Add types and specs for LastFm module

lib/last_fm/feed.ex

@@ -6,11 +6,14 @@ defmodule LastFm.Feed do
   is technically prone to collision, it's very unlikely for that to happen due to the
   nature of the data.
   """
+
+  @spec create_table!() :: :ok | no_return
   def create_table! do
     __MODULE__ = :ets.new(__MODULE__, [:ordered_set, :named_table, :public])
     :ok
   end
 
+  @spec update([LastFm.Track.t()]) :: :ok | no_return
   def update(tracks) do
     data = Enum.map(tracks, fn t -> {t.scrobbled_at_uts, t} end)
 
@@ -20,6 +23,7 @@ defmodule LastFm.Feed do
     Phoenix.PubSub.broadcast(LastFm.PubSub, "feed:update", %{tracks: tracks})
   end
 
+  @spec all() :: [LastFm.Track.t()]
   def all do
     m = [
       {
@@ -29,9 +33,11 @@ defmodule LastFm.Feed do
       }
     ]
 
+    # reversing to get tracks in DESC order
     :ets.select_reverse(__MODULE__, m)
   end
 
+  @spec subscribe() :: :ok
   def subscribe do
     Phoenix.PubSub.subscribe(LastFm.PubSub, "feed:update")
   end