OO solution to diamond kata

diamond.rb

require "forwardable"

class Diamond
  class Canvas
    def initialize(size)
      @size = size
    end

    def []=(x, y, value)
      lines[size - y][x + size] = value
    end

    def to_s
      lines.join("\n")
    end

    private

    attr_reader :size

    def lines
      @lines ||= Array.new(width) { " " * width }
    end

    def width
      size * 2 + 1
    end
  end

  class MirroredCanvas
    extend Forwardable

    def initialize(x_factor, y_factor, canvas)
      @canvas = canvas
      @x_factor = x_factor
      @y_factor = y_factor
    end

    def []=(x, y, value)
      canvas[x, y] = value
      canvas[x * x_factor, y * y_factor] = value
    end

    def_delegator :canvas, :to_s

    private

    attr_reader :canvas, :x_factor, :y_factor
  end

  ORIGIN = "A"

  def initialize(target)
    @target = target
  end

  def to_s
    draw
    canvas.to_s
  end

  private

  attr_reader :target

  def draw
    (ORIGIN..target).zip(0.upto(size), size.downto(0)).each do |letter, x, y|
      canvas[x, y] = letter
    end
  end

  def canvas
    @canvas ||=
      MirroredCanvas.new(
        -1, 1,
        MirroredCanvas.new(
          1, -1,
          Canvas.new(size)))
  end

  def size
    @size ||= target.ord - ORIGIN.ord
  end
end

puts Diamond.new(ARGV.fetch(0))