[NTG-context] Count (and limit) glyphs per line?

[Max Chernoff]

> It's also a very insightful example of how to use and inject Lua code in the TeX output routine. 

This is injecting Lua code before the paragraph builder, not in the 
output routine. Something like 
https://tex.stackexchange.com/a/644613/270600 or my module 
"lua-widow-control" would be an example of Lua code in the output routine.

> Do you mind if I add it to the wiki? (Probably under "Wrapping".)

Sure

> However, tex.localhsize (or tex.dimen["localhsize"]) is 0 when the document is initialized. (Maybe a more sensible default would be textwidth rather than 0?)
> 
> So, I added:
> 
> 	local localhsize = tex.dimen["textwidth"]
> 			
> 	if tex.dimen["localhsize"] > 0 then
> 		localhsize = tex.dimen["localhsize"]
> 	end
> 
>          if chars >= max_length or width > localhsize then

I don't think that's necessary. \hsize is a primitive TeX parameter that 
sets the width of the paragraph. It may be zero at the start of the 
document, but it is definitely non-zero by the end of every paragraph.

The Lua function gets the current value of \hsize at the end of every 
paragraph, so it should be using the exact same value that TeX's 
paragraph builder uses, meaning that it should account for itemizations 
and such. I'm not really sure what \localhsize is, but it's probably 
similar to \hsize.
> (2) I'm (now?) running into trouble with hyphenation.
> 
> In my own document, I also get lines with only a single character or hboxed group. I assume, this is because the hyphen is not counted and pushes the remainder to a new line where the intended breakpoint again starts another one.

Try this:

     \startluacode
         local max_length = 112

         local glyph_id = node.id "glyph"
         local disc_id = node.id "disc"
         local glue_id = node.id "glue"

         function userdata.limiter(head)
             language.hyphenate(head)

             local hyphen = node.new "glyph"
             hyphen.char = language.prehyphenchar(0)
             hyphen.font = font.current()
             local width = hyphen.width
             node.free(hyphen)

             local chars = 0
             local n = head
             while n do
                 if n.id == glyph_id or n.id == glue_id then
                     chars = chars + 1
                     width = width + n.width - (n.shrink or 0)
                 end

                 if chars >= max_length or width > tex.hsize then
                     local back_chars = 0
                     local end_disc = nil

                     while n do
                         if n.id == glue_id then
                             local penalty = node.new "penalty"
                             penalty.penalty = -10000
                             node.insertbefore(head, n, penalty)
                             break
                         end

                         if not end_disc and n.id == disc_id then
                             end_disc = n
                         end

                         if end_disc and back_chars >= 5 then
                             end_disc.penalty = -10000
                             break
                         end

                         if n.id == glyph_id then
                             back_chars = back_chars + 1
                         end

                         n = n.prev
                     end

                     width = 0
                     chars = 0
                 end

                 n = n.next
             end

             return head
         end

         nodes.tasks.appendaction(
             "processors",
             "before",
             "userdata.limiter"
         )
     \stopluacode

I've just added the width of a hyphen to the accumulated width. Let me 
know if this works; if not, there's a more complex fix that I can try.

> Unfortunately, I don't know what to change; I know a bit about "glyph" and "glue", but what is "disc" and would it help here?

"disc" nodes are "discretionaries", which are usually potential hyphens. 
See "The TeXbook" (page 95) or "TeX by Topic" 
(https://texdoc.org/serve/texbytopic/0#subsection.19.3.1) for details on 
the TeX side, or the LuaMetaTeX manual 
(https://www.pragma-ade.com/general/manuals/luametatex.pdf#%231205) for 
details on the Lua side.

-- Max