Test for \ucs and \toucs: check if a number is in the range of JIS X 0208

ucs_toucs.tex

%#!eptex -ini -etex
\let\dump\relax
\input plain
\newlinechar=`^^J

\newcount\tempcnta
\newcount\CharCount

\xdef\KINTERNAL{%
  \ifnum\ucs"3000="3000 \string\ucs
  \else\ifnum\euc"A1A1="A1A1 \string\euc
  \else\ifnum\sjis"8140="8140 \string\sjis
  \fi\fi\fi}

{\escapechar-1 \xdef\CHAR{\string\char}\xdef\OMCHAR{\string\omathchar}}
\def\CC#1{\expandafter\StripChar#1}
\expandafter\def\expandafter\StripChar\expandafter#\expandafter1\CHAR#2{#2}
\def\OC#1{\expandafter\StripOChar#1}
\expandafter\def\expandafter\StripOChar\expandafter#\expandafter1\OMCHAR#2{#2}

\CharCount=0\relax
\tempcnta=0\relax
%\ifnum\ucs"3000="3000 \else \tempcnta=256\relax \fi
\loop\ifnum\tempcnta<"110000\relax
 %\iffontchar\jfont\tempcnta\relax
  \ifnum\ucs\toucs\tempcnta=\tempcnta\relax
    \advance\CharCount by 1\relax
\ifnum\ucs"3000="3000 \else % boring for upTeX -> omitted
    \ifx\omathchardef\undefined      %%%%%
      \message{^^J\KINTERNAL\the\tempcnta\space
        = \string\ucs\toucs\tempcnta}
    \else                            %%%%%
      \chardef\KnjInternal=\tempcnta\relax
      \omathchardef\KnjUCS=\toucs\tempcnta\relax
      \message{^^J\KINTERNAL\CC{\meaning\KnjInternal}
        = \string\ucs\OC{\meaning\KnjUCS} (\Uchar\tempcnta)}
    \fi                              %%%%%
\fi
  \else
\ifnum\ucs"3000="3000 % should not happen for upTeX
    \errhelp{}\errmessage{Should not happen: char \the\tempcnta}
\fi
  \fi
 %\fi
  \advance\tempcnta by 1\relax
\repeat

\message{^^JNumber of valid characters: \the\CharCount^^J}
% => for pTeX, should be 6879 (JIS X 0208)
% => for upTeX, should be 1114112 (0..0x10FFFF)

\end

See texjporg/tex-jp-build#121