Add binary search to Util

For some reason there doesn't appear to be a binary search function in
Elixir's standard library, so this implements that.

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

lib/contrib/markov/chain.ex

@@ -0,0 +1,35 @@
+defmodule Omnibot.Contrib.Markov.Chain do
+  alias Omnibot.{Contrib.Markov.Chain, Util}
+
+  @enforce_keys [:order]
+  defstruct order: 2, chain: []
+
+  def train(%Chain {chain: chain, order: order}, words) when is_list(words) do
+
+    Enum.filter(words, &(String.length(&1) > 0))
+    |> Enum.chunk_every(order + 1, 1) # this gives us a "sliding window" effect
+    |> Enum.reduce(chain, &case Enum.split(words, order) do
+      {words, []} -> if length(&1) == order,
+          # Null case for the chain; this is an "end" state
+          do: train_one(%Chain {chain: &2, order: order}, words, nil)
+          # else: TODO ? train [a, nil] -> b ?
+      {words, [next]} ->
+        train_one(%Chain {chain: &2, order: order}, words, next)
+    end
+    )
+  end
+
+  def train_one(%Chain {chain: _chain, order: _order}, _key, _value) do
+  end
+
+  def lookup(%Chain {chain: chain, order: order}, key) do
+    if length(key) != order, do: raise(ArgumentError, message: "invalid key (length #{length(key)} vs. order #{order})")
+    case Util.binary_search(chain, key) do
+      {_index, value} -> value
+      nil -> nil
+    end
+  end
+
+  def put(%Chain {chain: _chain, order: _order}, _key, _value) do
+  end
+end

lib/contrib/markov/markov.ex

@@ -0,0 +1,22 @@
+defmodule Omnibot.Contrib.Markov do
+  use Omnibot.Plugin
+
+  alias Omnibot.Contrib.Markov.Chain
+
+  @default_config path: :"wordbot.ets", order: 2
+
+  @impl true
+  def on_init(cfg) do
+    # Create the markov database
+    path = if is_atom(cfg[:path]),
+      do: cfg[:path],
+      else: String.to_atom(cfg[:path])
+    {:ok, db} = :dets.open_file(path)
+    db
+  end
+
+  @impl true
+  def on_channel_msg(_irc, _channel, _nick, msg) do
+    _words = String.split(msg, ~r/\s+/)
+  end
+end