diff --git a/2017/ruby/day_16.rb b/2017/ruby/day_16.rb
new file mode 100644
index 0000000..ed2290b
--- /dev/null
+++ b/2017/ruby/day_16.rb
@@ -0,0 +1,37 @@
+require "strscan"
+
+@dance = ARGF.read.strip
+@programs = (?a..?p).to_a
+
+def run
+  ss = StringScanner.new(@dance)
+  until ss.eos?
+    case
+    when ss.scan(/s/)
+      spin = ss.scan(/\d+/).to_i
+      @programs.rotate!(@programs.size - spin)
+    when ss.scan(/x/)
+      a = ss.scan(/\d+/).to_i
+      ss.scan(/\//)
+      b = ss.scan(/\d+/).to_i
+      @programs[a], @programs[b] = @programs[b], @programs[a]
+    when ss.scan(/p/)
+      a = @programs.index(ss.scan(/\w/))
+      ss.scan(/\//)
+      b = @programs.index(ss.scan(/\w/))
+      @programs[a], @programs[b] = @programs[b], @programs[a]
+    when ss.scan(/,/)
+    end
+  end
+end
+
+@memoized = {}
+count = 0
+until @memoized.has_key?(@programs.join)
+  @memoized[@programs.join] = count
+  run
+  count += 1
+end
+
+mod = 1_000_000_000 % count
+p @memoized.find {|_,v| v == mod }.first