add factorial

lib/util/math.ex

@@ -33,4 +33,26 @@ defmodule AoC.Util.Math do
   def lcm(a, b) do
     div(a * b, gcd(a, b))
   end
+
+  @doc """
+  Returns the facorial of n.
+
+  iex> AoC.Util.Math.factorial(0)
+  1
+  iex> AoC.Util.Math.factorial(1)
+  1
+  iex> AoC.Util.Math.factorial(2)
+  2
+  iex> AoC.Util.Math.factorial(5)
+  120
+  iex> AoC.Util.Math.factorial(-1)
+  nil
+  """
+  def factorial(0), do: 1
+  def factorial(1), do: 1
+  def factorial(n) when n < 0, do: nil
+  def factorial(n), do: factorial(n-1, n)
+
+  defp factorial(1, k), do: k
+  defp factorial(n, k), do: factorial(n-1, k*n)
 end

test/util/math_test.exs

@@ -0,0 +1,4 @@
+defmodule AoC.Util.MathTest do
+  use ExUnit.Case
+  doctest AoC.Util.Math
+end