diff --git a/src/advent/util.cr b/src/advent/util.cr
new file mode 100644
index 0000000..3953353
--- /dev/null
+++ b/src/advent/util.cr
@@ -0,0 +1,13 @@
+class Array(T)
+  def union
+    reduce(T.new) do |l, r|
+      l | r
+    end
+  end
+
+  def intersect
+    reduce do |l, r|
+      l & r
+    end
+  end
+end