class FibonacciIterator(object):
	def __init__(self, max_n):
		self._precalculated = {0: 1, 1: 1, 2: 2, 3: 3, 4: 5}
		self._biggest = 4
		self._max_n = max_n
		self._i = 0

	def get_biggest_pair(self):
		return ((self._biggest - 1, self._precalculated[self._biggest - 1])
			, (self._biggest, self._precalculated[self._biggest]))
	def fibonacci_element(self, n):
		if(not isinstance(n, int)):
			raise TypeError("n must be integer")
		if(0 <= n <= self._biggest):
			return self._precalculated[n]

		(n_start_minus_one, b), (n_start, a) = self.get_biggest_pair()

		for i in range(n_start, n):
			swp = a
			a += b
			b = swp
			self._precalculated[n] = a

		self._biggest = n
		return a

	def __contains__(self, n):
		if(n > self._max_n):
			raise IndexError("{} > max_n({})".format(n, self._max_n))
		return n <= self._biggest

	def __getitem__(self, n):
		if(n > self._max_n):
			raise IndexError("{} > max_n({})".format(n, self._max_n))
		return self.fibonacci_element(n)

	def __iter__(self):
		self._i = -1
		return self

	def __next__(self):
		if(self._i > self._max_n):
			raise StopIteration()
		self._i += 1
		return self.fibonacci_element(self._i)

if( __name__ == "__main__"):
	i = FibonacciIterator(20)

	for j in i:
		print(j)

	print(15 in i)
	print(i[14])