feat: Support bold Fraktur

[ronkok]

Support bold Fraktur. Resolves #3776.

Note: KaTeX has a font for bold Fraktur, but no font metrics. That is why I left bold Fraktur out of the original PR for wide-character Unicode support. In this PR, I evade the lack of metrics by using the font metrics for regular Fraktur, not bold Fraktur.

[ronkok]

The second commit makes this PR more general in addressing the complaint of issue #3776. The results of function wideCharacterFont() are no longer used when inapplicable. That avoids all the bad error messages that would come from inapplicable use.

[codecov]

Codecov Report

Merging #3777 (29ee8da) into main (4f1d916) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #3777   +/-   ##
=======================================
  Coverage   92.98%   92.99%           
=======================================
  Files          91       91           
  Lines        6773     6779    +6     
  Branches     1575     1576    +1     
=======================================
+ Hits         6298     6304    +6     
  Misses        437      437           
  Partials       38       38           

Files	Coverage Δ
src/buildCommon.js	96.94% <100.00%> (+0.03%)	⬆️
src/symbols.js	100.00% <100.00%> (ø)
src/wide-character.js	70.00% <ø> (ø)

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[edemaine]

Sorry for the long delay in review here. Slowly getting back into things...

Overall, this seems like a good fix to bold Fraktur, so this PR should be probably be merged. However, I noticed one thing more generally about Fraktur support:

I just tested and \text{𝔄-ℨ} generates an error, because \u212D (Z), \u210C (H), and \u2128 (Z) are macro'd in src/macros.js to use \mathfrak but sometimes we want \textfrak. (Not sure why this doesn't generate an error when building the documentation. Perhaps it does and it's being silently ignored.)

Perhaps this is legacy code and can simply be removed?

KaTeX/src/macros.js

Lines 256 to 258 in 5dd9bc4

	defineMacro("\u212D", "\\mathfrak{C}"); // Fraktur
	defineMacro("\u210C", "\\mathfrak{H}");
	defineMacro("\u2128", "\\mathfrak{Z}");

[edemaine]

P.S. I also looked into generating actual metrics for the bold Fraktur font. (Part of the delay.) My attempts failed, though, so I think this should go as is; we can consider adding them later if I figure out how metrics get built.

[edemaine]

It would be nice to avoid the array creation in the negative case. How about this:

let wideFontName, wideFontClass;
if (text.charCodeAt(0) === 0xD835) {
  [wideFontName, wideFontClass] = wideCharacterFont(text, mode);
}
if (wideFontName) {

[ronkok]

I like it.

[edemaine]

Perhaps this is legacy code and can simply be removed?

@ronkok Do you agree with this assessment? If so, I can remove the code and make the other requested change.

[ronkok]

Perhaps this is legacy code and can simply be removed?

I don't think that is correct. Most Fractur characters are outside the Unicode Basic Multilingual Plane (BMP). They are addressed by the wide character function modified in this PR.

But a few Fractur characters are in the BMP. KaTeX defines them in these macros.

KaTeX/src/macros.js

Lines 247 to 258 in ad03d1e

	// Characters omitted from Unicode range 1D400–1D7FF
	defineMacro("\u212C", "\\mathscr{B}"); // script
	defineMacro("\u2130", "\\mathscr{E}");
	defineMacro("\u2131", "\\mathscr{F}");
	defineMacro("\u210B", "\\mathscr{H}");
	defineMacro("\u2110", "\\mathscr{I}");
	defineMacro("\u2112", "\\mathscr{L}");
	defineMacro("\u2133", "\\mathscr{M}");
	defineMacro("\u211B", "\\mathscr{R}");
	defineMacro("\u212D", "\\mathfrak{C}"); // Fraktur
	defineMacro("\u210C", "\\mathfrak{H}");
	defineMacro("\u2128", "\\mathfrak{Z}");

I suppose I could have instead defined them in symbols.js. That would probably be better than the macro method. It would be easier in symbols.js to define them both as a mathord and as a textord. The lack of a textord definition is causing the error.

I think a symbols.js definition of the BMP characters should be located below the loop where wide character Frakturs are defined.

KaTeX/src/symbols.js

Lines 815 to 817 in ad03d1e

	wideChar = String.fromCharCode(0xD835, 0xDD04 + i); // A-Z a-z Fractur
	defineSymbol(math, main, mathord, ch, wideChar);
	defineSymbol(text, main, textord, ch, wideChar);

Then the BMP definitions would overwrite the first definition.

[KaTeX-bot]

🎉 This PR is included in version 0.16.9 🎉

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