15924 tags, and then access that alist
> > > > from uni_script to get the correct script information to Harfbuzz.
> > > >
> > > > Patches implementing that are welcome.
> > > 
> > > I live to write awk scripts. I'm not 100% sure what you want, but as a
> > > first example, the following takes
> > > http://www.unicode.org/Public/UCD/latest/ucd/PropertyValueAliases.txt
> > > as input and outputs lines of the form "(gujr . gujarati)".
> > > 
> > > The aliases are so that the RHS matches charscript.el.
> > > 
> > > If this is not right, please clarify exactly what the inputs and output
> > > should be.
> > 
> > Thanks.
> > 
> > It turns out I didn't have this figured out completely, and your
> > proposal forced me to dig some more into the relevant parts of Unicode
> > and Emacs.  I found a few additional issues and considerations; for at
> > least some of them I'd like to hear the opinions of the Harfbuzz
> > developers.
> > 
> > Here are the issues:
> > 
> >  . Contrary to my original thoughts, I now tend to think that a
> >    separate char-table, say char-iso159240tag-table, that maps
> >    character codepoints directly to the script tags, is a better
> >    solution:
> >     - it will allow a faster look up, obviously
> >     - the subdivision of characters into scripts, as shown in
> >       Unicode's Scripts.txt, is slightly different from what
> >       char-script-table does, so a simple mapping from Emacs scripts
> >       to ISO 15924 script tag will not do.  For example, many
> >       characters Emacs puts into 'latin' or 'symbol' scripts are in
> >       the Common script according to Scripts.txt, and similarly for
> >       the Inherited script.  I imagine this is important for
> >       Harfbuzz.
> 
> Alternatively, we could just use HarfBuzz’s own built in ucdn-based
> Unicode function for this. The only reason for overriding this in Emacs
> was to keep HarfBuzz and Emacs Unicode support in sync, but if we are
> going to duplicate the Unicode script data then better use what HarfBuzz
> has.
> 
> I’m going to try this now.

I pushed a commit to harfbuzz branch that I think fixes this issue now.

Regards,
Khaled