In terminal, run: ruby turtle_tracks.rb, then type simple.txt or complex.txt, then open output.txt. More info at: http://www.puzzlenode.com/puzzles/9-turtle-tracks

complex.txt

1001

REPEAT 4 [ FD 200 LT 45 FD 100 LT 45 FD 50 LT 45 FD 25 LT 45 FD 12 LT 45 FD 6 LT 45 RT 90 LT 45 BK 6 RT 90 FD 6 LT 45 RT 90 LT 45 BK 6 LT 45 BK 12 RT 90 FD 12 LT 45 FD 6 LT 45 RT 90 LT 45 BK 6 RT 90 FD 6 LT 45 RT 90 LT 45 BK 6 LT 45 BK 12 LT 45 BK 25 RT 90 FD 25 LT 45 FD 12 LT 45 FD 6 LT 45 RT 90 LT 45 BK 6 RT 90 FD 6 LT 45 RT 90 LT 45 BK 6 LT 45 BK 12 RT 90 FD 12 LT 45 FD 6 LT 45 RT 90 LT 45 BK 6 RT 90 FD 6 LT 45 RT 90 LT 45 BK 6 LT 45 BK 12 LT 45 BK 25 LT 45 BK 50 RT 90 FD 50 LT 45 FD 25 LT 45 FD 12 LT 45 FD 6 LT 45 RT 90 LT 45 BK 6 RT 90 FD 6 LT 45 RT 90 LT 45 BK 6 LT 45 BK 12 RT 90 FD 12 LT 45 FD 6 LT 45 RT 90 LT 45 BK 6 RT 90 FD 6 LT 45 RT 90 LT 45 BK 6 LT 45 BK 12 LT 45 BK 25 RT 90 FD 25 LT 45 FD 12 LT 45 FD 6 LT 45 RT 90 LT 45 BK 6 RT 90 FD 6 LT 45 RT 90 LT 45 BK 6 LT 45 BK 12 RT 90 FD 12 LT 45 FD 6 LT 45 RT 90 LT 45 BK 6 RT 90 FD 6 LT 45 RT 90 LT 45 BK 6 LT 45 BK 12 LT 45 BK 25 LT 45 BK 50 LT 45 BK 100 RT 90 FD 100 LT 45 FD 50 LT 45 FD 25 LT 45 FD 12 LT 45 FD 6 LT 45 RT 90 LT 45 BK 6 RT 90 FD 6 LT 45 RT 90 LT 45 BK 6 LT 45 BK 12 RT 90 FD 12 LT 45 FD 6 LT 45 RT 90 LT 45 BK 6 RT 90 FD 6 LT 45 RT 90 LT 45 BK 6 LT 45 BK 12 LT 45 BK 25 RT 90 FD 25 LT 45 FD 12 LT 45 FD 6 LT 45 RT 90 LT 45 BK 6 RT 90 FD 6 LT 45 RT 90 LT 45 BK 6 LT 45 BK 12 RT 90 FD 12 LT 45 FD 6 LT 45 RT 90 LT 45 BK 6 RT 90 FD 6 LT 45 RT 90 LT 45 BK 6 LT 45 BK 12 LT 45 BK 25 LT 45 BK 50 RT 90 FD 50 LT 45 FD 25 LT 45 FD 12 LT 45 FD 6 LT 45 RT 90 LT 45 BK 6 RT 90 FD 6 LT 45 RT 90 LT 45 BK 6 LT 45 BK 12 RT 90 FD 12 LT 45 FD 6 LT 45 RT 90 LT 45 BK 6 RT 90 FD 6 LT 45 RT 90 LT 45 BK 6 LT 45 BK 12 LT 45 BK 25 RT 90 FD 25 LT 45 FD 12 LT 45 FD 6 LT 45 RT 90 LT 45 BK 6 RT 90 FD 6 LT 45 RT 90 LT 45 BK 6 LT 45 BK 12 RT 90 FD 12 LT 45 FD 6 LT 45 RT 90 LT 45 BK 6 RT 90 FD 6 LT 45 RT 90 LT 45 BK 6 LT 45 BK 12 LT 45 BK 25 LT 45 BK 50 LT 45 BK 100 LT 45 BK 200 RT 90 ]

simple.txt

61

RT 135
FD 5
REPEAT 2 [ RT 90 FD 15 ]
RT 90
FD 5
RT 45
FD 20

turtle_tracks.rb

class TurtleTracks

  def initialize(options = {})
    @inputs = parse_file(options[:file])
    @size = @inputs.shift.to_i
    @commands = @inputs
    @cursor = starting_point
    @facing_degrees = 0
  end


  def run
    center_turtle
    run_commands(@commands)
    output_to_file
  end

private

  def parse_file(file)
    array = []
    File.open(file).each do |line|
      val = line.chop.split("\t")
      array << val.first unless val.empty?
    end
    array
  end

  def run_commands(commands)
    commands.each do |command|
      args = command.split(" ")
      method = args.shift
      args = args.first.to_i unless method == "REPEAT"
      send(method.downcase, args)
    end
  end

  def output_to_file
    file = open("output.txt","w")
    file.write(to_s)
    file.close()
  end

  def grid
    @grid ||= Array.new(@size ** 2, ".")
  end

  def directions
    { 0 => -@size,
      45 => -@size + 1,
      90 => 1,
      135 => @size + 1,
      180 => @size,
      225 => @size - 1,
      270 => -1,
      315 => -@size - 1  }
  end

  def starting_point
    grid.size / 2
  end

  def center_turtle
    grid[starting_point] = "X"
  end

  def to_s
    out = ""
    grid.each_with_index do |i,n|
      out << sprintf("%s%s", @grid[n], (n+1) % @size == 0 ? "\n" : " ")
    end
    out
  end

  def rt(degrees)
    @facing_degrees += degrees
    @facing_degrees -= 360 if @facing_degrees >= 360
  end

  def lt(degrees)
    @facing_degrees -= degrees
    @facing_degrees += 360 if @facing_degrees < 0
  end

  def fd(dist)
    cursor_change = directions[@facing_degrees]
    dist.times do
      @cursor += cursor_change
      grid[@cursor] = "X"
    end
  end

  def bk(dist)
    about_face
    fd(dist)
    about_face
  end

  def about_face
    @facing_degrees += (@facing_degrees < 180) ? 180 : -180
  end

  def repeat(args)
    number_times = args.shift.to_i
    args = args[1..-2]
    commands = []
    args.each_with_index do |x,i|
      commands << (args[i-1] + " " + args[i]) if i.odd?
    end
    number_times.times do |i|
      run_commands(commands)
    end
  end

end

puts "Name of input file (for example: simple.txt)"
file = gets.strip

turtle_tracks = TurtleTracks.new(file: file)
turtle_tracks.run