longdiv.sty

%
% longdiv[ision].sty
%
\newcount\gpten % (global) power-of-ten -- tells which digit we are doing
\newcount\rtot % running total -- remainder so far
\newcount\scratch

\def\longdiv#1#2{% long division: #1/#2;  integers only, gives integer quotient and remainder
 \vtop{\offinterlineskip
   \setbox\strutbox\hbox{\vrule height 2.1ex depth .5ex width0ex}%
   \def\showdig{$\underline{\the\scratch\strut}$\cr\the\rtot\strut\cr
       \noalign{\kern-.2ex}}%
   \global\rtot=#1\relax
   \count0=\rtot\divide\count0by#2\edef\quotient{\the\count0}%\show\quotient
   % make list macro out of digits in quotient:
   \def\temp##1{\ifx##1\temp\else \noexpand\dodig ##1\expandafter\temp\fi}%
   \edef\routine{\expandafter\temp\quotient\temp}%
   % process list to give power-of-ten:
   \def\dodig##1{\global\multiply\gpten by10\relax}\global\gpten=1\relax\routine
   % to display effect of one digit in quotient (zero ignored):
   \def\dodig##1{\global\divide\gpten by10\relax
      \scratch =\gpten
      \multiply\scratch  by##1\relax
      \multiply\scratch  by#2\relax
      \global\advance\rtot-\scratch \relax
      \ifnum\scratch>0 \showdig \fi % must hide \cr in a macro to skip it
   }%
   \tabskip=0pt
   \halign{\hfil##\cr % \halign for entire division problem
     $\quotient$\strut\cr
     #2\kern.25em\smash{\raise.33ex\hbox{$\big)$}}$\mkern-8mu
        \overline{\enspace\the\rtot\strut}$\cr\noalign{\kern-.2ex}
     \routine \cr % do each digit in quotient
}}}
\endinput
%
%                TeX sample
%
\noindent Here are some long division problems

\indent
\longdiv{12345}{13}
\longdiv{123}{1234}
\longdiv{31415926}{2}
\longdiv{81}{3}
\longdiv{1132}{99}
\bye
%