Mdthbs

I'd like to draw the Chomsky hierarchy using tikz, similar to:

I have the following so far, for a subset of the hierarchy:

documentclass[11pt]{article}

usepackage{tikz}

usetikzlibrary{shapes,arrows,calc}

% ----------



begin{document}

    begin{tikzpicture}

        tikzstyle{venn} = [ellipse, minimum height=3em, minimum width=12em, draw]

        node [venn] (cs) at (0, $re.south$) {Context Sensitive};

        node [venn, minimum height=10em] (re) at (0,0)  {Recursively Enumerable};

    end{tikzpicture}

end{document}

How can I:

1. Anchor all of the shapes to the same point in the centre at the bottom of the diagram?




2. Align the text to the top of each ellipse - is shift appropriate here?

This is not a perfect solution, but it fits this particular case: write words above another node.

documentclass[tikz]{standalone}

usetikzlibrary{shapes}

begin{document}

begin{tikzpicture}

node[above,ellipse,minimum height=2em,minimum width=4em,draw] (a) {regular};

node[above,ellipse,minimum height=4em,minimum width=8em,draw] (b) {};

node[above,ellipse,minimum height=6em,minimum width=12em,draw] (c) {};

node[above,ellipse,minimum height=8em,minimum width=16em,draw] (d) {};

path (a.north) node[above] {context-free}

    (b.north) node[above] {context-sensitive}

    (c.north) node[above] {recursively enumerable};

end{tikzpicture}

end{document}

Naively one may think this should be simple with fit but unfortunately ellipse fits are not tight by default. (See here for a tighter fit, but my naive attempt to use it did not yield the desired results.) So one may do it differently. Note that this is not absolutely fool-proof but can be made so with more efforts. However, for the case at hand it works.

documentclass[11pt]{article}

usepackage{tikz}

usetikzlibrary{shapes.geometric,calc}

begin{document}

    begin{tikzpicture}[font=sffamily]

    foreach X [count=Y,remember=Y as LastY] in 

    {regular,context free,context sensitive,recursively enumerable}

     {ifnumY=1

      node[ellipse,draw,outer sep=0pt] (F-Y) {X};

     else

      node[anchor=south] (T-Y) at (F-LastY.north) {X};

      path let p1=($([yshift=1ex]T-Y.north)-(F-LastY.south)$),

      p2=($(F-1.east)-(F-1.west)$),p3=($(F-1.north)-(F-1.south)$)

      in ($([yshift=1ex]T-Y.north)!0.5!(F-LastY.south)$) 

      node[minimum height=y1,minimum width={y1*x2/y3},

      draw,ellipse,inner sep=0pt] (F-Y){};

     fi}

    end{tikzpicture}

end{document}