diff --git a/2020/ruby/day_10.rb b/2020/ruby/day_10.rb
index e562da7..ad67898 100644
--- a/2020/ruby/day_10.rb
+++ b/2020/ruby/day_10.rb
@@ -9,8 +9,21 @@ dist = (adapters + [outlet, device]).sort.each_cons(2).map {|a,b| b - a }
 # tally = dist.tally
 # p dist[1] * dist[3]
 
+# programmatically figuring out the arrangements
+# arrangements = Hash.new {|h,k|
+#   h[k] = (0..k-1).flat_map {|n| (2..k).to_a.combination(n).to_a }
+#     .reject {|c| (c << 1 << k+1).sort.each_cons(2).any? {|a,b| b-a > 3 }}
+#     .count
+# }
+
+# tribonacci
+arrangements = Hash.new {|h,k| h[k] = h[k-1] + h[k-2] + h[k-3] }
+arrangements[0] = 1
+arrangements[1] = 1
+arrangements[2] = 2
+
 p dist
   .slice_when {|a,b| a != b }
   .select {|run| run.first == 1 }
-  .map {|run| { 1 => 1, 2 => 2, 3 => 4, 4 => 7 }.fetch(run.size) }
+  .map {|run| arrangements[run.size] }
   .inject {|a,b| a*b }