Inconsistent TikZ multipart node anchors

Question

In the following example, why aren't the node anchors placed consistently?

\input tikz
\usetikzlibrary{arrows,shapes.multipart}
\tikzset{
  font=\tt, >= stealth, every picture/.style={thick}, pointer/.style={*->},
  box/.style={draw, inner sep=2ex, fill=black!10, rounded corners,
    rectangle, rectangle split, rectangle split parts=2,
    rectangle split ignore empty parts=false, rectangle split horizontal}}
\tikzpicture[every node/.append style={at end}]
  \node[box] (1) {};
  \draw[pointer] (1.text) -- +(0,-1) node[anchor=north,box] (2) {};
  \draw[pointer] (2.text) -- +(0,-1) node[anchor=north] {1};
  \draw[pointer] (2.two) -- +(0,-1) node[anchor=north] {2};
  \draw[pointer] (1.two) -- +(1,0) node[anchor=base west,box] (3) {};
  \draw[pointer] (3.text) -- +(0,-1) node[anchor=north] {3};
  \draw[pointer] (3.two) -- +(0,-1) node[anchor=north] {4};
\endtikzpicture
\bye

Ideally, I'd like the black dots to be exactly in the center.

Answer 1

I'm not sure you take the good way. In the pgfmanual, you can see that centers of the different parts are not given. I think the center of a part for example "one" is $(1.west)!.5!(1.one split)$. I write rapidly the next code and it's possible to write something more elegant.

Update version 2 : I found a way to create the coordinates of the centers for each part but I can't find how to create the circles with the style. (c1 3) désigne the center of the first part of the node (3). There are some modifications in the code like at end

\input tikz
\usetikzlibrary{arrows,shapes.multipart,calc}

\tikzset{
  font=\tt, 
  >= stealth, 
  every picture/.style={thick},
  box/.style={at end,
              draw, 
              inner sep=2ex, 
              fill=black!10, 
              rounded corners,
              rectangle, 
              rectangle split, 
              rectangle split parts=2,
              rectangle split ignore empty parts=false, 
              rectangle split horizontal,
              append after command={%
              \pgfextra{%
                \let\mainnode=\tikzlastnode} 
                coordinate (c1 \mainnode) at ($(\mainnode.west)!.5!(\mainnode.one split)$)
                coordinate (c2 \mainnode) at ($(\mainnode.one split)!.5!(\mainnode.east)$)}}}
\tikzpicture
  \node[box] (1) {};
   \draw[->] (c1 1) -- +(0,-1) node[anchor=north,box] (2) {};
   \draw[->] (c1 2) -- +(0,-1) node[anchor=north] {1};
   \draw[->] (c2 2) -- +(0,-1) node[anchor=north] {2};
   \draw[->] (c2 1) -- +(1,0)  node[anchor=west,box] (3) {};
   \draw[->] (c1 3) -- +(0,-1) node[anchor=north] {3};
   \draw[->] (c2 3) -- +(0,-1) node[anchor=north] {4};  
   \foreach \i in {1,2,3}{\draw[fill=black] (c1 \i)circle(2pt) (c2 \i)circle(2pt);}
\endtikzpicture
\bye 

Difference with Marc's solution

\node[box] (1) { };
\draw[] (1.one|-1.west) circle(2pt);
\draw[fill=black] ($(1.west)!.5!(1.one split)$)circle(2pt); 

gives

Update version 3. It's possible to draw the circle directly but in this case you can't draw the arrows at the same time. The code is interesting and perhaps someone can find a good idea for the arrows (only arrows when we use box)

\input tikz
\usetikzlibrary{arrows,shapes.multipart,calc}

\tikzset{
  font=\tt, 
  >= stealth, 
  every picture/.style={thick},
  box/.style={
              draw, 
              inner sep=2ex, 
              fill=black!10, 
              rounded corners,
              rectangle, 
              rectangle split, 
              rectangle split parts=2,
              rectangle split ignore empty parts=false, 
              rectangle split horizontal,
              append after command={%
                \pgfextra{\let\mainnode=\tikzlastnode 
                \coordinate (c1 \mainnode) at ($(\mainnode.west)!.5!(\mainnode.one split)$);
                \coordinate (c2 \mainnode) at ($(\mainnode.one split)!.5!(\mainnode.east)$);
                \draw[fill=black] (c1 \mainnode)circle(2pt) (c2 \mainnode)circle(2pt);}
                }}}
\tikzpicture
   \node[box] (1) {};
   \path (c1 1)     -- +(0,-1) node[anchor=north,box] (2) {};
   \draw[->] (c1 2) -- +(0,-1) node[anchor=north] {1};
   \draw[->] (c2 2) -- +(0,-1) node[anchor=north] {2};
   \path (c2 1)     -- +(1,0)  node[anchor=west,box] (3) {};
   \draw[->] (c1 3) -- +(0,-1) node[anchor=north] {3};
   \draw[->] (c2 3) -- +(0,-1) node[anchor=north] {4};
   \draw[->] (c1 1) --  (2.north);
   \draw[->] (c2 1) --  (3.west); 
\endtikzpicture
\bye 

We get the same result.

Answer 2

It looks as if this is a bug. Two things are going on. The pointer style results in the wrong positions for the start of the arrows. On top of that, the positioning inside the text parts of the boxes looks suspicious. In the following I show how you can get around these problems by using a normal arrow and by computing (what appears to be) the centre of the two text parts.

\usetikzlibrary{calc}

\usetikzlibrary{arrows,shapes.multipart}
\begin{document}
\tikzset{
  font=\tt, >= stealth, every picture/.style={thick}, pointer/.style={*->},
  box/.style={draw, inner sep=2ex, fill=black!10, rounded corners,
    rectangle, rectangle split, rectangle split parts=2,
    rectangle split ignore empty parts=false, rectangle split horizontal}}
\tikzpicture[every node/.append style={at end}]
  \node[box] (1) {};
  \draw[->] (1.mid) -- +(0,-1) node[anchor=north,box] (2) {};
  \draw[->] (2.mid) -- +(0,-1) node[anchor=north] {1};
  \draw[->] (2.two|-2.east) -- +(0,-1) node[anchor=north] {2};
  \draw[->] (1.two|-1.east) -- +(1,0) node[anchor=base west,box] (3) {};
  \draw[->] (3.mid) -- +(0,-1) node[anchor=north] {3};
  \draw[->] (3.two|-3.east) -- +(0,-1) node[anchor=north] {4};
\endtikzpicture