# L-10 MCS 275 Wed 3 Feb 2010 : memofib.py

# Memoization is a optimization technique to speedup programs by
# storing results of previous function calls.

# We illustrate how to memoize a recursive function to compute
# Fibonacci numbers using a dictionary.

from sys import setrecursionlimit

class MemoizedFibonacci():
   """
   The class exports a data attribute D that stores the results
   of all calls made to the functional attribute F.
   """

   def __init__(self):
      """
      Initializes the dictionary.
      """
      self.D = {}

   def F(self,n):
      """
      returns the n-th Fibonacci number
      """
      if self.D.has_key(n):
         return self.D[n]
      else:
         if n == 0:
            R = 0
         elif n == 1:
            R = 1
         else:
            R = self.F(n-1) + self.F(n-2)
         self.D[n] = R
         return R

def main():
   """
   Prompts the user for a number n,
   instantiates an object m of the class MemoizedFibonacci
   and applies the method F to the object m and shows m.D.
   """
   m = MemoizedFibonacci()
   n = input('Give a number : ')
   setrecursionlimit(n+2)
   f = m.F(n)
   print 'The %d-th Fibonacci number is %d' % (n,f)
   print 'The dictionary of function calls :'
   print m.D

if __name__=="__main__": main()