fibMortal.py

def fibBase(n,m):
	"""
	Calcultates the number of rabbit pairs alive at month n assuming
	rabbits live for m months. 
	Input: Number of months n and rabbit lifespan m
	Output: Number of rabbit pairs alive at month n
	"""
	memo = {0:0,1:1} # dict of (month:# of pairs) 
	return fibMortal(n,m,memo)

def fibMortal(n, m, memo):
	if not n in memo:
		if(n <= m):
			memo[n] = fibMortal(n-1, m, memo) + fibMortal(n-2, m, memo) # assume normal fib
		elif (n == (m + 1)):
			memo[n] = fibMortal(n-1, m, memo) + fibMortal(n-2, m, memo) - 1 # first rabbit dies
		else: 
			memo[n] = (fibMortal(n-1, m,memo) + fibMortal(n-2, m, memo)) - (fibMortal(n-(m + 1), m, memo)) # continuous death
	return memo[n]

print fibBase(91,20)