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/chain_server.ex

@@ -0,0 +1,86 @@
+defmodule Omnibot.Contrib.Markov.ChainServer do
+  use GenServer
+  alias Omnibot.Contrib.Markov
+  require Logger
+
+  ## Client API
+
+  def start_link(opts) do
+    {cfg, opts} = Keyword.pop(opts, :cfg)
+    {channel, opts} = Keyword.pop(opts, :channel)
+    {user, opts} = Keyword.pop(opts, :user)
+
+    chain = case load(channel, user) do
+      {:ok, chain} -> chain
+      {:error, _} -> %Markov.Chain{order: cfg[:order]}
+    end
+    GenServer.start_link(__MODULE__, {chain, channel, user}, opts)
+  end
+
+  @compile :inline
+  def user_path(channel, user), do: Path.join(channel_dir(channel), "#{user}.chain")
+
+  @compile :inline
+  def channel_dir(channel), do: Path.join(Markov.save_dir(), channel)
+
+  def load(channel, user) do
+    with {:ok, contents} <- user_path(channel, user) |> File.read(),
+         do: {:ok, :erlang.binary_to_term(contents)}
+  end
+
+  def save(server) do
+    GenServer.call(server, :save)
+  end
+
+  def train(server, msg) do
+    GenServer.call(server, {:train, msg})
+  end
+
+  def chain(server) do
+    GenServer.call(server, :chain)
+  end
+
+  def channel(server) do
+    GenServer.call(server, :channel)
+  end
+
+  def user(server) do
+    GenServer.call(server, :user)
+  end
+
+  ## Server callbacks
+
+  @impl true
+  def init({chain, channel, user}) do
+    {:ok, {chain, channel, user}}
+  end
+
+  @impl true
+  def handle_call(:save, _from, state = {chain, channel, user}) do
+    File.mkdir_p!(channel_dir(channel))
+    path = user_path(channel, user)
+    Logger.debug("Saving chain for #{user} on #{channel} to #{path}")
+    File.write!(path, :erlang.term_to_binary(chain))
+    {:reply, :ok, state}
+  end
+
+  @impl true
+  def handle_call({:train, msg}, _from, {chain, channel, user}) do
+    {:reply, :ok, {Markov.Chain.train(chain, msg), channel, user}}
+  end
+
+  @impl true
+  def handle_call(:chain, _from, state = {chain, _channel, _user}) do
+    {:reply, chain, state}
+  end
+
+  @impl true
+  def handle_call(:channel, _from, state = {_chain, channel, _user}) do
+    {:reply, channel, state}
+  end
+
+  @impl true
+  def handle_call(:user, _from, state = {_chain, _channel, user}) do
+    {:reply, user, state}
+  end
+end