Erlang Fast Fibonacci

fibonacci.erl

-module(fibonacci).

-export([fast_fib/1]).
-export([slow_fib/1]).

-spec slow_fib(integer()) -> integer().
-spec fast_fib(integer()) -> integer().

% Finds the Nth fibonacci number using standard recursion
% Gets unbearably slow to find around when N = 45
slow_fib(0) -> 0;
slow_fib(1) -> 1;
slow_fib(N) -> slow_fib(N - 1) + slow_fib(N - 2).

% Finds the Nth fibonacci number by counting up from 1 and adding the two
% previously found sums together
% Takes 10 seconds to find when N = 1,000,000
fast_fib(N) -> fast_fib(3, N, {1, 1}).
fast_fib(Step, Max, {_, X1}) when Step == Max + 1 -> X1;
fast_fib(Step, Max, {X0, X1}) -> fast_fib(Step + 1, Max, {X1, X0 + X1}).

Wow! This swept me off my feet. Don't tell me you're new to erlang

I would love to see your solution for the pieces problem