diff --git a/2021/ruby/day_15.rb b/2021/ruby/day_15.rb
index e22acb0..cd9a8c7 100644
--- a/2021/ruby/day_15.rb
+++ b/2021/ruby/day_15.rb
@@ -1,12 +1,12 @@
+require "set"
+
 cavern = ARGF.read.split("\n").map { _1.chars.map(&:to_i) }
-cavern = (0..4).flat_map {|i|
-  cavern.map {|row| row.map { (_1 + i - 1) % 9 + 1 }}
-}
-cavern = cavern.map {|row|
-  (0..4).flat_map {|i|
-    row.map { (_1 + i - 1) % 9 + 1 }
-  }
-}
+cavern = (0..4).flat_map {|i| cavern.map {|row|
+  row.map { (_1 + i - 1) % 9 + 1 }
+}}
+cavern = cavern.map {|row| (0..4).flat_map {|i|
+  row.map { (_1 + i - 1) % 9 + 1 }
+}}
 
 cavern = cavern
   .flat_map.with_index {|row,y| row.map.with_index {|risk,x| [[y,x], risk] }}
@@ -15,7 +15,7 @@ bottom_right = cavern.keys.max
 
 risks = Hash.new(0)
 risks[[0,0]] = 0
-edges = [[0,0]]
+edges = Set[[0,0]]
 
 until risks.has_key?(bottom_right)
   yx = edges.min_by { risks.fetch(_1) }
@@ -32,7 +32,7 @@ until risks.has_key?(bottom_right)
     risks[_1] = risk + cavern.fetch(_1)
   end
 
-  edges.concat(neighbors)
+  edges.merge(neighbors)
 end
 
 p risks[bottom_right]