Fast Fibonacci implementation in Elixir

fib_agent.exs

# Fast Fibonacci implementation (using a Map to cache previous results)
# This code comes from a mailing-list post by José Valim.
#
# Run iex
# $ iex
# > c("fib_agent.exs")
# > { :ok, agent } = FibAgent.start_link()
# > IO.puts FibAgent.fib(agent, 38)

defmodule FibAgent do
  def start_link do
    cache = Enum.into([ { 0, 0 }, { 1, 1 }], Map.new)
    Agent.start_link(fn -> cache end)
  end

  def fib(pid, n) when n >= 0 do
    Agent.get_and_update(pid, &do_fib(&1, n))
  end

  defp do_fib(cache, n) do
    if cached = cache[n] do
      { cached, cache }
    else
      { val, cache } = do_fib(cache, n - 1)
      result = val + cache[n - 2]
      { result, Map.put(cache, n, result)}
    end
  end
end