class Circle
  def initialize(radius)
    @radius = radius

    @points = []
    0.upto(radius) do |x|
      0.upto(radius / 2) do |y|
        @points << [x , y] if lies_on_circle?(x, y)
      end
    end
  end

  def draw
    unless @canvas
      @canvas = Array.new(diameter + 1){ Array.new(2 * (diameter + 1)){ ' ' } }
      @points.each do |p|
        set_symmetric_points(p[0], p[1])
      end
    end

    puts @canvas.map(&:join).join "\n"
  end

  def diameter
    @diameter ||= 2 * @radius
  end

  private

  # Detects if point is closest to the circle edge by checking the devitaion of points above and below
  def lies_on_circle?(x, y)
    cx, cy = @radius, @radius
    dx2, r2 = (x - cx) ** 2, @radius ** 2
    [y - 1, y, y + 1].min_by {|y| (dx2 + (y - cy) ** 2 - r2) ** 2 } == y
  end

  # Sets 8 points symmetrically to circle center
  def set_symmetric_points(x, y)
    [x, diameter - x].product([y, diameter - y]).each do |p|
      @canvas[p[0]][2 * p[1]] = @canvas[p[1]][2 * p[0]] = '*'
    end
  end
end

Circle.new(15).draw