How can I compare the width of a paragraph to 45% of the linewidth?

Question

Here's my pseudo latex code to try and show what I'm trying to do in my memo latex class:

\newcommand\headername{%should be the @memoto, or if too long, an abbreviated subject
    \ifthenelse{\lengthtest{\settowidth{\@memoto}>.45*\linewidth}}
        {\emph{dummy text here}}
        {\@memoto}
}

I'm probably missing some calculation package or other, but I'm looking for some kind of test to see if \@memoto is longer than 45% of the linewidth. How do I do this?

(ultimately I will use this latex class within LyX, but I hope this is general enough to be answered without regard to LyX)

Answer 1

Here is an option:

\documentclass{article}
\makeatletter
\newcommand{\memoto}{\@dblarg\memoto@aux}
\def\memoto@aux[#1]#2{%
  \def\@memoto@short{#1}% Short \@memoto
  \def\@memoto@long{#2}% Long \@memoto
}
\newcommand\headername{% should be \@memoto, or if too long, an abbreviated subject
  \setbox9=\hbox{\@memoto@long}%
  \ifdim\wd9>.45\linewidth
    \emph{\@memoto@short}%
  \else
    \@memoto@long%
  \fi
}
\makeatother
\setlength{\parindent}{0pt}% Just for this example
\begin{document}
\rule{.45\textwidth}{1pt}\par
\memoto[Here is some short text]{Here is some longer text}\headername\par
\memoto[Here is some short text]{Here is some much longer text}\headername\par
\memoto[Here is some short text]{Here is some very much longer text}\headername\par
\memoto[Here is some short text]{Here is some extremely much longer text}\headername\par
\rule[1ex]{.45\textwidth}{1pt}\par
\end{document}

The definition of \memoto uses \@dblarg to allow for easily specifying an optional argument (see What does the command \@dblarg in a class?) and uses this shorter text if the length of the longer text is greater than .45\textwidth.