prime-factors

elixir/prime-factors/lib/prime_factors.ex

@@ -0,0 +1,22 @@
+defmodule PrimeFactors do
+  @doc """
+  Compute the prime factors for 'number'.
+
+  The prime factors are prime numbers that when multiplied give the desired
+  number.
+
+  The prime factors of 'number' will be ordered lowest to highest.
+  """
+  @spec factors_for(pos_integer) :: [pos_integer]
+  def factors_for(number), do: do_factors_for(number)
+
+  defp do_factors_for(number, acc \\ [], divisor \\ 2)
+  defp do_factors_for(1, acc, _divisor), do: Enum.reverse(acc)
+  defp do_factors_for(number, acc, divisor) when rem(number, divisor) == 0 do
+    case divisor < trunc(number ** 0.5) + 1 do
+      true -> do_factors_for(div(number, divisor), [divisor | acc])
+      false -> Enum.reverse([divisor | acc])        
+    end
+  end
+  defp do_factors_for(number, acc, divisor), do: do_factors_for(number, acc, divisor + 1)
+end