Move more stuff from other places into Omnibot.Core

* Present rooms are tracked by Omnibot.Core now, instead of
  Omnibot.State
* Channel synchronization is done through Omnibot.Core instead of
  Omnibot.Irc
* Add Omnibot.Module.on_init/1 callback, which returns a "state" value
  for the module to keep track of. By default, the state is nil.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

lib/core.ex

@@ -2,42 +2,71 @@ defmodule Omnibot.Core do
   use Omnibot.Module
   alias Omnibot.State
 
+  @impl true
   def on_join(irc, channel, nick) do
     cfg = State.cfg()
     if nick == cfg.nick do
-      State.add_channel(channel)
+      add_channel(channel)
       # Sync if we join a channel we shouldn't be in
       if !Enum.member?(State.all_channels(), channel),
-        do: Irc.sync_channels(irc)
+        do: sync_channels(irc)
     end
   end
 
+  @impl true
   def on_part(irc, channel, nick) do
     cfg = State.cfg()
     if nick == cfg.nick do
-      State.remove_channel(channel)
+      remove_channel(channel)
       # Sync if we join a channel we forcibly part a channel we shouldn't leave
       if Enum.member?(State.all_channels(), channel),
-        do: Irc.sync_channels(irc)
+        do: sync_channels(irc)
     end
   end
 
+  @impl true
   def on_kick(irc, channel, _nick, target) do
     cfg = State.cfg()
     if target == cfg.nick do
-      State.remove_channel(channel)
+      remove_channel(channel)
       # Generally, being kicked is not intentionally leaving a channel, so always sync here
-      Irc.sync_channels(irc)
+      sync_channels(irc)
     end
   end
 
   @impl true
   def on_msg(irc, msg) do
     case String.upcase(msg.command) do
-      "001" -> Irc.sync_channels(irc)
+      "001" -> sync_channels(irc)
       "PING" -> Irc.send_msg(irc, "PONG", msg.params)
       _ -> route_msg(irc, msg)
     end
   end
+
+  @impl true
+  def on_init(_cfg) do
+    MapSet.new()
+  end
+
+  defp sync_channels(irc) do
+    desired = MapSet.new(State.all_channels())
+    present = state()
+
+    to_join = MapSet.difference(desired, present)
+      |> MapSet.to_list()
+    to_part = MapSet.difference(present, desired)
+      |> MapSet.to_list()
+
+    Enum.each(to_join, fn channel -> Irc.join(irc, channel) end)
+    Enum.each(to_part, fn channel -> Irc.part(irc, channel) end)
+  end
+
+  defp add_channel(channel) do
+    update_state(fn state -> MapSet.put(state, channel) end)
+  end
+  
+  defp remove_channel(channel) do
+    update_state(fn state -> MapSet.delete(state, channel) end)
+  end
 end