bug#54914: 28.1.50; Unable to form Brahmi conjuncts in Emacs

[Robert Pluim]

>>>>> On Wed, 13 Apr 2022 19:03:36 +0300, Eli Zaretskii <eliz@gnu.org> said:

    >> From: Sameer Singh <lumarzeli30@gmail.com>
    >> Date: Wed, 13 Apr 2022 16:25:41 +0530
    >> 
    >> Brahmi just like any other Indic script also has the ability to form 
conjuncts, but despite using "Segoe UI
    >> Historic" font which supports Brahmi Conjuncts, Emacs is not able to 
display them.

    Eli> This is a missing feature: we don't have composition rules for the
    Eli> Brahmi script, and more generally don't have any support for that
    Eli> script.  Patches to add that are welcome.

A brute-force example of what Eli's talking about would be:

(set-char-table-range composition-function-table
                      '(#x11000 . #x1107F)
                      (list (vector "[\x11000-\x1107F]+" 0 
#'font-shape-gstring)))

which essentially means "hand off any sequence of Brahmi codepoints to
harfbuzz", which might end up being slow. From your example, youʼre
requesting more something like "consonant, virami, consonant", which
is also possible, but we'll need your guidance to tell us when the
results are acceptable. (If anyone is interested, thereʼs section 14.1
of the Unicode 14 standard that describes how they think Brahmi should
be handled, but Iʼve not tried to understand it yet).

Robert
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