[HarfBuzz] [p-c] Perso-Arabic symbols for "year"

[Behdad Esfahbod]

On 05/28/2012 10:29 PM, Khaled Hosny wrote:
> 
>> This block:
>>
>> feature calt {
>>   sub                                              @YearSign @Digits' lookup digit2medium;
>>   sub @Digits.medium                               @YearSign @Digits' lookup digit2medium;
>>   sub @Digits.medium @Digits.medium                @YearSign @Digits' lookup digit2medium;
>>   sub @Digits.medium @Digits.medium @Digits.medium @YearSign @Digits' lookup digit2medium;
>> } calt;
>>
>> Looks completely wrong to me in that it matches the backtrack sequence in the
>> wrong order.  Ie, from what I understand, and I can be totally wrong, it
>> should have lines like this:
>>
>>   sub @YearSign @Digits.medium @Digits.medium    @Digits' lookup digit2medium;
>>
>> instead of this:
>>
>>   sub @Digits.medium @Digits.medium    @YearSign @Digits' lookup digit2medium;
> 
> AFAIK the backtrack coverage has to be in the reverse order, if I do as
> you suggest, only first digit will be matched. This have been tested
> with FireFox¹ (hb-ng), Pango (hb-old), LibreOffice (ICU) and MS Word
> (Uniscribe), so I think it is correct² :) 

Right.  That was my first guess.  I looked in Scheherazade to confirm but it
doesn't use more than one backtrack glyph for these characters so that didn't
help.  I then checked Adobe's documentation:

  https://www.adobe.com/devnet/opentype/afdko/topic_feature_file_syntax.html

It never defines how backtrack sequences are matched, but this example made me
think that it's matched in the straight order:

ignore substitute f [a e] d'
ignore substitute a d' d;
substitute [a e n] d' by d.alt;

It's the first line that suggests that it's trying to match and ignore 'fad'
and 'fed' as opposed to 'afd' and 'efd'.

Now, I know that in the OT tables backtrack is encoded in reverse order, so
this can quite possibly be a documentation issue on Adobe's side.

I don't seem to have the Adobe 'fea' parser source code to check, so I will
report this to Adobe in a separate thread and CC you.

behdad