prime

elixir/nth-prime/lib/prime.ex

@@ -0,0 +1,30 @@
+defmodule Prime do
+  @doc """
+  Generates the nth prime.
+  """
+  @spec nth(non_neg_integer) :: non_neg_integer
+  def nth(count) when count >= 1 do
+    0
+    |> Stream.iterate(&(&1 + 1))
+    |> Stream.filter(&prime?/1)
+    |> Stream.take(count + 1)
+    |> Enum.to_list()
+    |> List.last()
+  end
+
+  @spec prime?(non_neg_integer) :: boolean
+  defp prime?(number) when number in 1..3, do: true
+  defp prime?(number) when rem(number, 2) == 0, do: false
+  defp prime?(number) when number > 3, do: check_prime(number)
+
+  defp check_prime(number, prime \\ 3)
+  defp check_prime(number, number), do: true
+
+  defp check_prime(number, prime) do
+    if prime >= trunc(number ** 0.5) + 1,
+      do: true,
+      else: rem(number, prime) != 0 && check_prime(number, prime + 2)
+  end
+
+  # defp check_prime(number, prime), do: rem(number, prime) != 0 && check_prime(number, prime + 2)
+end