Cannot produce outlines for TrueType fonts with 65536 glyphs

[laurmaedje]

For TrueType fonts with the maximum number of glyphs, the code below early returns because the range of u16 is exceeded, leading to loca table parsing failing:

ttf-parser/src/tables/loca.rs

Lines 46 to 50 in 6e75b3c

	let actual_total = match format {
	IndexToLocationFormat::Short => data.len() / 2,
	IndexToLocationFormat::Long => data.len() / 4,
	};
	let actual_total = u16::try_from(actual_total).ok()?;

This in turn means that no outlines can be produced.

Having the maximum number of glyphs is not that uncommon in CJK fonts, e.g. Source Han Sans or Noto Serif CJK.

[laurmaedje]

In principle, this should not be that hard to fix because a usize (actual_total) is cast to a u16 just to be cast back to a usize in read_array. But I'm not sure what a clean fix would look like.

I'm also not sure what the idea behind this is:

ttf-parser/src/tables/loca.rs

Lines 30 to 37 in 6e75b3c

	// The number of ranges is `maxp.numGlyphs + 1`.
	//
	// Check for overflow first.
	let mut total = if number_of_glyphs.get() == u16::MAX {
	number_of_glyphs.get()
	} else {
	number_of_glyphs.get() + 1
	};

ttf-parser/src/tables/loca.rs

Lines 78 to 80 in 6e75b3c

	if glyph_id == u16::MAX {
	return None;
	}

At first glance, this seems like the last glyph simply cannot be outlined (which is better than no glyph being outlinable! but not ideal.)

[RazrFalcon]

Ugh... so we have to switch u16 to u32 for count, slices, etc. That's would be a lot of work.

I'm also not sure what the idea behind this is

0xFFFF + 1 would overflow. That's what this check for. Overly verbose, sure, but still.