"""This module contains code from
Think Python by Allen B. Downey
http://thinkpython.com

Copyright 2012 Allen B. Downey
License: GNU GPLv3 http://www.gnu.org/licenses/gpl.html

"""

cache = {}

def ackermann(m, n):
    """Computes the Ackermann function A(m, n)

    See http://en.wikipedia.org/wiki/Ackermann_function

    n, m: non-negative integers
    """
    if m == 0:
        return n+1
    if n == 0:
        return ackermann(m-1, 1)
    try:
        return cache[m, n]
    except KeyError:
        cache[m, n] = ackermann(m-1, ackermann(m, n-1))
        return cache[m, n]

print ackermann(3, 4)
print ackermann(3, 6)