diff --git a/2022/ruby/day_12.rb b/2022/ruby/day_12.rb
index 5f57404..6e2b77e 100644
--- a/2022/ruby/day_12.rb
+++ b/2022/ruby/day_12.rb
@@ -1,51 +1,49 @@
-heights = ARGF.read.lines(chomp: true).map(&:chars)
-  .each.with_index.with_object({}) do |(row,y),map|
-    row.each.with_index do |height,x|
-      map[[y,x]] = height
-    end
-  end
-
 class HeightMap
+  NEIGHBORS = [[0, -1], [-1, 0], [0, 1], [1, 0]]
+
   def initialize(heights)
     @heights = heights
   end
 
   def shortest(from:, to:, &cond)
-    queue = [from]
+    frontier = [from]
     visited = { from => 0 }
 
-    until queue.empty? || to.any? { visited.has_key?(_1) }
-      current = queue.shift
-      moves = visited.fetch(current)
-
-      neighbors = [
-        [-1,  0],
-        [ 1,  0],
-        [ 0, -1],
-        [ 0,  1],
-      ].map { current.zip(_1).map {|a,b| a + b } }
-        .select { @heights.has_key?(_1) }
-        .reject { visited.has_key?(_1) }
-        .select { cond.([@heights.fetch(current), @heights.fetch(_1)]) }
-
-      neighbors.each do |y,x|
-        visited[[y,x]] = moves + 1
-        queue << [y,x]
+    until frontier.empty? || to.any? { visited.has_key?(_1) }
+      current = frontier.shift
+
+      NEIGHBORS.each do |delta|
+        candidate = current.zip(delta).map { _1 + _2 }
+
+        next if visited.has_key?(candidate)
+        next unless cand_height = @heights[candidate]
+        next unless cond.(@heights.fetch(current), cand_height)
+
+        visited[candidate] = visited.fetch(current) + 1
+        frontier << candidate
       end
 
-      queue.sort_by { visited.fetch(_1) }
+      frontier.sort_by { visited.fetch(_1) }
     end
 
     visited.find {|k,v| to.include?(k) }.last
   end
 end
 
+heights = ARGF.read.lines(chomp: true).map(&:chars)
+  .flat_map.with_index {|row,y|
+    row.map.with_index {|height,x| [[y, x], height] }
+  }.to_h
+
 s, e = heights.invert.values_at(?S, ?E)
 heights[s] = ?a
 heights[e] = ?z
 
 hm = HeightMap.new(heights)
+
+# part one
 p hm.shortest(from: s, to: [e]) { _1.ord + 1 >= _2.ord }
 
+# part two
 as = heights.select { _2 == ?a }.map(&:first)
 p hm.shortest(from: e, to: as) { _1.ord - 1 <= _2.ord }