diff --git a/src/advent/util.cr b/src/advent/util.cr
index 52b8c3b..1bcce64 100644
--- a/src/advent/util.cr
+++ b/src/advent/util.cr
@@ -33,6 +33,14 @@ class Array(T)
       puts
     end
   end
+
+  def find_indices
+    into = [] of T
+    each_with_index do |v, i|
+      into << i if yield v
+    end
+    into
+  end
 end
 
 struct Tuple(*T)