Example: Towers of Hanoi

Published 2010-05-25 | Authors: Berteun Damman, Martin Hofmann

Towers of Hanoi illustrated and computed by TeX. The problem is solved in TeX and for every move the situation is drawn. The idea and visualization were by Martin Hofmann, Berteun Damman programmed the actual recursion.

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%Towers of Hanoi illustrated and computed by TeX. The problem is solved in
%TeX and for every move the situation is drawn. The idea and visualization were
%by Martin Hofmann, Berteun Damman programmed the actual recursion.
%
% Authors: Martin Hofmann and Berteun Damman

% This is needed for a hyperref/beamer interaction bug
\RequirePackage{atbegshi}
\documentclass{beamer}
\usepackage{scalefnt}

\hypersetup{pdfpagemode=FullScreen}
\usetheme{Warsaw}

\usepackage{tikz}

% The logic for Hanoi, we record the discs at every pole
% as a comma separated list ending with a '.'; i.e. the
% starting list for 4 discs would be 1,2,3,4,.
\newcount\ndiscs
\def\initpoles#1{
\def\disclist{}
\foreach \n in {1,...,#1} {
\xdef\disclist{\disclist\n,}
}
\expandafter\xdef\csname pole 1\endcsname{\disclist.}
\expandafter\gdef\csname pole 2\endcsname{.}
\expandafter\gdef\csname pole 3\endcsname{.}
}

% Delimited macro; #1 is everything up to the first ',' and
% #2 everything after it.
\def\tail#1,#2.{#2}

% This macro updates the disc lists, its arguments are the name
% names of the macro's corresponding to the poles, for example
% 'pole 1' and 'pole 3'.
\def\movedisc#1#2{
\edef\lista{\csname #1\endcsname}
\edef\listb{\csname #2\endcsname}
\expandafter\xdef\csname #1\endcsname{\expandafter\tail\lista.}
}

% Updates the lists and then draws a new frame.
\def\move#1#2{
\movedisc{pole #1}{pole #2}
\gdef\fmsg{\node[anchor=north] at (3.8,-.5) {Moved disc from pole #1 to pole #2.};}
\drawpoles
}

% This macro boils down to a well-known recursive solution, as given
% here for example: http://en.wikipedia.org/wiki/Towers_of_Hanoi#Recursive_solution
%
% #1 Pole to move from
% #2 Pole to move to
% #3 Pole to use as scratch
% #4 Number of disks
\def\rhanoi#1#2#3#4{
\ifnum#4>1
\move{#1}{#3}
\else
\move{#1}{#3}
\fi
}

% Below is the TikZ code to draw the towers:
\tikzset{
inner color=#1!20, outer color=#1!60!gray},
disc 1/.style={disc=yellow, minimum width=15mm},
disc 2/.style={disc=orange, minimum width=20mm},
disc 3/.style={disc=red, minimum width=25mm},
disc 4/.style={disc=green, minimum width=30mm},
disc 5/.style={disc=blue, minimum width=35mm},
disc 6/.style={disc=purple, minimum width=40mm},
}

% Define some colors, I don't like plain green and brown.
\definecolor{darkgreen}{rgb}{0.2,0.55,0}
\definecolor{darkbrown}{rgb}{0.375,0.25,0.125}

\newcommand{\pole}{
\fill[darkbrown] (-1.6cm, 0) rectangle (1.6cm,0.25cm)
(-1.25mm,2.5mm) rectangle (1.25mm,4.25cm);
}

% Because the list starts with the topmost disc, we
% use two recursive macro's to invert the drawing process.
\newcount\curlevel

% This macro checks whether the list is empty, if not,
% it calls \rdrawdiscs which removes one element and
% calls this one again.
\def\drawdiscs#1.{
% If #1 is empty, this expands to \if.. which is true, otherwise
% we're safe to assume there's at least one element.
\expandafter\if#1..\else
\rdrawdiscs#1.
\fi
}

\def\rdrawdiscs#1,#2.{
\drawdiscs#2.
{\edef\n{\the\curlevel}
% Draw the actual disk.
\node[disc #1,yshift={\n*5mm}] {#1};
}
}

\def\discs#1{
\curlevel=1
\expandafter\drawdiscs#1
}

% Draws the whole situation based on the lists.
\def\drawpoles{
\begin{frame}{\ftitle}
\begin{tikzpicture}
\foreach \n/\x in {1/0cm,2/3.8cm,3/7.6cm} {
\begin{scope}[xshift=\x]
\pole
\expandafter\discs\csname pole \n\endcsname
\end{scope}
}
% Macro that either contains something like OK or
% the last move.
\fmsg
% We use this to prevent the picture from jumping between
% frames.
\useasboundingbox (-1.6cm,-1.2cm) rectangle (9.2cm,4.25cm);
\end{tikzpicture}
\end{frame}
}

% Main macro, inits the lists for the current number, sets a title
% for the frame and starts the recursion.
\def\hanoi#1{
\ndiscs=#1
\initpoles{#1}
\ifnum\ndiscs=1
\xdef\ftitle{Tower of Hanoi -- \the\ndiscs\ Disc}
\else
\xdef\ftitle{Tower of Hanoi -- \the\ndiscs\ Discs}
\fi
\gdef\fmsg{}
% Starting frame.
\drawpoles
% Recursion draws a new frame for every step.
\rhanoi{1}{2}{3}{\ndiscs}
% Final frame.
\gdef\fmsg{\node[] at (3.8,2.5) {\Huge\scalefont{2}\color{darkgreen}OK};}
\drawpoles
}

\begin{document}
\hanoi{1}
\hanoi{2}
\hanoi{3}
\hanoi{4}

% For 5 discs we don't show everything (although it could be done).
\ndiscs=5
\initpoles{5}
\def\ftitle{Tower of Hanoi -- \the\ndiscs\ Disc}
\def\fmsg{\node[cloud, draw, fill=gray!20, aspect=2] at (3.8,2.5) {\Huge\scalefont{2}?};}
\drawpoles
\end{document}

• #1 Sebastian, May 29, 2012 at 3:50 p.m.

For some reason this does not work when trying to compile. (atbegshi.sty not found)

• #2 Mr. Platypus, June 27, 2012 at 6:24 p.m.

@Sebastian: Then install the package. :-) 