Paragraph break only if insufficient space to end of line

Question

I want to write source like

Lorem ipsum dolor sit amet. \smallnote{Smaller.}

and have it render so that

• if there is enough room for a \parindent plus the start of the \smallnote (the first word, or perhaps up to the first hyphenation point, in smaller type), then there will be a \parindent-sized whitespace and then the note will appear in the same paragraph;
• or, if there isn’t room, the note appears in a new paragraph entirely (or after a line-break with a \parindent space at the start of the new line).

The following illustrations (which I faked) show what I mean:

How is it possible to do this with TeX spacing?

Answer 1

you can use a hidden display math to measure the last line:

\documentclass{article}

\def\a{One two three four five six. }
\def\b{\a zzzzzz \a gg \a gg }

\def\smallnote#1{\xsmallnote#1 \zsmallnote}
\def\xsmallnote#1 #2\zsmallnote{%
{\abovedisplayshortskip0pt\abovedisplayskip0pt
 \belowdisplayshortskip0pt\belowdisplayskip0pt
$$\global\dimen1\predisplaysize$$\par}%
\sbox0{\small#1}%
\ifdim\dimexpr\dimen1-2em+\parindent+\wd0>\linewidth
\typeout{^^J!!! not enough room on \the\inputlineno^^J}%
\vskip-\baselineskip
{\small #1 #2\par}%
\else
\nobreak
\vskip-\glueexpr\baselineskip+\parskip\relax
\typeout{^^J!!! is enough room on \the\inputlineno^^J}%
{\small\vskip-\baselineskip\hspace*{\dimen1}#1 #2\par}
\fi}



\begin{document}


\b\b\smallnote{verylongword two three four five six seven eight
AAA BBB CCC DDD EE FF GG}

\bigskip

\b\b\a zz zz\smallnote{verylongword two three four five six seven eight}

\bigskip

\b\b\smallnote{verylongword two three four five six seven eight
AAA BBB CCC DDD EE FF GG with a long multiline not \b\b}

\end{document}

Answer 2

This question reminds me of the “Mathematical Reviews example” that you can find on page 106 of The TeXbook (second double dangerous bend paragraph): indeed, rather than the unnecessarily complicated and, above all, inherently unsafe test on \predisplaysize, I would use that approach, which is much more straightforward and reliable.

The reason for which the test for the value of \predisplaysize cannot be trusted, in general, is explained on page 188, second (double dangerous bend) paragraph. In particular, with e-TeX, simply activating the \lastlinefit feature can cause this test to fail.

In the following code, a command named \cleverparindentspace is defined: you use this command to insert into the current paragraph a space of width \parindent which is “clever” in the sense that it starts a new line if the text following it does not comfortably fit into the remainder of the same line -- just what you asked, if I understood well. Anyway, once you have got the general idea, you can fine tune this command to your needs; in particular, you can calibrate TeX’s propensity to choose the “new line” alternative by changing the value of the \penalty. The code also demonstrates the unreliability of the \predisplaysize test.

\documentclass[a4paper]{article}
\usepackage[T1]{fontenc}

\newcommand*{\cleverparindentspace}{%
  \unskip
  \nobreak \hfil \penalty1000 \hfilneg \indent
}

\lastlinefit = 750

\begin{document}

One two three four five six seven eight.  Some words.  Some more words.  How
long is this going to last?  One two three four five six seven eight.  This is
incredibly boring, isn't it?  (That was actually a quotation from \textsl{The
\TeX book}.)  One two three four five six seven eight.\cleverparindentspace
{\footnotesize\bfseries Here it is!}

One two three four five six seven eight.  Some words.  Some more words.  How
long is this going to last?  One two three four five six seven eight.  This is
incredibly boring, isn't it?  (That was actually a quotation from \textsl{The
\TeX book}.)  One two three four five six seven eight.  With a few words
added.\cleverparindentspace {\footnotesize\bfseries Here it is!}

One two three four five six seven eight.  Some words.  Some more words.  How
long is this going to last?  One two three four five six seven eight.  This is
incredibly boring, isn't it?  (That was actually a quotation from \textsl{The
\TeX book}.)  One two three four five six seven eight.  With some more words
added.\cleverparindentspace {\footnotesize\bfseries Here it is!}

One two three four five six seven eight.  Some words.  Some more words.  How
long is this going to last?  One two three four five six seven eight.  This is
incredibly boring, isn't it?  (That was actually a quotation from \textsl{The
\TeX book}.)  One two three four five six seven eight.  This one is just a tiny
bit longer:~a.\cleverparindentspace {\footnotesize\bfseries Here it is!}

% But consider the following:

One two three four five six seven eight.  Some words.  Some more words.  How
long is this going to last?  One two three four five six seven eight.  This is
incredibly boring, isn't it?  (That was actually a quotation from \textsl{The
\TeX book}.)  One two three four five six seven eight.  This one is just a tiny
bit longer: a.\cleverparindentspace {\footnotesize\bfseries Here it is!}

% You shouldn't trust \predisplaysize:

One two three four five six seven eight.  Some words.  Some more words.  How
long is this going to last?  One two three four five six seven eight.  This is
incredibly boring, isn't it?  (That was actually a quotation from \textsl{The
\TeX book}.)  The pre-display line.
$$\verb"\predisplaysize"=\mbox{\the\predisplaysize}$$

\end{document}

Output:

Answer 3

It's not really possible to reliable measuring the last line of a paragraph without doing it inside a \vbox.

Here's a solution with an environment.

\documentclass{article}

\newenvironment{addsmallnote}
 {\par\edef\tpd{\the\prevdepth}\setbox0=\vbox\bgroup\prevdepth=\tpd}
 {}
\makeatletter
\newcommand{\smallnote}[1]{\unskip\do@smallnote#1 \@nil}
\def\do@smallnote#1 #2\@nil{%
  \par
  \setbox2=\lastbox
  \setbox2=\hbox{\unhbox2\unskip}% remove the \parfillskip glue
  \setbox4=\hbox{\itshape#1\/}%
  \ifdim\dimexpr\wd2+\wd4+\parindent>\hsize
    \nointerlineskip\box2
    \nobreak
    {\interlinepenalty=\@M \footnotesize\itshape #1 #2\par}
  \else
    \parindent=\dimexpr\parindent+\wd2
    \nointerlineskip\box2
    \nobreak
    {\interlinepenalty=\@M \footnotesize\itshape #1\vadjust pre{\vskip-\baselineskip} #2\par}
  \fi
  \egroup
  \unvbox0
}
\makeatother

\begin{document}

This is a test.  This is a test.  This is a test.  This is a test. 
This is a test.  This is a test.  This is a test.  This is a test.
This is a test.  This is a test.  This is a test.  This is a test.
This is a test.  This is a test.  This is a test.

\begin{addsmallnote}
This is a test.  This is a test.  This is a test.  This is a test. 
This is a test.  This is a test.  This is a test.  This is a test.
This is a test.  This is a test.  This is a test.  This is a test.
This is a test.  This is a test.  This is a test.
\smallnote{Here is a small note}
\end{addsmallnote}

\begin{addsmallnote}
This is a test.  This is a test.  This is a test.  This is a test. 
This is a test.  This is a test.  This is a test.  This is a test.
This is a test.  This is a test.  This is a test.  This is a test.
This is a test.  This is a test.  This is a test.  A test.
\smallnote{Here is a small note}
\end{addsmallnote}

\begin{addsmallnote}
This is a test.  This is a test.  This is a test.  This is a test. 
This is a test.  This is a test.  This is a test.  This is a test.
This is a test.  This is a test.  This is a test.  This is a test.
This is a test.  This is a test.  This is a test.
\smallnote{Here is a small note that runs over and over in order to
fill much more than just the rest of a line and part of the next
line, I hope this is sufficient.}
\end{addsmallnote}

This is a test.  This is a test.  This is a test.  This is a test. 
This is a test.  This is a test.  This is a test.  This is a test.
This is a test.  This is a test.  This is a test.  This is a test.
This is a test.  This is a test.  This is a test.

\end{document}

The last line is split off, reboxed and measured. Then a decision is made based on its length and the length of the first word in the note. If the first word (plus the parindent) doesn't fit, a new paragraph is set normally at \footnotesize, otherwise the note is set with a parindent equal to the normal parindent plus the width of the last line; a negative skip equal to \baselineskip is inserted to cover our tracks.

No page break can be taken between the paragraph and the note, until the note itself has finished.

Answer 4

Here, I put a \kern of size \parindent and the first word of the small note into a box (to keep them together), and follow with the rest of the note. I also allow a break before the \kern, if needed.

I made the small note \bfseries just to help set it off.

EDITED, based on Gustavo's comment, to insert an \hfil prior to the \allowbreak, so that a linebreak would result in a ragged end prior to the small note.

\documentclass{article}
\newcommand\smallnote[1]{\smallnotehelp\expandafter\kern\the\parindent#1\relax}
\def\smallnotehelp#1#2\relax{\hfil\allowbreak%
  {\bfseries\small\mbox{#1}#2}}
\begin{document}
This is a test.  This is a test.  This is a test.  This is a test. 
This is a test.  This is a test.  This is a test.  This is a test.
This is a test.  This is a test.  This is a test.  This is a test.
This is a test.  This is a test.  This is a big test.%
\smallnote{here is the small note}

This is a test.  This is a test.  This is a test.  This is a test. 
This is a test.  This is a test.  This is a test.  This is a test.
This is a test.  This is a test.  This is a test.  This is a test.
This is a test.  This is a test.  This is a test.  A test.%
\smallnote{here is the small note}
\end{document}

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