The grammar in the OT spec, and the existing Windows implementation
seem to be confused around where to allow Asat around the medial
consonants.
The previous grammar for medial group was allowing an Asat after
the medial group only if there was a medial Wa or Ha, but not if
there was only a medial Ya. This doesn't make sense to me and
sounds reversed, as both medial Wa and Ha are below marks while
Asat is an above mark. An Asat can come before the medial group
already (in fact, multiple ones can. Why?!). The medial Ya
however is a spacing mark and according to Roozbeh it's valid
to want an Asat on the medial Ya instead of the base, so it looks
to me like we want to allow an Asat after the medial group if
there *was* a Ya but not if there wasn't any. Not wanting to
produce dotted-circle where Windows is not, this commit changes
the grammar to allow one Asat after the medial group no matter
what comes in the group.
Test: U+1002,103A,103B vs U+1002,103B,103A
The spec and Uniscribe don't allow these, but UTN#11
specifically says the sequence U+104B,U+1038 is valid.
As such, allow all "P V" sequences. There's about
eight sequences that match that structure, but Roozbeh
thinks it's fine to allow all of them.
Test case: U+104B, U+1038
https://bugs.freedesktop.org/show_bug.cgi?id=71947
This is broken sequence according to OpenType spec, Uniscribe,
and current HarfBuzz implementation. But Roozbeh says this
is a valid sequence, so allow it. There are multiple
"(DB As?)?" constructs in the grammar, but Roozbeh thinks only
this one needs changing.
Test case: 1014,1063,103A
Fixes https://bugs.freedesktop.org/show_bug.cgi?id=71949
Ebrahim Byagowi
David Corbett
Behdad Esfahbod