Memoization with and without context

gistfile1.rb

# Original idea probably due to Robert Feldt
module Memoize
  def remember(name, options = {})
    memory = {}
    original_method = instance_method(name)
    
    define_method(name) do |*args|
      lookup = options[:context] ? (memory[self] ||= {}) : memory

      if lookup.has_key?(args)
        lookup[args]
      else
        original = original_method.bind(self)
        lookup[args] = original.call(*args)
      end
    end
  end
end      

class Discounter
  extend Memoize
  
  def discount(*skus) 
    expensive_discount_calculation(*skus)
  end                                       

  alias_method :discount2, :discount
  
  remember :discount
  remember :discount2, :context => true
  
  private
  
  def expensive_discount_calculation(*skus)
    puts "Expensive calculation for #{skus.inspect}"
    skus.inject {|m,n| m + n }
  end
end

puts '------- discount ---------'

puts 'Using d:'
d = Discounter.new
puts d.discount(1,2,3)
puts d.discount(1,2,3)
puts d.discount(2,3,4)
puts d.discount(2,3,4)

puts 'Now using e:'
e = Discounter.new
puts e.discount(1,2,3)
puts e.discount(1,2,3)
puts e.discount(2,3,4)
puts e.discount(2,3,4)

puts '------- discount2 ---------'

puts 'Using d:'
d = Discounter.new
puts d.discount2(1,2,3)
puts d.discount2(1,2,3)
puts d.discount2(2,3,4)
puts d.discount2(2,3,4)

puts 'Now using e:'
e = Discounter.new
puts e.discount2(1,2,3)
puts e.discount2(1,2,3)
puts e.discount2(2,3,4)
puts e.discount2(2,3,4)

------- discount ---------
Using d:
Expensive calculation for [1, 2, 3]
6
6
Expensive calculation for [2, 3, 4]
9
9
Now using e:
6
6
9
9
------- discount2 ---------
Using d:
Expensive calculation for [1, 2, 3]
6
6
Expensive calculation for [2, 3, 4]
9
9
Now using e:
Expensive calculation for [1, 2, 3]
6
6
Expensive calculation for [2, 3, 4]
9
9