How to go through a vbox line by line in Lua?

How to go through a vbox line by line in Lua?

[Henri Menke]

Dear list,

Recently I answered a question on TeX.SX where it was necessary to go
through an already typeset vbox and examine it line by line.  Therefore
I used the macro \eatlines from Viktor Ei­jkhout's "TeX by Topic", here
a version tailored to my needs:

\def\eatlines{%
  \setbox\scratchbox=\lastbox
  \ifvoid\scratchbox\else
    \unskip\unpenalty
    {\eatlines}%
    \setbox\scratchbox=\hbox{\unhbox\scratchbox}%
    % Do something with scratchbox, e.g. examine width
  \fi
}

It is then used like

\vbox{ ... some content ... \par\eatlines}

The macro goes through all hboxes (lines) in the vertical list until
there is none left (\ifvoid) and unpacks each hbox into a new one.

I was wondering, can the same thing be done in Lua (with probably less
and more readable code)?

Kind regards,
Henri
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Re: How to go through a vbox line by line in Lua?

[Hans Hagen]

On 4/26/2017 11:10 PM, Henri Menke wrote:

> Recently I answered a question on TeX.SX where it was necessary to go
> through an already typeset vbox and examine it line by line.  Therefore
> I used the macro \eatlines from Viktor Ei­jkhout's "TeX by Topic", here
> a version tailored to my needs:
>
> \def\eatlines{%
>   \setbox\scratchbox=\lastbox
>   \ifvoid\scratchbox\else
>     \unskip\unpenalty
>     {\eatlines}%
>     \setbox\scratchbox=\hbox{\unhbox\scratchbox}%
>     % Do something with scratchbox, e.g. examine width
>   \fi
> }
>
> It is then used like
>
> \vbox{ ... some content ... \par\eatlines}
>
> The macro goes through all hboxes (lines) in the vertical list until
> there is none left (\ifvoid) and unpacks each hbox into a new one.
>
> I was wondering, can the same thing be done in Lua (with probably less
> and more readable code)?

it depends on what you want to do. In any case you neet to be aware of 
possible side effects then.

Hans


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If your question is of interest to others as well, please add an entry to the Wiki!

maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
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