Add a generator of a SafeString.

lib/link_shortener/generators/safe_string.ex

@@ -0,0 +1,15 @@
+defmodule LinkShortener.Generators.SafeString do
+  @moduledoc """
+  This module provides a generator.
+  """
+
+  @doc """
+  Generate a random string with the length.
+  The string can be used as part of a url.
+  """
+  def generate(length) do
+    :crypto.strong_rand_bytes(length)
+    |> Base.url_encode64()
+    |> binary_part(0, length)
+  end
+end