diff --git a/2021/ruby/day_14.rb b/2021/ruby/day_14.rb
new file mode 100644
index 0000000..97c9d9c
--- /dev/null
+++ b/2021/ruby/day_14.rb
@@ -0,0 +1,41 @@
+template, rules = ARGF.read.split("\n\n")
+template = template.chars
+rules = rules.scan(/(\w{2}) -> (\w)/).to_h
+
+# polymer = template
+# 10.times do
+#   polymer = polymer.each_cons(2).flat_map {|a,b|
+#     [a, rules.fetch("#{a}#{b}")]
+#   } << polymer.last
+# end
+
+# min, max = polymer.tally.values.minmax
+# p max - min
+
+tally = Hash.new {|h,k|
+  polymer, steps = k
+  new_elements = polymer.each_cons(2).map(&:join).map { rules.fetch(_1) }
+  h[k] = case steps
+         when 1
+           (polymer + new_elements).tally
+         when (2..)
+           initial = new_elements.tally.transform_values { -_1 }
+           polymer.zip(new_elements)
+             .flatten[0..-2]
+             .each_cons(2)
+             .map { h[[_1, steps-1]] }
+             .inject(initial) {|final,tally| final.merge(tally) { _2 + _3 } }
+         else
+           fail
+         end
+}
+
+final = tally[[template, 40]]
+
+# correct for internal elements being double-counted
+template[1..-2].each do
+  final[_1] -= 1
+end
+
+min, max = final.values.minmax
+p max - min