memoization with sub-class

moving_memoization_to_subclass.rb

class Discounter
  def discount(*skus)
    expensive_calc(skus)
  end

  private
  def expensive_calc(*skus)
    puts "in expensive calc for #{skus}"
    skus.inject{|m,n| m + n}
  end
end

class MemoDiscounter < Discounter
  def discount(*skus)
    @memory ||= {}
    @memory[skus] = super unless @memory.has_key?(skus)
    @memory[skus]
  end
end

d = MemoDiscounter.new
d.discount(1,2,3)
d.discount(1,2,3)
d.discount(4,2,3)
d.discount(4,2,3)