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>

test/contrib/markov/chain_test.exs

@@ -0,0 +1,12 @@
+defmodule MarkovChainTest do
+  use ExUnit.Case
+  alias Omnibot.Contrib.Markov.Chain
+
+  test "chain train_one works correctly" do
+    chain = %Chain {order: 2}
+            |> Chain.train_one(["foo", "bar"], "baz")
+    #assert chain.chain == [
+      #{["foo", "bar"], {"baz", 1}}
+      #]
+  end
+end

test/util_test.exs

@@ -12,4 +12,21 @@ defmodule Omnibot.UtilTest do
     assert Util.string_or_nil("") == nil
     assert Util.string_or_nil("asdf") == "asdf"
   end
+
+  test "binary_search" do
+    indexes = 0..10 |> Enum.to_list()
+    values = indexes |> Enum.map(&({&1, &1 * 2}))
+    assert Enum.map(indexes, &(Util.binary_search(values, &1))) == values
+
+    indexes = 0..101 |> Enum.to_list()
+    values = indexes |> Enum.map(&({&1, &1 * 2}))
+    assert Enum.map(indexes, &(Util.binary_search(values, &1))) == values
+
+    values = [a: 15, b: 22, c: -1, d: 0]
+
+    assert Util.binary_search(values, :a) == {0, 15}
+    assert Util.binary_search(values, :b) == {1, 22}
+    assert Util.binary_search(values, :c) == {2, -1}
+    assert Util.binary_search(values, :d) == {3, 0}
+  end
 end