Conditional centering if single line

Question

I would like to define a new environment that centers its contents if it can be typeset on a single line, but justifies the contents if it requires multiple lines.

\documentclass{article}
\usepackage{lipsum}
\newenvironment{mycenter}{\noindent}{}
\begin{document}

\begin{mycenter}%
This should be centered since it is a single line
\end{mycenter}

\begin{mycenter}%
This should be justified since it is multiple lines. \lipsum[1]
\end{mycenter}

\end{document}

I have a convoluted idea using the Environ package to capture the contents, then sticking them in a box and trying to check the height of the box. Is there an easier way?

Answer 1

\documentclass{article}
\usepackage{lipsum}
\newenvironment{mycenter}{\vbox\bgroup\bgroup
\parindent0pt\relax}{%
\par
\ifnum\prevgraf=1
\setbox0\lastbox
\noindent\hskip\parfillskip\hbox{\unhbox0}\par
\fi
\egroup\egroup}
\begin{document}

\begin{mycenter}%
This should be centered since it is a single line
\end{mycenter}

\begin{mycenter}%
This should be justified since it is multiple lines. \lipsum[1]
\end{mycenter}

\end{document}