diff --git a/2016/ruby/day_21.rb b/2016/ruby/day_21.rb
new file mode 100644
index 0000000..8b9c137
--- /dev/null
+++ b/2016/ruby/day_21.rb
@@ -0,0 +1,37 @@
+input = 'abcdefgh'
+ARGF.read.split("\n").each do |instruction|
+  case instruction
+  when /swap position (\d+) with position (\d+)/
+    x = $1.to_i
+    y = $2.to_i
+    input[x], input[y] = input[y], input[x]
+  when /swap letter (\w+) with letter (\w+)/
+    x = $1
+    y = $2
+    input.tr!("#{x}#{y}", "#{y}#{x}")
+  when /rotate (left|right) (\d+) steps?/
+    steps = $2.to_i
+    steps = -steps if $1 == 'right'
+    input = input.chars.rotate(steps).join
+  when /rotate based on position of letter (\w+)/
+    index = input.index($1)
+    index += 1 if index >= 4
+    index += 1
+    index = -index
+    input = input.chars.rotate(index).join
+  when /reverse positions (\d+) through (\d+)/
+    x = $1.to_i
+    y = $2.to_i
+    input[x..y] = input[x..y].reverse
+  when /move position (\d+) to position (\d+)/
+    x = $1.to_i
+    x = input[x]
+    input[x] = ''
+
+    y = $2.to_i
+    input.insert(y, x)
+  else
+    raise "invalid instruction: '#{instruction}'"
+  end
+end
+puts input