The BrickpigScript class for creating and evaluating random arithmetic expressions in a simple Cartesian GP language, and some accompanying files from the book I'm working on.

brickpig.rb

class Float
  alias :add :+
  alias :subtract :-
  alias :multiply :*
  alias :negate :-@
  def divide(arg2)
    arg2==0 ? 0.0 : self / arg2
  end
end

module Brickpig
  class BrickpigCell
    attr_accessor :current_value, :script, :inputs
    
    def initialize(script)
      @script = script
      @inputs = script.scan(/cells\[([0-9]+)\]/).flatten.collect {|link| link.to_i}
      @current_value = 0.0
    end
  end
  
  
  class BrickpigScript
    attr_accessor :script
    
    FUNCTION_SET = [
      "constant",
      "cells[#].negate",
      "cells[#].add( cells[#] )", "cells[#].subtract( cells[#] )",
        "cells[#].multiply( cells[#] )", "cells[#].divide( cells[#] )"]
    
    
    def initialize(number_of_cells=20)
      script = "x\n"
      number_of_cells.times do 
        cell_function = FUNCTION_SET.sample
        cell_function = cell_function.gsub("#") {|v| Random.rand(number_of_cells).to_s}
        cell_function = cell_function.gsub("constant") {(Random.rand()*20-10).round(3)}
        script += "#{cell_function}\n"
      end
      @script = script
    end
    
    
    def evaluate(x, iterations = 1)
      x = x.to_f
      graph = script.split("\n").collect {|line| BrickpigCell.new(line)}
      cells = graph.collect {|node| node.current_value}
      iterations.times do
        graph.each_with_index do |node,idx|
          old_value = cells[idx]
          result = self.instance_eval(node.script)
          cells[idx] = result
        end
      end
      return cells[-1]
    end
  end
  
  # brick = BrickpigScript.new(20)
  # (-5..5).each do |x|
  #   puts (1..30).inject("#{x}") {|line,iters| line + "\t#{brick.evaluate(x, iters).round(5)}"}
  # end
end

example_2.rb

require "./brickpig"
include Brickpig

module Brickpig
  class BrickpigScript
  
    def as_dotfile
      line_count = script.lines.to_a.length-1
      dotfile = "digraph Brickpig {"
      @script.split("\n").each_with_index do |cell,idx|
        links = cell.scan(/cells\[([0-9]+)\]/).flatten
        unless links == []
          dotfile += " node_#{idx} [label=\"#{idx}\\n #{cell}\", rank=#{idx}, shape=box]; "
          links.each {|link| dotfile += " node_#{link} -> node_#{idx};"}
        else
          dotfile +=" node_#{idx} [rank=min, shape=invhouse, label=\"#{idx}\\n #{cell}\", style=filled, color=\"palegreen\"];"
        end
      end
      dotfile += " node_#{line_count}->output; output [rank=max,shape=house,label=\"RESULT\",style=filled, color=\"coral2\"]; }"
    end
  end
end

# brick = BrickpigScript.new(30)
# puts brick.as_dotfile