Factorial with tail-call optimization

Section

defmodule Math do
  def factorial(0), do: 0

  # Recursive factorial
  #
  # def factorial(n) do
  #  factorial(n - 1) * n
  # end

  # Tail-call optimization
  def factorial(n), do: factorial(n, 1)
  def factorial(1, acc), do: acc

  def factorial(n, acc) do
    factorial(n - 1, acc * n)
  end
end

ExUnit.start(autorun: false)

defmodule Math.Test do
  use ExUnit.Case, async: false

  test "Factorial from 0 to 5" do
    assert 0 == Math.factorial(0)
    assert 1 == Math.factorial(1)
    assert 2 == Math.factorial(2)
    assert 6 == Math.factorial(3)
    assert 24 == Math.factorial(4)
    assert 120 == Math.factorial(5)
  end
end

ExUnit.run()