diff --git a/2021/ruby/day_12.rb b/2021/ruby/day_12.rb
new file mode 100644
index 0000000..41612db
--- /dev/null
+++ b/2021/ruby/day_12.rb
@@ -0,0 +1,25 @@
+cave = Hash.new {|h,k| h[k] = [] }
+ARGF.read.scan(/(\w+)-(\w+)/).each do
+  cave[_1] << _2
+  cave[_2] << _1
+end
+
+paths = [ %w[start] ]
+
+loop do
+  closed, open = paths.partition { _1.last == "end" }
+  break if open.empty?
+
+  paths = closed + open.flat_map {|path|
+    cave.fetch(path.last)
+      # .reject { _1 =~ /^[a-z]+$/ && path.include?(_1) }
+      .reject { _1 == "start" }
+      .map { path + [_1] }
+      .reject {|path|
+        small = path.tally.select { _1 =~ /^[a-z]+$/ }
+        (small.count { _2 > 1 } > 1) || small.any? { _2 > 2 }
+      }
+  }
+end
+
+p paths.count