Example: Fibonacci spiral

% Fibonacci spiral
% Author: Andrew Mertz
\documentclass{minimal}
\usepackage{amsmath,tikz}
\usetikzlibrary{backgrounds,calc}
\begin{document}
% I have seen many beautiful depictions of Fibonacci spirals and
% golden spirals. So I thought it would be nice to make a Fibonacci
% spiral in TikZ. I like the look of white on black so here I define a
% black background rectangle.
\begin{tikzpicture}[background rectangle/.style={fill=black},
                    show background rectangle] 
  % Create some counters for holding the Fibonacci numbers
  \newcounter{a}
  \newcounter{b}
  \newcounter{temp}

  % Initialize the counters
  \setcounter{a}{0}
  \setcounter{b}{1}

  % The spiral will start at the origin
  \coordinate (0) at (0,0);

  % This loop defines the number of turns in the spiral. Note that we
  % will have to be careful not to overflow our counters or make the
  % spiral too large for TeX to handle. This is easy to do as the
  % Fibonacci sequence grows exponentially.
  \foreach \i in {1,...,18}
  {
    % Get the "name" of the last point on the spiral
    \pgfmathsetmacro{\lastpoint}{\i-1}

    % Compute the angle for this turn of the spiral
    \pgfmathsetmacro{\startangle}{mod(\i-1,4) * 90}

    % Draw this turn of the spiral and remember the point where we end 
    \draw[white] (\lastpoint) arc 
      (\startangle : \startangle + 90 : \value{b}/10.0pt) coordinate (\i);

   % Compute the next Fibonacci number
    \setcounter{temp}{\value{b}}
    \addtocounter{b}{\value{a}}
    \setcounter{a}{\value{temp}}
 }

 % Add some framing for the spiral while at the same time not "boxing"
 % it in. Note that to put a square around each turn of the spiral we
 % could have just used the command \draw[white] (\lastpoint)
 % rectangle (\i); after drawing each turn in the loop above.
 \foreach \i in {1,3,...,17}
 {
   \pgfmathsetmacro{\lastpoint}{\i-1}
   \draw[white] (\lastpoint) -| (\i);
 }

 \foreach \i in {2,4,...,16}
 {
   \pgfmathsetmacro{\lastpoint}{\i-1}
   \draw[white] (\lastpoint) |- (\i);
 }

 \draw[white] (17) -- (17 |- 18);
 
 % Add some text displaying the formula for the Fibonacci numbers
 \node(eq) at ($(18) + (2.5,1)$) 
   [white,text width = 2cm,font=\fontsize{8}{8}\selectfont] {
   \begin{displaymath}
     f(n) = \left\{
       \begin{array}{lr}
         0 & \text{~~if } n = 0\\
         1 & \text{~~if } n = 1\\
         f(n-1) + f(n-2) & \text{~~if } n \geq 2\\
      \end{array}
     \right.
   \end{displaymath}
  };
\end{tikzpicture}
\end{document}
% LocalWords:  tikzpicture eq lr TikZ