Add Markov.ChainServer for Markov plugin

* Markov chains each have their own process keeping it updated and
  synchronized. The markov plugin has been upgraded to account for this.

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

lib/contrib/markov/markov.ex

@@ -1,12 +1,27 @@
 defmodule Omnibot.Contrib.Markov do
   use Omnibot.Plugin
-  alias Omnibot.{Contrib.Markov.Chain, Util}
+  alias Omnibot.{Contrib.Markov.ChainServer, Util}
   require Logger
 
-  @default_config path: "markov.ets", order: 2, save_every: 5 * 60
+  @default_config save_dir: "markov", order: 2, save_every: 5 * 60
+
+  @registry __MODULE__.Registry
+  @supervisor __MODULE__.ChainSupervisor
+
+  @impl true
+  def children(cfg) do
+    [
+      {Task, fn -> Stream.timer(cfg[:save_every] * 1000)
+        |> Stream.cycle()
+        |> Stream.each(fn _ -> save_chains() end)
+        |> Stream.run()
+      end},
+      {Registry, keys: :unique, name: @registry},
+      {DynamicSupervisor, name: @supervisor, strategy: :one_for_one},
+    ]
+  end
 
   command "!markov", ["force"] do
-    # Choose a random value from the sender
     Irc.send_to(irc, channel, "TODO")
   end
 
@@ -18,27 +33,8 @@ defmodule Omnibot.Contrib.Markov do
     Irc.send_to(irc, channel, "TODO")
   end
 
-  @impl true
-  def children(cfg) do
-    [
-      {Task, fn ->
-        Stream.timer(cfg[:save_every] * 1000)
-        |> Stream.cycle()
-        |> Stream.each(fn _ -> save_chains() end)
-        |> Stream.run()
-      end}
-    ]
-  end
-
-  @impl true
-  def on_init(_cfg) do
-    # Create the markov database
-    path = String.to_atom(cfg()[:path])
-    {:ok, db} = :dets.open_file(path, [])
-    chains = :ets.new(:markov_chains, [:named_table, :public])
-    :dets.to_ets(db, chains)
-    :ok = :dets.close(db)
-    chains
+  def save_dir() do
+    cfg()[:save_dir]
   end
 
   @impl true
@@ -47,39 +43,57 @@ defmodule Omnibot.Contrib.Markov do
   end
 
   def train(channel, user, msg) do
-    chain = (user_chain(channel, user) || create_user_chain(channel, user))
-            |> Chain.train(msg)
-    true = update_user_chain(channel, user, chain)
+    server = ensure_chain_server(channel, user)
+    ChainServer.train(server, msg)
+  end
+
+  def ensure_chain(channel, user) do
+    ensure_chain_server(channel, user)
+    |> ChainServer.chain()
   end
 
   def user_chain(channel, user) do
-    db = state()
-    case :ets.lookup(db, {channel, user}) do
+    chain_server(channel, user) |> ChainServer.chain()
+  end
+
+  def chain_server(:all) do
+    # See https://hexdocs.pm/elixir/Registry.html#select/2-examples to understand what the hell is going on here
+    # (it just selects the PID of all chain_server processes)
+    for {pid} <- Registry.select(@registry, [{{:_, :"$1", :_}, [], [{{:"$1"}}]}]),
+      do: pid
+  end
+
+  def chain_server(channel, user) do
+    case Registry.lookup(@registry, {channel, user}) do
       [] -> nil
-      [{{^channel, ^user}, chains}] -> chains
+      [{pid, _} | _] -> pid
     end
   end
 
-  def update_user_chain(channel, user, chain) do
-    db = state()
-    case user_chain(channel, user) do
-      nil -> :ets.insert_new(db, {{channel, user}, chain})
-      _old_chain -> :ets.insert(db, {{channel, user}, chain})
+  def ensure_chain_server(channel, user) do
+    case chain_server(channel, user) do
+      nil -> start_chain!(channel, user)
+      pid -> pid
     end
   end
 
-  defp create_user_chain(channel, user) do
-    true = update_user_chain(channel, user, %Chain{order: cfg()[:order]})
-    user_chain(channel, user)
+  defp start_chain!(channel, user) do
+    {:ok, chain} = start_chain(channel, user)
+    chain
+  end
+
+  defp start_chain(channel, user) do
+    DynamicSupervisor.start_child(
+      @supervisor,
+      {ChainServer, cfg: cfg(), channel: channel, user: user, name: {:via, Registry, {@registry, {channel, user}}}}
+    )
   end
 
   def save_chains() do
     start = Util.now_unix()
     Logger.debug("Saving markov chains")
 
-    {:ok, db} = :dets.open_file(cfg()[:path], [])
-    :ets.to_dets(state(), db)
-    :ok = :dets.close(db)
+    chain_server(:all) |> Enum.each(&ChainServer.save/1)
 
     stop = Util.now_unix()
     Logger.info("Saved markov chains in #{stop - start} seconds")