Ruby Self-Modification Example - Memoize

ruby-memoization.rb

require 'benchmark'

class Fib
  def fib(n)
    return n if n < 2
    return fib(n-1) + fib(n-2)  
  end
end

def memoize(className, methodName)
  className.class_eval do 
    alias_method("original_memoize_#{methodName}", methodName)
    
    define_method(methodName) do |*args|
      hash = instance_variable_get("@memoize_#{methodName}_hash")
      
      if hash.nil?
        hash = Hash.new
        instance_variable_set("@memoize_#{methodName}_hash", hash) 
      end
      
      return hash[args] if hash.has_key?(args)
      res = self.send("original_memoize_#{methodName}", *args)
      hash[args] = res
      return res
    end
  end
end

if ARGV.length != 1
  puts "Usage: meta.rb <number>"
  exit
end

num = ARGV[0].to_i
puts "Running Fib with #{num}"

res1 = res2 = 0

a = Fib.new
puts "Without Memoization:"
puts Benchmark.measure {
  res1 = a.fib(num)
}

memoize(Fib, :fib)

a = Fib.new
puts "With Memoization:"
puts Benchmark.measure {
  res2 = a.fib(num)
}