TikZ - Placing several numbers in one cell of a Sudoku grid

Question

I've put at the end of this post one code coming from this page which is a pretty easy way to display Sudoku grid.

I would like to have help so as to add the possibility to indicate several entries in one cell like in the following screenshot ? This is to indicate how to solve the sudoku without giving the answer.

My question is about one TiKz way to produce at least the first preceding case with nine possibilities.

Code, from TeXample.net, that displays a Sudoku grid

\documentclass{article}
    \usepackage{tikz}

    \newcounter{row}
    \newcounter{col}

    \newcommand\setrow[9]{
        \setcounter{col}{1}
        \foreach \n in {#1, #2, #3, #4, #5, #6, #7, #8, #9} {
            \edef\x{\value{col} - 0.5}
            \edef\y{9.5 - \value{row}}
            \node[anchor=center] at (\x, \y) {\n};
            \stepcounter{col}
        }
        \stepcounter{row}
    }


\begin{document}

\begin{tikzpicture}[scale=.5]
  \begin{scope}
    \draw (0, 0) grid (9, 9);
    \draw[very thick, scale=3] (0, 0) grid (3, 3);

    \setcounter{row}{1}
    \setrow { }{2}{ }  {5}{ }{1}  { }{9}{ }
    \setrow {8}{ }{ }  {2}{ }{3}  { }{ }{6}
    \setrow { }{3}{ }  { }{6}{ }  { }{7}{ }

    \setrow { }{ }{1}  { }{ }{ }  {6}{ }{ }
    \setrow {5}{4}{ }  { }{ }{ }  { }{1}{9}
    \setrow { }{ }{2}  { }{ }{ }  {7}{ }{ }

    \setrow { }{9}{ }  { }{3}{ }  { }{8}{ }
    \setrow {2}{ }{ }  {8}{ }{4}  { }{ }{7}
    \setrow { }{1}{ }  {9}{ }{7}  { }{6}{ }

    \node[anchor=center] at (4.5, -0.5) {Unsolved Sudoku};
  \end{scope}

  \begin{scope}[xshift=12cm]
    \draw (0, 0) grid (9, 9);
    \draw[very thick, scale=3] (0, 0) grid (3, 3);

    \setcounter{row}{1}
    \setrow { }{2}{ }  {5}{ }{1}  { }{9}{ }
    \setrow {8}{ }{ }  {2}{ }{3}  { }{ }{6}
    \setrow { }{3}{ }  { }{6}{ }  { }{7}{ }

    \setrow { }{ }{1}  { }{ }{ }  {6}{ }{ }
    \setrow {5}{4}{ }  { }{ }{ }  { }{1}{9}
    \setrow { }{ }{2}  { }{ }{ }  {7}{ }{ }

    \setrow { }{9}{ }  { }{3}{ }  { }{8}{ }
    \setrow {2}{ }{ }  {8}{ }{4}  { }{ }{7}
    \setrow { }{1}{ }  {9}{ }{7}  { }{6}{ }

    \node[anchor=center] at (4.5, -0.5) {Solved Sudoku};

    \begin{scope}[blue, font=\sffamily]
      \setcounter{row}{1}
      \setrow {4}{ }{6}  { }{7}{ }  {3}{ }{8}
      \setrow { }{5}{7}  { }{9}{ }  {1}{4}{ }
      \setrow {1}{ }{9}  {4}{ }{8}  {2}{ }{5}

      \setrow {9}{7}{ }  {3}{8}{5}  { }{2}{4}
      \setrow { }{ }{3}  {7}{2}{6}  {8}{ }{ }
      \setrow {6}{8}{ }  {1}{4}{9}  { }{5}{3}

      \setrow {7}{ }{4}  {6}{ }{2}  {5}{ }{1}
      \setrow { }{6}{5}  { }{1}{ }  {9}{3}{ }
      \setrow {3}{ }{8}  { }{5}{ }  {4}{ }{2}
    \end{scope}
  \end{scope}
\end{tikzpicture}

\end{document}

Answer 1

I introduced some new commands to help explaining sudokus:

UPDATE: Highlights are put in background as suggested by Claudio Fiandrino. Also a new \highligthrectangle was added to highlight arbitrary rectangular sections, such as quadrants. Note how several highligths can overlap.

    % Highlight some cells
    \highlightcell{2}{8}
    \highlightrow{8}
    \highlighcolumn{5}
    \highlightrectangle{4}{1}{6}{3}

    % Other marks
    \circlecell{5}{2}
    \crosscell{8}{1}

    % Hints
    \hintcell{5}{5}{2,7,8}
    \hintcell{1}{1}{3,4,6,7}

Resulting in:

The code is as follows.

\usetikzlibrary{backgrounds}
% Some customizable styles
\tikzset {
  highlight/.style = {yellow, opacity=0.3},
  digit/.style = { minimum height = 5mm, minimum width=5mm, anchor=center },
  circle/.style = {draw=green!80!black, dotted, very thick},
  cross/.style = {red, opacity=.5, shorten >=1mm, shorten <=1mm, very thick, line cap=round},
  hint/.style={blue, font=\sf, minimum width=3mm, minimum height=3mm}
}

% Original code-----------------------------------------------------------
% Modified the \node to give a unique name to each one, which is the
% row number, a dash and the column number. E.g: 1-1, 4-5, etc.
\newcounter{row}
\newcounter{col}

\newcommand\setrow[9]{
    \setcounter{col}{1}
    \foreach \n in {#1, #2, #3, #4, #5, #6, #7, #8, #9} {
        \edef\x{\value{col} - 0.5}
        \edef\y{9.5 - \value{row}}
        \node[digit,name={\arabic{row}-\arabic{col}}] at (\x, \y) {\n};
        \stepcounter{col}
    }
    \stepcounter{row}
}

% New code -------------------------------------------------------------
\def\highlightcell#1#2{
\fill[highlight] (#1-#2.north west) rectangle (#1-#2.south east);
}

\def\circlecell#1#2{
\draw[circle] (#1-#2) circle(4mm);
}

\def\crosscell#1#2{
\draw[cross] (#1-#2.north west) -- (#1-#2.south east);
\draw[cross] (#1-#2.north east) -- (#1-#2.south west);
}

\def\highlightrow#1{
\fill[highlight] (#1-1.north west) rectangle (#1-9.south east);
}

\def\highlighcolumn#1{
\fill[highlight] (1-#1.north west) rectangle (9-#1.south east);
}

\def\hintcell#1#2#3{
\node at (#1-#2) {\hintbox{#3}};
}

\def\highlightrectangle#1#2#3#4{
\begin{pgfonlayer}{background}
\fill[highlight] (#1-#2.north west) rectangle (#3-#4.south east);
\end{pgfonlayer}
}

% UGLY code. Do not read :-)
\def\hintbox#1{
\resizebox{4.5mm}{4.5mm}{%
\tikz[scale=0.3]{%
  \def\auxc{0}
  \foreach \m in {1,...,9} {
    \pgfmathparse{mod(\auxc,3)}
    \xdef\x{\pgfmathresult}
    \pgfmathparse{-floor(\auxc/3)}
    \xdef\y{\pgfmathresult}
    \xdef\hintprinted{0}
    \foreach \n in {#1} {
      \ifnum\n=\m
       \node[hint] at (\x,\y) {\n};
       \xdef\hintprinted{1}
      \fi
     }
   \ifnum\hintprinted=0
      \node[hint, opacity=0.1] at (\x,\y) {\m};
   \fi
   \pgfmathparse{\auxc+1} 
   \xdef\auxc{\pgfmathresult}   }
  }%
 }
}


\begin{tikzpicture}[scale=.5]
  \begin{scope}
    \draw (0, 0) grid (9, 9);
    \draw[very thick, scale=3] (0, 0) grid (3, 3);

    \setcounter{row}{1}
    \setrow { }{2}{ }  {5}{ }{1}  { }{9}{ }
    \setrow {8}{ }{ }  {2}{ }{3}  { }{ }{6}
    \setrow { }{3}{ }  { }{6}{ }  { }{7}{ }

    \setrow { }{ }{1}  { }{ }{ }  {6}{ }{ }
    \setrow {5}{4}{ }  { }{ }{ }  { }{1}{9}
    \setrow { }{ }{2}  { }{ }{ }  {7}{ }{ }

    \setrow { }{9}{ }  { }{3}{ }  { }{8}{ }
    \setrow {2}{ }{ }  {8}{ }{4}  { }{ }{7}
    \setrow { }{1}{ }  {9}{ }{7}  { }{6}{ }

    \node[anchor=center] at (4.5, -0.5) {Unsolved Sudoku};

    % Highlight some cells
    \highlightcell{2}{8}
    \highlightrow{8}
    \highlighcolumn{5}
    \highlightrectangle{4}{1}{6}{3}

    % Other marks
    \circlecell{5}{2}
    \crosscell{8}{1}

    % Hints
    \hintcell{5}{5}{2,7,8}
    \hintcell{1}{1}{3,4,6,7}
  \end{scope}
\end{tikzpicture}