How to find the width of the longest line in multi-paragraph text

Question

Question

Is it possible to get the width of a multi-paragraph piece of text? For example, if I want to get the width of a single piece of text, I can do the following:

\documentclass[a4paper]{article}
\begin{document}
\newlength{\mytextwidth}
\settowidth{\mytextwidth}{some text}
\end{document}

However, if that text has multiple paragraphs, that doesn't work. E.g. If I do something like the following I get the error Paragraph ended before \@settodim was complete. <to be read again>:

\documentclass[a4paper]{article}
\begin{document}
\newlength{\testlen}
\settowidth{\testlen}{
    some text
some longer text

}
\end{document}

What I'm wondering is if there is a way to get the widest width of text content (even if it's multi-paragraph text). So, for the first example it would still give the length of some text and for the second example, it would give the length of some longer text.

---

Context

Above is what I think I need for what I'm trying to do, but in case it's helpful, I'll also add the context for why I'm interested in getting this.

I'm attempting to write a command that will take in text content for a left side and right side. The content will be split into two columns, with the left side content being flush left and the right side content being flush right. The columns will adjust themselves to fit as much text on a line as they can before wrapping to the next line.

So if the left side content is much longer than the right side content, the right column will be much shorter in width than the left column and vice-versa. If neither side's content requires half the line width, each column's width will simply be half the line width. Similarly, if both side's content requires at least half the line width, each column's width will just be half the line width.

Here's what I have, which is working for single paragraph text:

\documentclass[a4paper]{article}
\begin{document}
% length variables for the command below
\newlength{\leftwidth}
\newlength{\rightwidth}
\newlength{\halfline}
\newlength{\leftexcess}
\newlength{\rightexcess}
% arg1: flush left minipage content
% arg2: flush right minipage content
\newcommand{\mycmd}[2]{
    \settowidth{\leftwidth}{#1}
    \settowidth{\rightwidth}{#2}
    \setlength{\halfline}{0.5\linewidth}
% if the left side content doesn't need at least half of the 
% line width, store the excess leftover space
\ifdim\leftwidth<\halfline\relax
    \setlength{\leftexcess}{\dimexpr(\halfline-\leftwidth)}
\else
    \setlength{\leftexcess}{0pt}
\fi

% if the right side content doesn't need at least half of the 
% line width, store the excess leftover space
\ifdim\rightwidth<\halfline\relax
    \setlength{\rightexcess}{\dimexpr(\halfline-\rightwidth)}
\else
    \setlength{\rightexcess}{0pt}
\fi

% neither side has any leftover space, so each column just gets half the line
\ifdim\rightexcess=0pt\relax
    \ifdim\leftexcess=0pt\relax
        \setlength{\leftwidth}{\halfline}
        \setlength{\rightwidth}{\halfline}
    \fi
\fi

% neither side needs any extra space to fit, so each column just gets half the line
\ifdim\leftexcess>0pt\relax
    \ifdim\rightexcess>0pt\relax
        \setlength{\leftwidth}{\halfline}
        \setlength{\rightwidth}{\halfline}
    \fi
\fi

% if the right side has space to spare and the left side needs more space than
% just half the line, give the left side the excess space from the right side.
\ifdim\rightexcess>0pt\relax
    \ifdim\leftwidth>\halfline\relax
        \setlength{\leftwidth}{\dimexpr(\halfline+\rightexcess)}
    \fi 
\fi

% if the left side has space to spare and the right side needs more space than
% just half the line, give the right side the excess space from the left side.
\ifdim\leftexcess>0pt\relax
    \ifdim\rightwidth>\halfline\relax
        \setlength{\rightwidth}{\dimexpr(\halfline+\leftexcess)}
    \fi
\fi

% command output
\par\noindent%
\begin{minipage}[t]{\leftwidth}%
    \begin{flushleft}%
        #1
    \end{flushleft}%
\end{minipage}%
\begin{minipage}[t]{\rightwidth}%
    \begin{flushright}%
        #2
    \end{flushright}%
\end{minipage}%

}
% works fine
\mycmd
{
    left left left left left left left left left left left left left left left
}
{
    right right
}
% errors with: Paragraph ended before @settodim was complete.
% \mycmd
% {
%     left left left left left left left left left left left left left left left
%     left left left
% }
% {
%     right right
% }
\end{document}

Answer 1

The varwidth package provides a similarly-named varwidth environment that acts like minipage. It takes a width argument like minipage, but instead of fixing the output to this width, it reduces the width of the resulting box to something smaller if the contents can fit within this smaller width. So you can specify the maximum width possible (\maxdimen) and varwidth should shrink that down to what you need - the maximum width of any paragraph within the environment body.

\documentclass{article}
\usepackage{varwidth}
\newsavebox{\mybox}
\newlength{\mylen}
\begin{document}
\settowidth{\mylen}{left left left left left left left left left left left left left left left}%
1: \the\mylen
\begin{lrbox}{\mybox}
\begin{varwidth}{\maxdimen}
left left left left left left left left left left left left left left left
right right
\end{varwidth}
\end{lrbox}%
2: \the\wd\mybox
\end{document}

Answer 2

tabulary can be used to get a similar effect.

\documentclass{article}
\usepackage{tabulary}
\newcommand\mycmd[2]{%
  \begin{center}
    \begin{tabulary}{\textwidth}{@{}LR@{}}
     #1&#2
    \end{tabulary}
  \end{center}}
\begin{document}
\mycmd
 {
     left left left left left left left left left left left left left left left
 left left left

}
 {
     right right
 }
\mycmd
 {
     left left left left left
 left left left

}
 {
     right right
 }
\mycmd
 {
     left left left left left left left left left left left left left left left
     left left left left left left left left left left left left left left left
 left left left

}
 {
     right right
 }
\mycmd
 {
     left left left left left left left left left left left left left left left
 left left left

}
 {
     right right right right right right right right right right right right right right
     right right right right right right right right right right right right right right
     right right right right right right right right right right right right right right
     right right right right right right right right right right right right right right
     right right right right right right right right right right right right right right
 }
\end{document}

Answer 3

Here, I count the width of all cat-11 and 12 tokens, as well as \parindent associated with paragraphs (I bypass any expansion of macros and ignore their width). In \balance, I do the math to calculate the ratio of each width to the total width.

I create minipages, using a specified usable fraction of linewidth to set the parts in minipages based on those ratios. Not knowing how much blank space is available at the end of a paragraph, I add .25\usablewidth as a very approximate average. So if the two sides vary in numbers of paragraphs, that can screw things up a little.

I also, at this point do not account for extremely short entries, less than a line in width...I'm not sure how a general algorithm should handle that case anyhow, if one entry is very short and the other very long.

\documentclass{article}
\usepackage{tokcycle}
\newlength\Xwd
\newlength\Ywd
\edef\svparindent{\the\parindent}
\newcommand\addtowd[2]{\sbox0{#2}\addtolength #1{\wd0}}
\newcommand\countwidth[3]{%
  \def#1{#3}%
  \setlength#2{0pt}%
  \tokcycle
  {\tctestifcatnx A##1{\addtowd{#2}{##1}}
    {\tctestifcatnx.##1{\addtowd{#2}{##1}}{}}\addcytoks{##1}}
  {\processtoks{##1}}
  {\tctestifx\par##1{\addtolength{#2}{%
  \dimexpr\parindent+.25\usablewidth}}{}\addcytoks{##1}}
  {\addtowd{#2}{##1}\addcytoks{##1}}
  {#3}%
} 
\newlength\Awd
\newlength\Bwd
\newlength\ABwd
\newlength\usablewidth
\usablewidth=.9\textwidth
\def\truncdec#1.#2\relax{#1}
\makeatletter
\newcommand\balance[4]{%
  \Awd=#2\relax
  \Bwd=#4\relax
  \ABwd=\dimexpr\Awd+\Bwd\relax
  \edef\ABcnt{\strip@pt\ABwd} 
  \divide\Awd by \expandafter\truncdec\ABcnt\relax
  \divide\Bwd by \expandafter\truncdec\ABcnt\relax
  \edef\Apct{\strip@pt\Awd}%
  \edef\Bpct{\strip@pt\Bwd}%
  \par\noindent
  \begin{minipage}[t]{\Apct\usablewidth}
    \parindent=\svparindent#1\end{minipage}%
  \hfill
  \begin{minipage}[t]{\Bpct\usablewidth}
    \parindent=\svparindent#3\end{minipage}%
  \bigskip
}
\makeatother
\begin{document}   
\countwidth{\X}{\Xwd}{This is a test of blah bla blah.
This is a test of blah bla blah.
This is a test of blah bla blah.
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\countwidth{\Y}{\Ywd}{This is a test of blah bla blah.
This is a test of blah bla blah.
This is a test of blah bla blah.}
\balance{\X}{\Xwd}{\Y}{\Ywd}
\countwidth{\Y}{\Ywd}{This is a test of blah bla blah.
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\balance{\X}{\Xwd}{\Y}{\Ywd}
\countwidth{\Y}{\Ywd}{This is a test of blah bla blah.
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\balance{\X}{\Xwd}{\Y}{\Ywd}
\end{document}