結城浩さんの CodeIQ の アルゴリズム問題「ジェムストリング問題」（ https://codeiq.jp/ace/yuki_hiroshi/q684 ）をTeX言語(TeX on LaTeX)で解きました。TeX Liveなどで pdflatex gemstring とすればコンパイルできます。

gemstring.tex

%! pdflatex gemstring
\documentclass{article}
\usepackage[nomessages]{fp}
\usepackage{pgffor}
\usepackage{xstring}
\usepackage{ifthen}

\makeatletter

\newif\ifdebugmode

\def\shiftarray#1{%
  \@shiftarray#1,\relax
}

\def\@shiftarray#1,#2\relax{%
  \def\@shiftarray@temp{#2}%
  \ifx\@shiftarray@temp\@empty
    \def\result{}%
  \else
    \removelastcomma{#2}%
  \fi
}

\def\removelastcomma#1{%
  \@removelastcomma#1\relax
}
\def\@removelastcomma#1,\relax{%
  \def\result{#1}%
}

\def\firstcount#1{%
  \expandafter\@firstcount#1\relax
}

\def\@firstcount#1/#2#3\relax{%
  \def\result{#2}%
}

\def\decarray#1#2{%
  \foreach \char/\num in {#1}{%
    \expandafter\xdef\csname gem\char\endcsname{\num}%
  }%
  \@decarray#2\relax
  \def\result{}%
  \foreach \char/\num in {#1}{%
    \xdef\result{\result,\char/\csname gem\char\endcsname}%
  }%
  \expandafter\shiftarray\expandafter{\result}%
  \edef\result{\result}%
}

\def\@decarray#1{%
  \ifx#1\relax
    \let\@decarraynext\relax
  \else
    \let\@decarraynext\@decarray
    \FPeval\@result{trunc(\csname gem#1\endcsname - 1:0)}%
    \expandafter\xdef\csname gem#1\endcsname{\@result}%
  \fi
  \@decarraynext
}

\def\factorial#1{%
  \def\result{1}%
  \@gobble\if\FPifeq{#1}{0}%
    \def\result{1}%
  \else
    \@gobble\if\FPifeq{#1}{1}%
      \def\result{1}%
    \else
      \foreach \i in {1,2,...,#1}{%
        \FPeval\@result{trunc(\result*\i:0)}%
        \xdef\result{\@result}%
      }%
    \fi
  \fi
}

\def\countstring#1{%
  \def\sumresult{0}%
  \@countstring{#1}{}%
}

\def\@countstring#1#2{%
  \def\@temp{#1}%
  \ifx\@temp\@empty
    \shiftarray{#2}%
    \def\@sumresult{0}%
    \edef\ary{\result}%
    \foreach \i in \ary{%
      \FPeval\@sumresult{trunc(\@sumresult + \i:0)}%
      \xdef\@sumresult{\@sumresult}%
    }%
    \factorial{\@sumresult}%
    \edef\@factresult{\result}%
    \def\@prodresult{1}%
    \foreach \i in \ary{%
      \factorial{\i}%
      \FPeval\@prodresult{trunc(\@prodresult*\result:0)}%
      \xdef\@prodresult{\@prodresult}%
    }%
    \FPeval\sumresult{trunc(\sumresult + \@factresult / \@prodresult:0)}%
    \xdef\sumresult{\sumresult}%
  \else
    \firstcount{#1}%
    \edef\@@temp{\result}%
    \shiftarray{#1}%
    \edef\@nextary{\result}%
    \foreach \i in {0,...,\@@temp}{%
      \edef\@@@temp{#2,\i}%
      \expandafter\expandafter\expandafter\@countstring
      \expandafter\expandafter\expandafter{%
      \expandafter\@nextary\expandafter}\expandafter{\@@@temp}%
    }%
  \fi
}

\newcount\position
\newcount\charnum

\def\search#1#2{%
  \def\searchresult{0}%
  \StrLen{#2}[\strlen]%
  \position=0\relax
  \whiledo{\position<\strlen}{%
    \advance\position by 1\relax
    \StrChar{#2}{\position}[\charAt]%
    \if\charAt a\def\charNum{1}\fi
    \if\charAt b\def\charNum{2}\fi
    \if\charAt c\def\charNum{3}\fi
    \if\charAt d\def\charNum{4}\fi
    \if\charAt e\def\charNum{5}\fi
    \if\charAt f\def\charNum{6}\fi
    \if\charAt g\def\charNum{7}\fi
    \charnum=1\relax
    \whiledo{\charnum<\charNum}{%
      \@tempcntb=\the\position\relax
      \advance\@tempcntb by -1\relax
      \StrLeft{#2}{\the\@tempcntb}[\strleft]%
      \def\@temp{#1}%
      \expandafter\expandafter\expandafter\decarray
      \expandafter\expandafter\expandafter{%
      \expandafter\@temp\expandafter}\expandafter{\strleft}%
      \edef\@counttarget{\result}%
      \gdef\sumflag{T}%
      \def\next@ary{}%
      \foreach \char/\num in \@counttarget{%
        \ifnum\charnum=1\relax\gdef\thechar{a}\fi
        \ifnum\charnum=2\relax\gdef\thechar{b}\fi
        \ifnum\charnum=3\relax\gdef\thechar{c}\fi
        \ifnum\charnum=4\relax\gdef\thechar{d}\fi
        \ifnum\charnum=5\relax\gdef\thechar{e}\fi
        \ifnum\charnum=6\relax\gdef\thechar{f}\fi
        \ifnum\charnum=7\relax\gdef\thechar{g}\fi
        \if\thechar\char
          \@tempcnta=\num\relax
          \ifnum\@tempcnta=0\relax
            \gdef\sumflag{F}%
          \else
            \advance\@tempcnta by -1\relax
            \edef\num{\the\@tempcnta}%
          \fi
        \fi
        \xdef\next@ary{\next@ary,\char/\num}%
      }%
      \expandafter\shiftarray\expandafter{\next@ary}%
      \edef\next@ary{\result}%
      \if\sumflag T%
        \expandafter\countstring\expandafter{\next@ary}%
        \FPeval\searchresult{trunc(\sumresult+\searchresult:0)}%
        \ifdebugmode
          \typeout{\strleft\thechar\space + [\next@ary] => \sumresult}%
        \fi
      \fi
      \advance\charnum by 1\relax
    }%
    \ifdebugmode
      \StrLeft{#2}{\position}[\@strleft]%
      \typeout{\@strleft\space => 1}%
    \fi
    \FPeval\searchresult{trunc(\searchresult+1:0)}%
  }%
  \typeout{Index of "#2" in [#1] => \searchresult}%
  \searchresult
}
\makeatother
\begin{document}
\debugmodetrue
\search{a/1,b/4,c/1,d/4,e/2,f/1,g/3}{eagcdfbe} % => 5578864439
\end{document}