numbers.rb

require "ruby_badge_numbers/version"

module RubyBadgeNumbers
  # Your code goes here...
  class PositiveNumber
    attr_reader :zero, :pos

    def initialize(zero, pos)
      @pos = pos # su posicion en la cadena
      @zero = zero # de esta forma todos las intancias tienen referencia al mismo objeto zero
      @succ = nil
    end

    def succ
      @succ ||= PositiveNumber.new(@zero, @pos + 1)
    end

    def +(number)
      sum = @zero
      (self.pos + number.pos).times do
        sum = sum.succ
      end
      sum
    end

    def -(number)
      subs = @zero
      (self.pos - number.pos).times do
        subs = subs.succ
      end
      subs
    end
  end

  class Zero
    attr_reader :pos

    def initialize
      @pos = 0
      @succ = nil
    end

    def succ
      @succ ||= PositiveNumber.new(self, @pos + 1)
    end
  end
end