diff --git a/2016/input/day_21.txt b/2016/input/day_21.txt
new file mode 100644
index 0000000..4870930
--- /dev/null
+++ b/2016/input/day_21.txt
@@ -0,0 +1,100 @@
+move position 0 to position 3
+rotate right 0 steps
+rotate right 1 step
+move position 1 to position 5
+swap letter h with letter b
+reverse positions 1 through 3
+swap letter a with letter g
+swap letter b with letter h
+rotate based on position of letter c
+swap letter d with letter c
+rotate based on position of letter c
+swap position 6 with position 5
+rotate right 7 steps
+swap letter b with letter h
+move position 4 to position 3
+swap position 1 with position 0
+swap position 7 with position 5
+move position 7 to position 1
+swap letter c with letter a
+move position 7 to position 5
+rotate right 4 steps
+swap position 0 with position 5
+move position 3 to position 1
+swap letter c with letter h
+rotate based on position of letter d
+reverse positions 0 through 2
+rotate based on position of letter g
+move position 6 to position 7
+move position 2 to position 5
+swap position 1 with position 0
+swap letter f with letter c
+rotate right 1 step
+reverse positions 2 through 4
+rotate left 1 step
+rotate based on position of letter h
+rotate right 1 step
+rotate right 5 steps
+swap position 6 with position 3
+move position 0 to position 5
+swap letter g with letter f
+reverse positions 2 through 7
+reverse positions 4 through 6
+swap position 4 with position 1
+move position 2 to position 1
+move position 3 to position 1
+swap letter b with letter a
+rotate based on position of letter b
+reverse positions 3 through 5
+move position 0 to position 2
+rotate based on position of letter b
+reverse positions 4 through 5
+rotate based on position of letter g
+reverse positions 0 through 5
+swap letter h with letter c
+reverse positions 2 through 5
+swap position 7 with position 5
+swap letter g with letter d
+swap letter d with letter e
+move position 1 to position 2
+move position 3 to position 2
+swap letter d with letter g
+swap position 3 with position 7
+swap letter b with letter f
+rotate right 3 steps
+move position 5 to position 3
+move position 1 to position 2
+rotate based on position of letter b
+rotate based on position of letter c
+reverse positions 2 through 3
+move position 2 to position 3
+rotate right 1 step
+move position 7 to position 0
+rotate right 3 steps
+move position 6 to position 3
+rotate based on position of letter e
+swap letter c with letter b
+swap letter f with letter d
+swap position 2 with position 5
+swap letter f with letter g
+rotate based on position of letter a
+reverse positions 3 through 4
+rotate left 7 steps
+rotate left 6 steps
+swap letter g with letter b
+reverse positions 3 through 6
+rotate right 6 steps
+rotate based on position of letter c
+rotate based on position of letter b
+rotate left 1 step
+reverse positions 3 through 7
+swap letter f with letter g
+swap position 4 with position 1
+rotate based on position of letter d
+move position 0 to position 4
+swap position 7 with position 6
+rotate right 6 steps
+rotate based on position of letter e
+move position 7 to position 3
+rotate right 3 steps
+swap position 1 with position 2
\ No newline at end of file
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