Adjusting end of paragraph with start of the next one

I had to compile a selection of articles to create so-called "proceedings" for a workshop. One problem was to join together multiple pdf files; I will address this problem in a later post.
The other problem I had is I I wanted to display the list of pairs article title / article authors in a similar style to a "table of contents", i.e., the title on the left with a line of dots (a.k.a. "leaders" in TeX) joining the names of authors on the right.

If everything fits on one line

Here is such an example:

My dummy title . . . . . . . . . . . . . . . . . . . . . . A. Nonymous

It this case, it is very easily achieved using the following code:

\noindent
My dummy title\dotfill {\it A. Nonymous}

Note that if you want more control over the way leaders are displayed, you can use the generic macro, \leaders of which \dotfill is just a specialization:

\noindent
My dummy title\leaders\hbox to 1em{\hss.\hss}
\hfill {\it A. Nonymous}

The \leaders macro takes first a box, and then "repeat" this box over the length of the next \hskip, i.e., some horizontal "glue". In this case, \hfill is a shortcut for

\hskip 0pt plus 1fill

which means "a horizontal space of length 0 and infinite stretchability". In our case, since the title and author do not fill completely the line, the \hfill will take up all the remaining space between the two, and then the \leaders macro will fill this space with horizontal boxes of length exactly 1em (i.e., approximately the length of an "M") containing a centered dot "."

When one line is not enough

In that case, we start to have some problems. Consider the following example:

Lorem ipsum dolor sit amet, consectetur.
\dotfill{\it A. Nonymous and U. Known}

This produces the following

Lorem ipsum dolor sit amet, consectetur. A. Nonymous
and U. Known

But we in fact want:

Lorem ipsum dolor sit amet, consectetur. . . . . . . .
. . . . . . . . . . . . . . . . . . A. Nonymous and U. Known

Because all the line where the \dotfill appears is already full, there is no more space to "fill"!

The idea to solve this problem is to separate the title from the authors by two such fills, and incent TeX to break between those fills. Here is the solution:

\noindent
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do.\dotfill
\penalty0\hskip 0pt plus -1fill%
\vphantom{x}\dotfill{\it A. Nonymous and U. Known}\break

Let us review this code in details:

• The \penalty0 tells TeX that there is no penalty in breaking a line here.
• The \hskip 0pt plus -1fill is a negative fill and has the following effect: if TeX does not break at the penalty, (because there is enough space on the line), then it will cancel the first \dotfill; else, it will do nothing (there is nothing to cancel at the beginning of a new line). This is to avoid a blank inserted between the two dotfill if they fit into a single line.
• The \vphantom{x} before the second \dotfill is so that there will be something at the beginning of the second line after which to insert the dotfill. This macro insert a "phantom" box of width 0 (and height the same as an "x" but this is a side-effect).
• Finally, we end the paragraph with \break, which makes TeX break the paragraph without adding any glue, i.e., the end of the paragraph will be stuck to the right. This is what forces the second \dotfill to take the remaining space.

To conclude, here is the final code embedded in a macro:

\def\alignpars#1#2{
\noindent \ignorespaces #1
\dotfill%
\penalty0\hskip 0pt plus -1fill\relax
\vphantom{x}\dotfill
#2\unskip\break
}

I added in this macro \ignorespaces so that any spaces at the beginning of the first arguments will not produce blanks, and \unskip after the second arguments for the same reason (remove spaces at the end so there is no blank).

You can try this macros on variable-length paragraphs and see for yourself:

\alignpars{
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor
fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in
laborum.}{
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor
fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in
laborum.
}

\alignpars{
Lorem ipsum dolor sit amet.
laborum.}{
Excepteur sint proident, sunt in laborum.
}

\alignpars{
Lorem ipsum dolor sit amet snatohu sntahx laborum.}{
Excepteur sint proident, sunt in laborum.}


\alignpars{
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor
fugiat nulla pariatur. Excepteur sint occaecat.
laborum.}{
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor
fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in
laborum.
}

\alignpars{
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor
fugiat nulla pariatur. Excepteur sint occaecat.
laborum.}{
Lorem ipsum dolor sit amet, 
fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in
laborum.
}

\alignpars{
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor
fugiat nulla pariatur. Excepteur sint occaecat.
laborum.}{
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor
fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident.
}

\alignpars{
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor
fugiat nulla pariatur. Excepteur sint occaecat. santoheu satoheu snthao eusnth
aoeu laborum.}{
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor
fugiat nulla pariatur. Excepteur.
}

\alignpars{
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor
toto nulla pariatur. Excepteur sint occaecat. santoheu satoheu snthao eusnth
lfugiat aborum.}{
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor
nulla pariatur. Excepteur.
}

\alignpars{
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor
toto nulla pariatur. Excepteur sint occaecat. santoheu satoheu snthao eusnth
sotnuh saousnth lfugiat aborum.}{
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor
pariatur. Excepteur.
}