fib_stream.ex

defmodule Fibonacci do
  defrecordp :fib_state, Fibonacci, current: 0, previous1: nil, previous2: nil

  def new do
    fib_state()
  end

  def value(fib_state(current: current)) do
    current
  end

  def next(state) do
    state
    |> shift
    |> compute_current
  end

  defp shift(fib_state(current: current, previous1: previous1) = state) do
    fib_state(state, previous1: current, previous2: previous1)
  end

  defp compute_current(fib_state(previous2: nil) = state) do
    fib_state(state, current: 1)
  end

  defp compute_current(fib_state(previous1: previous1, previous2: previous2) = state) do
    fib_state(state, current: (previous1 || 0) + (previous2 || 0))
  end

  def lazy(initial_state // fib_state()) do
    Stream.iterate(initial_state, next(&1))
  end
end

# Basic usage
fib0 = Fibonacci.new
fib1 = Fibonacci.next(fib0)
fib2 = Fibonacci.next(fib1)
fib3 = Fibonacci.next(fib2)
IO.inspect fib3
IO.puts ""


# Lazy sequence
Fibonacci.lazy
|> Enum.take(5)
|> IO.inspect
IO.puts ""

# Advanced usage:
Fibonacci.lazy
|> Stream.drop(3)                                           # drop first three
|> Enum.first                                               # take the first
|> Fibonacci.lazy                                           # start lazy from it
|> Stream.filter(&(rem(Fibonacci.value(&1), 2) == 1))       # take only odd values
|> Stream.map(&(Fibonacci.value(&1)))                       # extract values from it
|> Enum.take(10)                                            # take first 10
|> IO.inspect                                               # print it