Max Chernoff mseven at telus.net
Fri Jul 8 09:40:33 CEST 2022

> This is because you need to pass a list that conforms to what the
> builder expects and the callback that you use doesn't do that for you
> (after all, it also gets hbox content).

Isn't "processors/after" the same as "pre_linebreak_filter"? I thought
that only "hpack_filter" gets \hbox content? Anyways, my actual function
has this guard at the very start:

if (head.id ~= par_id and context) or -- Ensure that we were
actually given a par
status.output_active or -- Don't run during the output routine
tex.nest.ptr > 1 -- Don't run inside boxes
then
end

so I think that I'm only processing actual top-level paragraphs here.

> There is no 'callback sequence handler' for the par builder (currently i
> see no need for it, also given the extra overhead involved) but this
> what what you can do:
>
> \startluacode
>          local result, info = tex.linebreak(head)
>          tex.prevdepth = info.prevdepth
>          tex.prevgraf = info.prevgraf
>          return result
>      end
>          local result, info = tex.linebreak(nodes.nuts.copylist(head))
>          inspect(info)
>          return true
>      end
> \stopluacode
>
> \defineparbuilder[preroll_a]
> \defineparbuilder[preroll_b]
>
> \starttext
>      \setmainparbuilder[default]   \input tufte \par \input tufte \page
>      \setmainparbuilder[preroll_a] \input tufte \par \input tufte \page
>      \setmainparbuilder[preroll_b] \input tufte \par \input tufte \page
> \stoptext

I think that that code is for replacing the linebreaker entirely, like
with using "linebreak_filter"? My goal isn't to replace the linebreaker;
I just want to be able to inspect the paragraph before it is broken,
without modifying anything.

What I'm trying to do *very* roughly looks like the following:

paragraphs = {}
attribute = 1234

end

node.freelist(nat_node)

local long_node, long_info = tex.linebreak(
)

if long_info.prevgraf == nat_info.prevgraf + 1 then
table.insert(paragraphs, long_node)
end

end

end

if tex.outputpenalty ~= tex.widowpenalty then
end

-- Pick a paragraph from paragraphs somehow

-- Replace that paragraph on the page with the one
-- from paragraphs

-- Move the last line of the page onto the top of

end

(The full implementation is in the module "lua-widow-control" on CTAN,
TeX Live, and modules.contextgarden.net, or directly at
"https://github.com/gucci-on-fleek/lua-widow-control/blob/master/source/lua-widow-control.lua".
It's pretty long though, so I'm just trying to summarize here.)

This works pretty well with Plain LuaTeX, LuaLaTeX, OpTeX, MkIV, and
MkXL before the latest upload, but something broke with the latest
upload in MkXL. I understand that I'm mucking around with volatile
interfaces, and I have no problem making a bunch of changes whenever the
engine/format changes; the problem is that I'm not too sure what changed
in the engine, so I don't know what I need to change in my code.

> On the to do is a to add a 'prepare' helper that adds
> the mandate nodes (par fillers etc)

Maybe that's all that I need? If that's the case, I have no problem
coding my own "prepare helper" if you think that it'll be awhile before
you get around to it; the problem is that I'm not entirely sure what
nodes I would need to add. These new nodes aren't added until after
"pre_linebreak_filter", and they're gone after the linebreaker runs, so
I can't inspect a "regular" paragraph to see where these nodes belong.

> but even then one has to be careful
> where linebreak hackery is applied.

Oh yes, I'm well aware :) Earlier versions of my module would silently
eat entire paragraphs, which was very not good. Luckily that problem is
fixed now.

And finally, thanks for all your work with LuaMetaTeX. This new engine
is quite nice to work with and has some pretty cool new features.

Thanks,
-- Max