Gsub lookup 71 writing #563
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Connum
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help wanted
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Description
THIS IS A DRAFT ONLY, SEE BELOW!
Motivation and Context
Based on #486 adding read support, I thought I could implement writing support for LookupType 7 (Extension Substitution).
However it seems I'm way out of my comfort zone and stuck on finding the right approach to this.
What I've done so far is adding a subtableMaker for ExtensionSubstFormat1 tables and returning the extension as a new table to append after all the other subtables.
However, I'm not sure if the extensions are appended in the correct location (the spec is not clear on where they are supposed to be located - after the LookupList? Or at the end of the GSUB table? -, or at least I don't understand it) and how to replace the offset value in the corresponding extension substitution table afterwards. I tried to do it similar to the subsitution table offsets, but I wasn't able to figure it out and gave up.
Maybe this is still a good starting point for someone more clever than me to implement this.