improve functions definitions

lib/broadway_kafka/producer/buffer.ex

@@ -29,11 +29,13 @@ defmodule BroadwayKafka.Producer.Buffer do
 
   @spec enqueue_with_key(t, key, list :: [Message.t()], at :: :rear | :front) :: t
   @doc false
+  def enqueue_with_key(buffer, key, list, at \\ :rear)
+
   def enqueue_with_key(buffer, _key, [], _at) do
     buffer
   end
 
-  def enqueue_with_key(buffer, key, list, at \\ :rear) do
+  def enqueue_with_key(buffer, key, list, at) do
     if at == :rear do
       :queue.in({key, list}, buffer)
     else
@@ -85,19 +87,19 @@ defmodule BroadwayKafka.Producer.Buffer do
     end
   end
 
-  def split(list, count) do
+  defp split(list, count) do
     split_list(list, count, nil, 0, [])
   end
 
-  def split_list(rest, 0, last_item, length_of_new_list, acc) do
+  defp split_list(rest, 0, last_item, length_of_new_list, acc) do
     {Enum.reverse(acc), last_item, length_of_new_list, rest}
   end
 
-  def split_list([], _count, last_item, length_of_new_list, acc) do
+  defp split_list([], _count, last_item, length_of_new_list, acc) do
     {Enum.reverse(acc), last_item, length_of_new_list, []}
   end
 
-  def split_list([head | tail], count, _last_item, length_of_new_list, acc) do
+  defp split_list([head | tail], count, _last_item, length_of_new_list, acc) do
     split_list(tail, count - 1, head, length_of_new_list + 1, [head | acc])
   end
 end