diff --git a/2020/ruby/day_22.rb b/2020/ruby/day_22.rb
new file mode 100644
index 0000000..76c76b7
--- /dev/null
+++ b/2020/ruby/day_22.rb
@@ -0,0 +1,46 @@
+require "set"
+
+decks = ARGF.read.split("\n\n").map {|players| players.split("\n")[1..].map(&:to_i) }
+
+# part one
+
+# until decks.any?(&:empty?)
+#   p decks
+#   top_cards = decks.map(&:shift)
+#   fail if top_cards.tally.size == 1
+
+#   if top_cards.first > top_cards.last
+#     decks.first.concat(top_cards)
+#   else
+#     decks.last.concat(top_cards.reverse)
+#   end
+# end
+
+SEEN = Set.new
+def recursive_combat!(decks)
+  until decks.any?(&:empty?)
+    return 0 if SEEN.include?(decks)
+    SEEN << decks
+
+    top_cards = decks.map(&:shift)
+    winner = if decks.zip(top_cards).all? {|d,tc| d.size >= tc }
+               recursive_combat!(decks.zip(top_cards).map {|d,tc| d[0,tc] })
+             elsif top_cards.first > top_cards.last
+               0
+             else
+               1
+             end
+
+    if winner == 0
+      decks.first.concat(top_cards)
+    else
+      decks.last.concat(top_cards.reverse)
+    end
+  end
+
+  decks.index {|d| !d.empty? }
+end
+
+recursive_combat!(decks)
+
+p decks.flatten.reverse.map.with_index {|v,i| v * (i+1) }.sum