How do I typeset interleaved streams of text, with independent line breaks?

Question

I am writing scripts to produce synchronized texts as an aid to language study. I would like multiple streams of text to flow together in paragraphs, synchronizing where I indicate and otherwise flowing independently.

This second example is not an example of linguistic "gloss", and cannot be produced using the linguistics packages that have been suggested so far. Note how the two parallel streams of text have independent line breaks:

I know enough TeX and LaTeX to believe that this would be very difficult to do in one pass, entirely in TeX. Were I to code this now, I'd measure every word in a first LaTeX pass, then use an external programming language to plan the paragraphs for a second LaTeX pass. The idea of expressing the necessary logic in TeX sounds excruciating to me.

It would not surprise me, however, if someone knows or can imagine how to do this entirely in TeX or LaTex. Is this possible?

This may be a "hardware store" puzzle: The part I want is labeled for a different purpose, and I won't know what to ask for until I can guess that purpose. What's another problem like this one, that has already been solved? In-place footnotes?

I am aware of parallel text, side by side. That is what I do now. It is too much eye travel and alignment stress. If QWERTY were that much finger travel, everyone would have switched to DVORAK by now.

I am aware of stacking boxes, to flow as indivisible entities in a paragraph. That may be what I do as a first draft, but it wastes space. Interleaved text that flows can borrow space from each other, and can break where needed.

In some applications, the interleaved texts might be several languages, synchronized as closely as the translations allow. There are off-the-shelf tools such as "hunalign" for synchronizing texts, as this is a standard step in AI language learning. For that application, I would surely end up with some blocks that don't fit on a line. I need the streams to independently flow.

Answer 1

Yes, coding seems the pragmatic way. Coordinates on the page have to be transferred between two iterators.

TikZ allows a proof of concept of free-stream text interleaving at paragraph-level, with the overlay option (one picture on top of the other):

Raw interleaving:

With (manual) spacing added:

Coding, with Lua (or expl3?), will probably be the easiest method to synchronise sentence starts.

MWE

\documentclass{article}
\usepackage{xcolor}
\usepackage{fontspec}
\setmainfont{Noto Serif}
\setsansfont{Noto Sans}
\usepackage{tikz}
\usepackage{setspace}
\begin{document}
\doublespacing
\fbox{%
\begin{tikzpicture}[remember picture]
\node (a) [text width=0.98\textwidth, minimum width=0.98\textwidth] 
{the objects in space and time are what first give rise to human reason.
Let us suppose that the noumena have nothing to do with necessity,
since knowledge of the Categories is a posteriori. Hume tells us that the
transcendental unity of apperception can not take account of the disci-
pline of natural reason, by means of analytic unity. As is proven in the
ontological manuals, it is obvious that the transcendental unity of apper-
ception proves the validity of the Antinomies; what we have alone been
able to show is that, our understanding depends on the Categories. It
remains a mystery why the Ideal stands in need of reason. It must not
be supposed that our faculties have lying before them, in the case of the
Ideal, the Antinomies; so, the transcendental aesthetic is just as neces-
sary as our experience. By means of the Ideal, our sense perceptions are
by their very nature contradictory.\ \  \ \ \ \ };
\end{tikzpicture}\ 
}%fbox
\color{red}\sffamily
\begin{tikzpicture}[remember picture,overlay]
\node (b) at (a) [yshift=1.6ex,text width=0.98\textwidth, minimum width=0.98\textwidth] 
{Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus
lit, vestibulum ut, placerat ac, adipiscing vitae, felis. Curabitur dictum
ravida mauris. Nam arcu libero, nonummy eget, consectetuer id, vulpu
ate a, magna. Donec vehicula augue eu neque. Pellentesque habitan
morbi tristique senectus et netus et malesuada fames ac turpis egestas
Mauris ut leo. Cras viverra metus rhoncus sem. Nulla et lectus vestibulum
rna fringilla ultrices. Phasellus eu tellus sit amet tortor gravida placerat
nteger sapien est, iaculis in, pretium quis, viverra ac, nunc. Praesent ege
em vel leo ultrices bibendum. Aenean faucibus. Morbi dolor nulla, male
uada eu, pulvinar at, mollis ac, nulla. Curabitur auctor semper nulla
onec varius orci eget risus. Duis nibh mi, congue eu, accumsan eleifend
agittis quis, diam. Duis eget orci sit amet orci dignissim rutrum. Nam du
gula, fringilla a, euismod sodales, sollicitudin vel, wisi. Morbi auctor
orem non justo. Nam lacus libero, pretium at, lobortis vitae, ultricies et
%ellus. Donec aliquet, tortor sed accumsan bibendum, erat ligula alique
%magna, vitae ornare odio metus a mi. Morbi ac orci et nisl hendrerit mol
%s. Suspendisse ut massa. Cras nec ante. Pellentesque a nulla. Cum sociis
%atoque penatibus et magnis dis parturient montes, nascetur ridiculus
%mus. Aliquam tincidunt urna. Nulla ullamcorper vestibulum turpis. Pel
%entesque cursus luctus mauris. Nulla malesuada porttitor diam. Donec
%elis erat, congue non, volutpat at, tincidunt tristique, libero. Vivamus
%iverra fermentum felis. Donec nonummy pellentesque ante. Phasel
%us adipiscing semper elit. Proin fermentum massa ac quam. Sed diam
};
\end{tikzpicture}
\color{black}\rmfamily
\newpage
\hfill Project Gutenberg: Jules Verne
\fbox{%
\begin{tikzpicture}[remember picture]
\node (c) [text width=0.98\textwidth, minimum width=0.98\textwidth] 
{«Remontons-nous?
-- Non! Au contraire! Nous descendons!
-- Pis que cela, monsieur Cyrus! Nous tombons!
-- Pour Dieu! Jetez du lest!
-- Voilà le dernier sac vidé!
-- Le ballon se relève-t-il?
-- Non!
-- J'entends comme un clapotement de vagues!
-- La mer est sous la nacelle!
-- Elle ne doit pas être à cinq cents pieds de nous!»
Alors une voix puissante déchira l'air, et ces mots retentirent:
«Dehors tout ce qui pèse!... tout! et à la grâce de Dieu!» \ \ };
\end{tikzpicture}
}%fbox
\color{red}\sffamily
\begin{tikzpicture}[remember picture,overlay]
\node (d) at (c) [yshift=1.5ex,text width=0.98\textwidth, minimum width=0.98\textwidth] 
{“Are we rising again?” “No. On the contrary.” “Are we descending?”
 “Worse than that, captain! we are falling!” “For Heaven’s sake heave out
the ballast!” “There! the last sack is empty!” “Does the balloon rise?”
 “No!” “I hear a noise like the dashing of waves. The sea is below the
car! It cannot be more than 500 feet from us!” “Overboard with every
weight! ... everything!”};
\end{tikzpicture}
\color{black}\rmfamily
\newpage
\hfill Project Gutenberg: Jules Verne
\fbox{%
\begin{tikzpicture}[remember picture]
\node (e) [text width=0.98\textwidth, minimum width=0.98\textwidth] 
{«Remontons-nous?
-- Non! Au contraire! Nous descendons!
-- Pis que cela, monsieur Cyrus! Nous tombons!
-- Pour Dieu! Jetez du lest!
\hspace{7em}-- Voilà le dernier sac vidé!
-- Le ballon se relève-t-il?
-- Non!
-- J'entends comme un clapotement de vagues!
-- La mer est sous la nacelle!
-- Elle ne doit pas être à cinq cents pieds de nous!»
Alors une voix puissante déchira l'air, et ces mots retentirent:
«Dehors tout ce qui pèse!... tout! et à la grâce de Dieu!» \ \ };
\end{tikzpicture}
}%fbox
\color{red}\sffamily
\begin{tikzpicture}[remember picture,overlay]
\node (f) at (e) [yshift=-0.2ex,text width=0.98\textwidth, minimum width=0.98\textwidth] 
{“Are we rising again?” “No. On the contrary.” “Are we descending?”
 “Worse than that, captain! we are falling!” “For Heaven’s sake heave out
the ballast!” “There! the last sack is empty!” “Does the balloon rise?”
 “No!” “I hear a noise like the dashing of waves. \hspace{4em}The sea is below the
car! \hspace{4.5em}It cannot be more than 500 feet from us!” \hspace{5em}\ \ \ \ “Overboard with every
weight! ... everything!”};
\end{tikzpicture}
\end{document}

---

Addendum

Experimenting a bit in TikZ, starting sentences flush left for a paragraph seems more readable, and also automatically synchronizes them x-wise (obviously). Perhaps an array/matrix/grid structure could be used for the start position of run-in sentences.

MWE for the above, partially macro-ised:

\documentclass{article}
\usepackage{xcolor}
\usepackage{fontspec}
\setmainfont{Noto Serif}
\setsansfont{Noto Sans}
\usepackage{tikz}
\usepackage{setspace}
\usepackage{pgffor}
%============================================
\newcommand\doubleb[3]{%
\color{black}\rmfamily
\newpage
\fbox{%
\begin{tikzpicture}[remember picture]
\node (i) [text width=0.98\textwidth, minimum width=0.98\textwidth] 
{#1\foreach \x in {1,...,#3}{\ \ }};
\end{tikzpicture}
}%fbox
\color{red}\sffamily
\begin{tikzpicture}[remember picture,overlay]
\node (j) at (i) [,text width=0.98\textwidth, minimum width=0.98\textwidth] 
{#2};
\end{tikzpicture}
}
%============================================
\begin{document}
\doublespacing
\doubleb{Or, au moment même où tant de catastrophes s'accomplissaient sur
terre et sur mer, un drame, non moins saisissant, se jouait dans
les airs bouleversés. \
En effet, un ballon, porté comme une boule
au sommet d'une trombe, et pris dans le mouvement giratoire de la
colonne d'air, parcourait l'espace avec une vitesse de quatre-vingt-dix milles à l'heure, en tournant sur lui-même, comme s'il
eût été saisi par quelque maelström aérien. \
Au-dessous de
l'appendice inférieur de ce ballon oscillait une nacelle, qui
contenait cinq passagers, à peine visibles au milieu de ces
épaisses vapeurs, mêlées d'eau pulvérisée, qui traînaient jusqu'à
la surface de l'Océan.}{But while so many catastrophes were taking place on land and at sea, a
drama not less exciting was being enacted in the agitated air.\ \ \ 
In fact, a balloon, as a ball might be carried on the summit of a
waterspout, had been taken into the circling movement of a column of
air and had traversed space at the rate of ninety miles an hour, turning
round and round as if seized by some aerial maelstrom.\ \ \ 
Beneath the lower point of the balloon swung a car, containing five
passengers, scarcely visible in the midst of the thick vapor mingled
with spray which hung over the surface of the ocean.\ \ }{1}
\end{document}