Finish markov chain generation impl

* Markov chains will train and generate chains correctly now
* Implement Markov.save_chains/0
* Add a couple more utils that help accomplish the above

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

lib/util.ex

@@ -14,4 +14,27 @@ defmodule Omnibot.Util do
   def denotify_nick(nick) do
     String.graphemes(nick) |> Enum.join("\u200b")
   end
+
+  def weighted_random(items) when is_map(items) do
+    Enum.to_list(items) |> weighted_random()
+  end
+
+  def weighted_random([]), do: nil
+
+  def weighted_random(items) do
+    value = items
+            |> Enum.reduce(0, fn {_, weight}, total -> total + weight end)
+            |> :rand.uniform()
+    select_item(items, value)
+  end
+
+  defp select_item([{item, _}], _), do: item
+
+  defp select_item([{item, weight} | _], index) when weight >= index, do: item
+
+  defp select_item([{_, weight} | tail], index), do: select_item(tail, index - weight)
+
+  def pad_trailing(list, _what, len) when length(list) >= len, do: list
+
+  def pad_trailing(list, what, len), do: pad_trailing(list ++ [what], what, len)
 end