using Tail Call Optimization pattern

fibonacci.ex

defmodule Fibonacci do
  def start(n) do
    start = :os.system_time(:seconds)
    fibonacciNumber = getNumber(n)
    finish = :os.system_time(:seconds)
    totalTime = finish - start
    IO.puts("The fibonacci number was #{fibonacciNumber}")
    IO.puts("Fibonacci for number placed #{n} finished in #{totalTime} seconds!")
  end

  def getNumber(n) when n < 0,
    do: raise("You're trying to find a Fibonacci number under zero. Please try again")

  def getNumber(n), do: getNumber(n, 1, 0)
  defp getNumber(0, _, result), do: result
  defp getNumber(n, next, result), do: getNumber(n - 1, next + result, next)
end