Litherum

(Not a language or Unicode expert, the following likely has important mistakes.) > Off the top of my head, I don't know of a terminal that actually implements the entire (very complex) set of Unicode text rendering behaviors There are at least two reasons for this: First, nobody actually seems to know how bidirectional text should interact with terminal control sequences, or indeed how it should be typeset on a terminal in the first place (what is the primary direction? where are the reordering boundaries?). There is the pre-Unicode bidirectional support mode (BDSM, I kid you not) in ECMA-48[1] and TR/53[2], which AFAIK nobody implements nor cares about; there are terminal emulators endorsed by bidi-language users[3], which AFAIK nobody has written down the behaviour of; there is the Freedesktop bidi terminal spec[4], which is a draft and AFAIK nobody implements yet either but at least some people care about; finally, there are bidi-language users who say that spec is a mistake[5]. Second, aside from bidi and a smattering of other things such as emoji, there is no detailed “Unicode text rendering behaviour”, only standards specific to font formats—the most recent among them being OpenType, which is dubiously compatible across implementations, decently documented only through painstaking reverse engineering (sometimes in words[6], sometimes only in Freetype library code), and generally full of snakes[7]. And it has no notion of a monospace font—only of a (proportional)…

Given that the OpenType font format is Turing-complete [0], I would challenge the “without code” claim. ;) [0] https://litherum.blogspot.com/2019/03/addition-font.html

Hinting might be a red herring here: these font formats are unsafe almost by design, since they consistently rely on a pattern of reading a value out of a chunk of memory and then treating that value as a pointer offset where additional values are read. The proposed WASM shaping implementation would be a replacement for traditional OpenType layout[1] which is already turing complete[2]), and in this sense using WASM is significantly safer than the alternative. Your comment suggests that "execution of arbitrary WASM code" is something that we should be scared of, but the entire point of WASM is that it allows for safe/isolated compilation of arbitrary code. [1] https://simoncozens.github.io/fonts-and-layout/features.html [2] https://litherum.blogspot.com/2019/03/addition-font.html

OpenType is basically turing-complete via glyph shaping. There's an explanation here: https://litherum.blogspot.com/2019/03/addition-font.html

https://litherum.blogspot.com/2019/03/addition-font.html "So this is pretty cool, but it turns out that the contextual substitution lookup type is really powerful. This is because the table that it references can be itself, which means it can be recursive." […] "So thats a pretty powerful virtual machine. I think the above is sufficient to prove Turing complete-ness."

You may not have an explicit rand(), but with the ligatures & substitution rules, you can add so much context sensitivity that no one will ever spot any duplications. That's how you can do things like https://litherum.blogspot.com/2019/03/addition-font.html https://pagedout.institute/download/PagedOut_002_beta2.pdf#page=42 https://aftertheflood.com/journal/the-worlds-first-code-free-sparkline… https://www.coderelay.io/fontemon.html (most of these will work in a browser). I've also suggested that you can create 'prank fonts' which add in subtle typos sporadically. Less evilly, this is what calligraphy handwriting fonts do to get convincing variation.

I think that is thanks to the Knuth-Plass algorithm, with Knuth being the inventor of Tex, which powers Latex. The problem seems not trivial indeed. http://www.eprg.org/G53DOC/pdfs/knuth-plass-breaking.pdf https://tex.stackexchange.com/questions/230668/any-progress-on-knuth-p… http://litherum.blogspot.com/2015/07/knuth-plass-line-breaking-algorit…

Yes: https://litherum.blogspot.com/2019/03/addition-font.html

Fonts are not like photos, they have hinting and kerning and conditional code that distinguishes a high quality font from crap. The glyph substitution table in OpenType fonts is of such complexity that it's been shown to be Turing complete. [https://litherum.blogspot.com/2019/03/addition-font.html]

I suspect the opposite situation is what you run into: it is possible to write programs that are valid and out to be accepted by the type system, but get rejected because you blew the stack of the type checker. Had the type checker been allocated more memory / time, it could have accepted the program, but it crashed before that. This reminds me of that: http://litherum.blogspot.com/2019/03/addition-font.html You can craft font files that perform arbitrary calculations… if you give the font shaping engine a (practically) unlimited stack, where it usually limits recursion to 6 levels. If you do so, you can make a font that does math (or whatever you want) as part of computing ligatures. But in practice, you hardly ever hit those kinds of limits unless you're trying to do something silly on purpose. Sure you cannot port Doom to font substitution lookup tables, despite them being Turing complete. But that doesn't make your font shaping engine useless, or even broken.

I really enjoyed this author's "Complex Text Handling in WebKit" series from a few years back. They're spread over several months on the blog, so here are the direct links to each part, if anyone is interested: Part 1: Encoding Systems, Code Points, and Code Units <https://litherum.blogspot.com/2013/11/complex-text-handling-in-webkit-… Part 2: Fonts <https://litherum.blogspot.com/2013/11/complex-text-handling-in-webkit-… Part 3: Codepoint to Glyph Mapping <https://litherum.blogspot.com/2014/02/complex-text-handling-in-webkit-… Part 4: Line Breaking <https://litherum.blogspot.com/2014/04/complex-text-handling-in-webkit-… Part 5: Bidirectional Processing <https://litherum.blogspot.com/2014/11/complex-text-handling-in-webkit-… Part 6: Run Layout <https://litherum.blogspot.com/2014/11/complex-text-handling-in-webkit-… Part 7: Width Calculations <https://litherum.blogspot.com/2014/11/complex-text-handling-in-webkit-…