feat: Add basic queueing functionality

lib/sergei/consumer.ex

@@ -14,9 +14,14 @@ defmodule Sergei.Consumer do
     opt.(3, "url", "URL of the audio to play", [])
   ]
 
+  @queue_opts [
+    opt.(3, "url", "URL of the audio to queue", required: true)
+  ]
+
   @slash_commands [
     {"ping", "Pong", []},
     {"play", "Play a song or resume playback", @play_opts},
+    {"queue", "Queue a song to play next", @queue_opts},
     {"pause", "Pause media playback", []},
     {"stop", "Stop media playback and leave the voice channel", []},
     {"song", "What song is currently playing?", []}
@@ -127,6 +132,24 @@ defmodule Sergei.Consumer do
     end
   end
 
+  # /queue <url>
+  def do_command(%{
+        guild_id: guild_id,
+        data: %{name: "queue", options: [%{name: "url", value: url}]}
+      }) do
+    case Sergei.Player.queue(guild_id, url) do
+      :ok ->
+        {:ok, "Song queued."}
+
+      :not_playing ->
+        {:ok, "I'm not playing anything right now."}
+
+      {:error, err} ->
+        Logger.error("Failed to queue media: #{err}")
+        {:error, "This is embarassing..."}
+    end
+  end
+
   # /pause
   def do_command(%{guild_id: guild_id, data: %{name: "pause"}}) do
     case Sergei.Player.pause(guild_id) do

lib/sergei/player.ex

@@ -24,6 +24,11 @@ defmodule Sergei.Player do
     GenServer.call(__MODULE__, {:play, guild_id, channel_id, url})
   end
 
+  @spec queue(integer(), String.t()) :: :ok | :not_playing | {:error, String.t()}
+  def queue(guild_id, url) do
+    GenServer.call(__MODULE__, {:queue, guild_id, url})
+  end
+
   @spec pause(integer()) :: :ok | :not_playing | {:error, String.t()}
   def pause(guild_id) do
     GenServer.call(__MODULE__, {:pause, guild_id})
@@ -50,8 +55,21 @@ defmodule Sergei.Player do
     state
     |> Enum.filter(fn {_id, %{paused: paused} = _state} -> not paused end)
     |> Enum.filter(fn {guild_id, _data} -> not Voice.playing?(guild_id) end)
-    |> Enum.each(fn {guild_id, %{url: url}} = _state ->
+    |> Enum.map(fn {guild_id, %{queue: queue} = state} ->
+      state =
+        case :queue.out(queue) do
+          {{:value, url}, queue} ->
+            %{state | url: url, queue: queue}
+
+          {:empty, _} ->
+            state
+        end
+
+      {guild_id, state}
+    end)
+    |> Enum.map(fn {guild_id, %{url: url}} = state ->
       Voice.play(guild_id, url, :ytdl)
+      state
     end)
 
     Process.send_after(self(), :check_repeat, @check_repeat_interval)
@@ -100,12 +118,28 @@ defmodule Sergei.Player do
     state =
       Map.put(state, guild_id, %{
         url: url,
-        paused: false
+        paused: false,
+        queue: :queue.new()
       })
 
     {:reply, res, state}
   end
 
+  @impl true
+  def handle_call({:queue, guild_id, url}, _from, state) do
+    %{queue: queue} = instance = Map.fetch!(state, guild_id)
+
+    {
+      :reply,
+      :ok,
+      Map.put(
+        state,
+        guild_id,
+        %{instance | queue: :queue.in(url, queue)}
+      )
+    }
+  end
+
   @impl true
   def handle_call({_, guild_id}, _from, state) when not is_map_key(state, guild_id) do
     {:reply, :not_playing, state}
@@ -113,13 +147,14 @@ defmodule Sergei.Player do
 
   @impl true
   def handle_call({:pause, guild_id}, _from, state) do
-    %{url: url} = Map.fetch!(state, guild_id)
+    %{url: url, queue: queue} = Map.fetch!(state, guild_id)
     Voice.pause(guild_id)
 
     state =
       Map.put(state, guild_id, %{
         url: url,
-        paused: true
+        paused: true,
+        queue: queue
       })
 
     {:reply, :ok, state}
@@ -127,13 +162,14 @@ defmodule Sergei.Player do
 
   @impl true
   def handle_call({:resume, guild_id}, _from, state) do
-    %{url: url} = Map.fetch!(state, guild_id)
+    %{url: url, queue: queue} = Map.fetch!(state, guild_id)
     Voice.resume(guild_id)
 
     state =
       Map.put(state, guild_id, %{
         url: url,
-        paused: false
+        paused: false,
+        queue: queue
       })
 
     {:reply, :ok, state}