chessboard.rb

class Color
  def initialize
    @colors = ["R", "G", "B"]  # => ["R", "G", "B"], ["R", "G", "B"]
  end

  def get
    c = @colors.first         # => "R", "G", "B", "R", "G", "B", "R", "G", "B", "R", "G", "B", "R", "G", "B", "R", "G", "B", "R", "G", "B", "R", "G", "B", "R"
    @colors = @colors.rotate  # => ["G", "B", "R"], ["B", "R", "G"], ["R", "G", "B"], ["G", "B", "R"], ["B", "R", "G"], ["R", "G", "B"], ["G", "B", "R"], ["B", "R", "G"], ["R", "G", "B"], ["G", "B", "R"], ["B", "R", "G"], ["R", "G", "B"], ["G", "B", "R"], ["B", "R", "G"], ["R", "G", "B"], ["G", "B", "R"], ["B", "R", "G"], ["R", "G", "B"], ["G", "B", "R"], ["B", "R", "G"], ["R", "G", "B"], ["G", "B", "R"], ["B", "R", "G"], ["R", "G", "B"], ["G", "B", "R"]
    c                         # => "R", "G", "B", "R", "G", "B", "R", "G", "B", "R", "G", "B", "R", "G", "B", "R", "G", "B", "R", "G", "B", "R", "G", "B", "R"
  end
end

class Chessboard

  def initialize(size)
    @board = Array.new(size) { Array.new(size,0) }  # => [[0, 0, 0], [0, 0, 0], [0, 0, 0]], [[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
    @size = size                                    # => 3, 4
    @color = Color.new                              # => #<Color:0x007fa7518bbcd0 @colors=["R", "G", "B"]>, #<Color:0x007fa7518b4e80 @colors=["R", "G", "B"]>
    @board[0][0] = @color.get                       # => "R", "R"
    @direction = :to_right                          # => :to_right, :to_right
    @x = 0                                          # => 0, 0
    @y = 1                                          # => 1, 1
  end

  def out_of_board?
    @x < 0 || @y < 0 || @board[@x].nil? || @board[@x][@y].nil? || ["R", "G", "B"].include?(@board[@x][@y])  # => false, false, true, false, false, true, false, false, true, false, true, false, false, false, false, true, false, false, false, true, false, false, false, true, false, false, true, false, false, true, false, true, false
  end

  def solve
    counter = 0                        # => 0, 0
    while counter < @size**2 - 1       # => true, true, true, true, true, true, true, true, true, true, true, true, false, true, true, true, true, true, true, true, true, true, true, true, true, true, true, true, true, true, true, true, true, true, false
      if @direction == :to_right       # => true, true, true, false, false, false, false, false, false, false, false, true, true, true, true, true, false, false, false, false, false, false, false, false, false, false, false, true, true, true, false, false, false
        if out_of_board?               # => false, false, true, false, false, false, false, true, false, false, true
          @direction = :to_bottom      # => :to_bottom, :to_bottom, :to_bottom
          @y -= 1                      # => 2, 3, 2
          @x += 1                      # => 1, 1, 2
        else
          @board[@x][@y] = @color.get  # => "G", "B", "B", "G", "B", "R", "R", "G"
          @y += 1                      # => 2, 3, 2, 2, 3, 4, 2, 3
          counter += 1                 # => 1, 2, 8, 1, 2, 3, 12, 13
        end                            # => 1, 2, 1, 8, 1, 2, 3, 1, 12, 13, 2
      elsif @direction == :to_bottom   # => true, true, true, false, false, false, false, false, true, true, true, true, false, false, false, false, false, false, false, true, true, false
        if out_of_board?               # => false, false, true, false, false, false, true, false, true
          @direction = :to_left        # => :to_left, :to_left, :to_left
          @x -= 1                      # => 2, 3, 2
          @y -= 1                      # => 1, 2, 1
        else
          @board[@x][@y] = @color.get  # => "R", "G", "G", "B", "R", "B"
          @x += 1                      # => 2, 3, 2, 3, 4, 3
          counter += 1                 # => 3, 4, 4, 5, 6, 14
        end                            # => 3, 4, 1, 4, 5, 6, 2, 14, 1
      elsif @direction == :to_left     # => true, true, true, false, false, true, true, true, true, false, false, false, true
        if out_of_board?               # => false, false, true, false, false, false, true, false
          @direction = :to_top         # => :to_top, :to_top
          @x -= 1                      # => 1, 2
          @y += 1                      # => 0, 0
        else
          @board[@x][@y] = @color.get  # => "B", "R", "G", "B", "R", "R"
          @y -= 1                      # => 0, -1, 1, 0, -1, 0
          counter += 1                 # => 5, 6, 7, 8, 9, 15
        end                            # => 5, 6, 0, 7, 8, 9, 0, 15
      elsif @direction == :to_top      # => true, true, true, true, true
        if out_of_board?               # => false, true, false, false, true
          @direction = :to_right       # => :to_right, :to_right
          @x += 1                      # => 1, 1
          @y += 1                      # => 1, 1
        else
          @board[@x][@y] = @color.get  # => "G", "G", "B"
          @x -= 1                      # => 0, 1, 0
          counter += 1                 # => 7, 10, 11
        end                            # => 7, 1, 10, 11, 1
      end                              # => 1, 2, 1, 3, 4, 1, 5, 6, 0, 7, 1, 8, 1, 2, 3, 1, 4, 5, 6, 2, 7, 8, 9, 0, 10, 11, 1, 12, 13, 2, 14, 1, 15
    end                                # => nil, nil
  end

  def to_s
    solve                                # => nil, nil
    @board.map { |l| l.join }.join "\n"  # => "RGB\nGBR\nRBG", "RGBR\nBRGG\nGRBB\nRBGR"
  end
end

puts Chessboard.new(3)  # => nil
puts ""                 # => nil
puts Chessboard.new(4)  # => nil

# >> RGB
# >> GBR
# >> RBG
# >>
# >> RGBR
# >> BRGG
# >> GRBB
# >> RBGR