autofill with square-pattern (leaders vs. tikz)

Question

I want to autofill the rest of the line with a gray square-pattern in order give students an area to submit their mathimatical thoughts.

My solution is so far:

\newcommand\mySE{\vrule height 1pt width 5mm depth 0pt
     \vrule height 5mm width 1pt depth 0pt}
\newcommand\myNW{\vrule height 5mm width 1pt depth 0pt
     \raise5mm\hbox{\vrule height 0pt width 5mm depth 1pt}}

\newcommand\mysquarefill[1]{{\color{black!50}%
\mbox{}\leaders\hbox{\vbox to #1{\cleaders\hbox{\mySE}\vfil}}\hfill}}

The result of

B\mysquarefill{2.5cm}E

results in

, which is quite nice exept the missing lines.

If I write \hbox{\framebox(5,5){}} instead of \hbox{\mySE} the middlelines a drawn twice and thus darkgray.

Does anyone has an idea how to use \leaders to draw the missing vertical und horizontal line in \mysquarefillcorrectly?

Update and refraised:

In the meantime I learned from "How to align consecutive \leaders properly?" that \[x|c]leaders aren't the optimal solution for my problem, because they somehow align to a global grid. That's why I couldn't find a raggedleft- / raggedright-solution.

Because I would like to measure the remaining space and to calcute how many complete squares to use, I think this is a better way: Is there a way to measure the remaining space of a line of text?

My solution so far:

    \documentclass{article}

    \usepackage{tikzpagenodes}
    \usepackage{etoolbox}

    \usetikzlibrary{calc}       

    \newlength{\whatsleft}
    \newcommand{\measureremainder}[2]{%
        \begin{tikzpicture}[overlay,remember picture]
        % Measure distance to right text border
        \path let \p0 = (0,0), \p1 = (current page text area.east) in
        [/utils/exec={\pgfmathsetlength#1{floor((\x1-\x0)/#2)*#2}\global#1=#1}];
        \end{tikzpicture}%
    }

    \newcommand{\mysquarefill}[4][r]{%
        \measureremainder{\whatsleft}{#2}%
        \ifstrequal{#1}{l}{}{\hfill}%
        \lower#4\hbox{\begin{tikzpicture}%
        \draw[step=5mm,color=gray](0,0) grid (\whatsleft,#3);
        \end{tikzpicture}}%
        \ifstrequal{#1}{c}{\hfill\mbox{}}{}%
    }   


    \begin{document}

    \begin{itemize}
    \item $\frac{8}{15} + \frac{7}{12}+ \frac{5}{12} +2=$\mysquarefill[l]{5mm}{10mm}{4mm}
    \item $\frac{3}{4}$ von $ \frac{2}{5}=$\mysquarefill[r]{5mm}{15mm}{7mm}
    \item $\frac{15}{28}\cdot\frac{14}{30}=$\mysquarefill[r]{5mm}{10mm}{4mm}
    \end{itemize}

    \end{document}

My new question is: How could I directly use (current page text area.east) ? If I all do it in one macro, then sometimes the calculations aren't correctly updated, thus producing to small boxes.

It should be possible to use something like

\draw[step=5mm,color=gray](0,0) grid (floor(x((current page text area.east))/#2)*#2,#3);

And does exist an easier solution for the raggedleft/raggedright/center-mechanism?

Answer 1

I propose the following:

\documentclass{article}
\usepackage{xcolor}

\makeatletter
\newcommand*{\mysquarefill}[3]{%
  \begingroup
  \color{black!50}\mbox{}%
  \cleaders\vbox to #1{%
      \cleaders\hbox to #2{%
        \vrule height #2 width #3 depth \z@
        \kern -#3\relax
        \vbox to #2{\hrule width \dimexpr #2+#3 height #3\vfil
                    \hrule width \dimexpr #2+#3 height #3%
                    \kern -#3}%
        \kern -#3\relax
        \vrule height #2 width #3 depth \z@
        \hss}%
        \vfil
      \kern #3\relax
  }\hfill\kern #3\relax
  \endgroup
}
\makeatother

\newcommand*{\test}[1]{%
  \makebox[0pt][r]{B}% protrudes to the left, takes no space
  #1%
  \makebox[0pt][l]{E}% protrudes to the right, takes no space
}

\setlength{\parindent}{0pt}

\begin{document}

Exactly 10 squares of size $5\,$mm and border width one point fit in
an \verb|\hsize| of $5\,$cm plus one point (this is not the \verb|plus|
keyword of \TeX!):

\smallskip \hsize=\dimexpr 5cm+1pt\relax
\test{\mysquarefill{\dimexpr 2.5cm+1pt\relax}{5mm}{1pt}}

\bigskip \hsize=5.3cm
\test{\mysquarefill{2.8cm}{5mm}{1pt}}

\bigskip \hsize=5.6cm
\test{\mysquarefill{3.1cm}{5mm}{1pt}}

\bigskip \hsize=5.9cm
\test{\mysquarefill{3.4cm}{5mm}{1pt}}

\bigskip \hsize=\textwidth
\mysquarefill{\dimexpr 2.8cm+10pt\relax}{2.8cm}{10pt}%

\end{document}