diff --git a/src/days.rs b/src/days.rs
index f591d7d..f74b0af 100644
--- a/src/days.rs
+++ b/src/days.rs
@@ -18,7 +18,7 @@ pub mod d17;
 pub mod d18;
 pub mod d19;
 pub mod d20;
-// pub mod d21;
+pub mod d21;
 // pub mod d22;
 // pub mod d23;
 // pub mod d24;
diff --git a/src/days/d21.rs b/src/days/d21.rs
new file mode 100644
index 0000000..42da274
--- /dev/null
+++ b/src/days/d21.rs
@@ -0,0 +1,151 @@
+#![allow(non_upper_case_globals)]
+#![allow(dead_code)]
+
+use crate::prelude::*;
+
+pub type I = Vec<Inst>;
+pub type O = String;
+
+#[derive(Debug, Clone)]
+pub enum Inst {
+    SwapPos (usize, usize),
+    SwapLetter (char, char),
+    RotateLeft (usize),
+    RotateRight (usize),
+    RotatePos (char),
+    Reverse (usize, usize),
+    Move (usize, usize)
+}
+
+// const _rotations: [usize; 8] = [1,2,3,4,6,7,0,1];
+// ↓
+const _rotations: [usize; 8] = [1,1,6,2,7,3,0,4];
+
+
+impl Inst {
+    fn from_str (i: &str) -> Self {
+        let i = i.trim();
+        if let Some(sfx) = i.strip_prefix("swap position ") {
+            let (l, r) = sfx.split_once(" with position ").unwrap();
+            Inst::SwapPos(p!(l), p!(r))
+        } else if let Some(sfx) = i.strip_prefix("swap letter ") {
+            let (l, r) = sfx.split_once(" with letter ").unwrap();
+            Inst::SwapLetter(l.trim().chars().next().unwrap(), r.trim().chars().next().unwrap())
+        } else if let Some(sfx) = i.strip_prefix("rotate left ") {
+            let (l, _) = sfx.split_once(' ').unwrap();
+            Inst::RotateLeft(p!(l))
+        } else if let Some(sfx) = i.strip_prefix("rotate right ") {
+            let (l, _) = sfx.split_once(' ').unwrap();
+            Inst::RotateRight(p!(l))
+        } else if let Some(sfx) = i.strip_prefix("rotate based on position of letter ") {
+            Inst::RotatePos(sfx.chars().next().unwrap())
+        } else if let Some(sfx) = i.strip_prefix("reverse positions ") {
+            let (l, r) = sfx.split_once(" through ").unwrap();
+            Inst::Reverse(p!(l), p!(r))
+        } else if let Some(sfx) = i.strip_prefix("move position ") {
+            let (l, r) = sfx.split_once(" to position ").unwrap();
+            Inst::Move(p!(l), p!(r))
+        } else {
+            panic!("unknown instruction")
+        }
+    }
+
+    fn apply (self, target: &mut Vec<char>) {
+        use Inst as I;
+        match self {
+            I::SwapPos(l, r) => target.swap(l, r),
+            I::SwapLetter(l, r) => {
+                let lpos = target.iter().position(|&c| c == l);
+                let rpos = target.iter().position(|&c| c == r);
+                if let Some(lpos) = lpos && let Some(rpos) = rpos {
+                    target.swap(lpos, rpos);
+                }
+            },
+            I::RotateLeft(times) => target.rotate_left(times),
+            I::RotateRight(times) => target.rotate_right(times),
+            I::RotatePos(needle) => {
+                let pos = target.iter().position(|&c| c == needle);
+                if let Some(pos) = pos {
+                    let rotations = pos + 1 + if pos >= 4 { 1 } else { 0 };
+                    let len = target.len(); // % len is useless in my input but needed for the general correctness of the algorithm
+                    target.rotate_right(rotations % len);
+                }
+            },
+            I::Reverse(from, to) => (&mut target[from..=to]).reverse(),
+            I::Move(from, to) => {
+                let val = target.remove(from);
+                target.insert(to, val);
+            }
+        }
+    }
+
+    fn undo (self, target: &mut Vec<char>) {
+        use Inst as I;
+        match self {
+            I::SwapPos(l, r) => target.swap(l, r),
+            I::SwapLetter(l, r) => {
+                let lpos = target.iter().position(|&c| c == l);
+                let rpos = target.iter().position(|&c| c == r);
+                if let Some(lpos) = lpos && let Some(rpos) = rpos {
+                    target.swap(lpos, rpos);
+                }
+            },
+            I::RotateLeft(times) => target.rotate_right(times),
+            I::RotateRight(times) => target.rotate_left(times),
+            I::RotatePos(needle) => {
+                let pos = target.iter().position(|&c| c == needle);
+                if let Some(pos) = pos {
+                    target.rotate_left(_rotations[pos]);
+                }
+            },
+            I::Reverse(from, to) => (&mut target[from..=to]).reverse(),
+            I::Move(from, to) => {
+                let val = target.remove(to);
+                target.insert(from, val);
+            }
+        }
+    }
+}
+
+fn _parse (data: &str) -> I {
+    data.trim().lines().map(Inst::from_str).collect()
+}
+
+fn _silver (start: &'static str, list: I) -> O {
+    let mut state = start.chars().collect_vec();
+    list.into_iter().for_each(|i| i.apply(&mut state));
+    state.into_iter().collect()
+}
+
+fn _gold (start: &'static str, mut list: I) -> O {
+    list.reverse();
+    let mut state = start.chars().collect_vec();
+    list.into_iter().for_each(|i| i.undo(&mut state));
+    state.into_iter().collect()
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    fn read () -> I {
+        let data = inc!(21);
+        _parse(data)
+    }
+
+    #[test]
+    fn silver () {
+        let data = read();
+        let ans = _silver("abcdefgh", data);
+
+        assert_eq!(ans, s!("bfheacgd"))
+    }
+
+    #[test]
+    fn gold () {
+        let data = read();
+        let ans = _gold("fbgdceah", data);
+
+        assert_eq!(ans, s!("gcehdbfa"))
+    }
+}