vector2d.rb

class Vector2D < Vector
  class Pixel < Vector2D; end
  class Millimeter < Vector2D; end
  class Viewbox < Vector2D; end

  def initialize(*)
    super
    raise ArgumentError unless size == 2
  end

  # 自分と同じクラスか、無次元のVector2Dが相手であれば加算や乗算ができる
  # Millimeter * Pixel などはエラーになる
  concerning :SameUnit do
    def +(other)
      case other
      in self.class | Vector2D
        super(other)
      end
    end

    def *(other)
      case other
      in self.class | Vector2D
        super(other)
      end
    end
  end

  def x
    self[0]
  end

  alias width x

  def y
    self[1]
  end

  alias height y

  def to_s
    "#{x}x#{y}"
  end

  def to_h
    { x:, y: }
  end

  def to_a
    [x, y]
  end
end

def canvas_size_mm
  Vector2D::Millimeter[100, 200]
end