Creating an ellipse-rectangle shaped node in tikz

Question

I would like to create a new node shape. The left half of the node is shaped like a rectangle and the right half is shaped like an ellipse. I should be able to insert text in my node and the dimensions of the node should adapt automatically.

One way of doing it, I thought, would be to draw three nodes, one as rectangle with my phantom text, one as an ellipse with phantom text, and one with no shape but with the text appearing. The first two nodes would be clipped into halves. However, with this method I have no guarantees that the boundaries of the two half-shapes will touch.

Answer 1

I never use \pgfdeclareshape before…

I took the elements from this question and this answer

\documentclass[border=10pt,tikz,multi]{standalone}
\usetikzlibrary{positioning}
\makeatletter%
\pgfdeclareshape{rectell}{%
    \inheritsavedanchors[from=rectangle]
    \inheritanchorborder[from=rectangle]
    \inheritbackgroundpath[from=rectangle]
    \inheritanchor[from=rectangle]{center}
    \inheritanchor[from=rectangle]{north}
    \inheritanchor[from=rectangle]{north east}
    \inheritanchor[from=rectangle]{north west}
    \inheritanchor[from=rectangle]{south}
    \inheritanchor[from=rectangle]{south east}
    \inheritanchor[from=rectangle]{south west}
    \inheritanchor[from=rectangle]{east}
    \inheritanchor[from=rectangle]{west}
    \inheritanchor[from=rectangle]{text}
    \backgroundpath{
        % store lower right in xa/ya and upper right in xb/yb
        \southwest \pgf@xa=\pgf@x \pgf@ya=\pgf@y
        \northeast \pgf@xb=\pgf@x \pgf@yb=\pgf@y
        % construct main path
        \pgfpathmoveto{\pgfpoint{(\pgf@xa+\pgf@xb)/2}{\pgf@ya}}
        \pgfpathlineto{\pgfpoint{\pgf@xa}{\pgf@ya}}
        \pgfpathlineto{\pgfpoint{\pgf@xa}{\pgf@yb}}
        \pgfpathlineto{\pgfpoint{(\pgf@xa+\pgf@xb)/2}{\pgf@yb}}
        \pgfpatharcto{(\pgf@xa+\pgf@xb)/8}{(\pgf@ya+\pgf@yb)/4}{0}{0}{0}{\pgfpoint{(\pgf@xa+\pgf@xb)/2}{\pgf@ya}}
    }
    \foregroundpath{
        \pgfusepath{stroke}
    }
}
\makeatother
\begin{document}
    \begin{tikzpicture}
%   \draw [help lines] (-2,-4) grid (10,2);
    \node (a) [rectell, draw] at (0, 0) {$\bullet\bullet\bullet$};
    \node (b) [right=5pt of a.east, rectell, anchor=north west, draw=magenta,very thick, fill=gray] {text};
    \node (c) [right=5pt of b.east, rectell, anchor=south west, draw=blue] {$\bullet$};
    \node (d) [right=5pt of c.south east, rectell, anchor=south east, draw, draw=red, scale=-.5] {*};
    \end{tikzpicture}
\end{document}

And the result is

Answer 2

One possible solution:

\documentclass[tikz,border=3mm]{standalone}
\usetikzlibrary{calc}
\tikzset{
    pics/mypic/.style args={#1/#2/#3}{
      code = {
       \draw (0,0)coordinate(A) arc(90:-90:#1cm and #2cm)coordinate(B);
 \draw (A)--++(180:#1)--++(-90:#1)--(B);
 \node at  ($(A)!0.5!(B)$) {#3};
    }}}
\begin{document}

\begin{tikzpicture}
\draw pic {mypic=2/1/dada}; 
\end{tikzpicture}
\end{document}