fib_with_agents.exs

defmodule FibAgent do

  defstruct cache: nil, highest_n: 0

  def start_link do
    initial_cache = Enum.into([ {0, 0}, {1, 1}], HashDict.new)
    state = %__MODULE__{cache: initial_cache, highest_n: 1}
    Agent.start_link(fn -> state end, name: __MODULE__)
  end

  def fib(n) do
    Agent.update(__MODULE__, &fill_cache_up_to(&1, n))
    Agent.get(__MODULE__, fn state -> Dict.get(state.cache, n) end)
  end

  # If the cache is already full at least as far as n, no action is required
  def fill_cache_up_to(state = %__MODULE__{highest_n: highest_n}, n)
  when n <= highest_n,
  do: state

  # otherwise fill in entries from highest+1 to n
  def fill_cache_up_to(%__MODULE__{cache: cache, highest_n: highest_n}, n) do
    next  = highest_n + 1
    fib1  = Dict.get(cache, next - 2)
    fib2  = Dict.get(cache, next - 1)
    cache = Dict.put cache, next, fib1 + fib2
    fill_cache_up_to(%__MODULE__{cache: cache, highest_n: next}, n)
  end
end