Memoization by module mixin

memoizing_mixing_in_module.rb

module Memoize

  def remember(method)
    orig_method_name = "_original_#{method.to_s}_"
    alias_method orig_method_name.to_sym, method

    define_method(method) do |*args|
      @memory ||= {}
      @memory[args] = send(orig_method_name,args) unless @memory.has_key?(args)
      @memory[args]
    end

  end
end

class Discounter
  extend Memoize

  def discount(*skus)
    expensive_calc(skus)
  end

  remember :discount

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

d = Discounter.new

d.discount(1,2,3)
d.discount(1,2,3)
d.discount(4,2,3)
d.discount(4,2,3)