Find the line number inside a vbox (at end of paragraph), and change the behavior of a macro that follows next

Question

Edit: I have added a luatex solution as an answer to my question.

I have a \vbox in which I plan to typeset long material (long poetry), the way it is typeset is slightly unique. There is a special highlight macro that puts 3 red colored asterisks at the end of some special stanzas (which end with \par). Though if the end of a special stanza (each stanza can be different number of lines) falls at a multiple of some number (lets say 100) from the top of the poetry then we do something different (like add extra line break, or some other marker like a colored rule) instead of putting these asterisks.

The way I imagine this could work is if I can find, and pass the line number of the last line of previous paragraph to my macro that prints special marker. I have not come across any macro to know the line number within the text about to be typeset. From my fringe understanding of tex, given tex engine puts lines of paragraph on the vertical list at the end of processing a paragraph, it seems like there would be a way to know this at the beginning of next paragraph? Ideally I would like plain tex solution. If that is impossible to achieve in plain tex, then luatex solution based on line-break-filter would be great (assuming from the name it won't re-typeset the entire \vbox again?)

Corollary-1: I guess in plain tex it is not possible to get line number of the line within the paragraph while the paragraph is being typeset, though is it possible using luatex's line break filter? If it is possible in luatex, then for luatex solution it would be nice to just have a macro that works on any line, not just to know the line number of the last line of previous paragraph.

% Macro that returns line number from top of vbox
\currentlineno

% Macro that consumes this line number
\myspecialasterisk{\currentlineno} % I can write this, am looking for the definition of \currentlineno

Answer 1

I don't say that it is full solution of your question, but you can start with experimenting wit this code:

\newcount\globlines

\hsize=10cm
\def\text{fghs sdsdvjd gdj dhad dhd fda dfgdfd dhd dfshk sdhsds
fghs sdsdvjd gdj dhad dhd fda dfgdfd dhd dfshk sdhsds
fghs sdsdvjd gdj dhad dhd fda dfgdfd dhd dfshk sdhsds}

\def\par{\ifhmode\endgraf \global\advance\globlines by\prevgraf \fi}

\def\showlines{\vskip-\baselineskip \llap{\the\globlines\qquad}}

\text \par \text \par
\showlines
\text \par \text \par \text\text \par
\showlines

\bye

Answer 2

Here's my luatex solution that seems to work in all of my testing.

% Post line break filter to find number of lines from a startpoint
\directlua{
  % Global linecount, usage:
  %     Set linecount to a start value at whatever place you want to start counting lines from (like beginning of \vbox):
  %         \directlua{linecount=0}
  %     Get value within luatexcode (linecount valid only after \par, gets updated upon every new \par, till then it holds previous value)
  %         \directlua{tex.sprint(linecount)}
  linecount=0 % global variable (change name if you think there can be conflict with other lualatex packages)
    function my_post_lb_filter(head,groupcode)
      local HLIST = node.id("hlist") % node.id for a line of text in vertical list
      for n in node.traverse(head) do % For every subnode within node head
        if n.id==HLIST then % If n is a line, increment linecount
          linecount=linecount+1
        end
      end
      return head
    end
  luatexbase.add_to_callback('post_linebreak_filter', my_post_lb_filter, 'Increment a count at eol to track number of lines')
}

% Set start value to 0 at beginning 
\directlua{linecount=0}

% Required macro that works when used right after \par
\newcommand{\currentlineno}{\directlua{tex.sprint(linecount)}}

Note: If you are a luatex expert, please leave a code review comment.