diff --git a/y2024/resources/21_input.txt b/y2024/resources/21_input.txt
new file mode 100644
index 0000000..dddcb20
--- /dev/null
+++ b/y2024/resources/21_input.txt
@@ -0,0 +1,5 @@
+340A
+149A
+582A
+780A
+463A
diff --git a/y2024/src/bin/d21.rs b/y2024/src/bin/d21.rs
new file mode 100644
index 0000000..fe9a2c1
--- /dev/null
+++ b/y2024/src/bin/d21.rs
@@ -0,0 +1,27 @@
+use std::{fs, time::Instant};
+
+use utils::time::get_elapsed_string;
+use y2024::days::d21;
+
+fn main() {
+    let now = Instant::now();
+    println!("Part 1:");
+    part1();
+    println!("Ran in {}", get_elapsed_string(now.elapsed()));
+    let now = Instant::now();
+    println!("Part 2:");
+    part2();
+    println!("Ran in {}", get_elapsed_string(now.elapsed()));
+}
+
+fn part1() {
+    let root = env!("CARGO_MANIFEST_DIR");
+    let content = fs::read_to_string(format!("{root}/resources/21_input.txt")).unwrap();
+    println!("{}", d21::process_part1(&content));
+}
+
+fn part2() {
+    let root = env!("CARGO_MANIFEST_DIR");
+    let content = fs::read_to_string(format!("{root}/resources/21_input.txt")).unwrap();
+    println!("{}", d21::process_part2(&content));
+}
diff --git a/y2024/src/days/d21.rs b/y2024/src/days/d21.rs
new file mode 100644
index 0000000..a2baabe
--- /dev/null
+++ b/y2024/src/days/d21.rs
@@ -0,0 +1,523 @@
+use core::panic;
+use std::{char, error::Error};
+
+use itertools::Itertools;
+
+pub fn process_part1(input: &str) -> usize {
+    input
+        .lines()
+        .map(|line| {
+            let numpad = Numpad::from(line);
+            println!("first robot {}", numpad.sequence);
+            let _ = Numpad::revert_from(&numpad.sequence);
+            let keypad_1 = Keypad::from(numpad.sequence.as_str());
+            println!("second robot {}", keypad_1.sequence);
+            let _ = Keypad::revert_from(&keypad_1.sequence);
+            let keypad_2 = Keypad::from(keypad_1.sequence.as_str());
+            println!("me {}", keypad_2.sequence);
+            let _ = Keypad::revert_from(&keypad_2.sequence);
+            keypad_2.sequence.len() * numeric_part(line)
+        })
+        .sum()
+}
+
+pub fn process_part2(input: &str) -> usize {
+    0
+}
+
+fn numeric_part(code: &str) -> usize {
+    let code = &code[..code.len() - 1];
+    code.parse().unwrap()
+}
+
+#[derive(Debug, Clone, Copy)]
+enum NumpadState {
+    Seven,
+    Eight,
+    Nine,
+    Four,
+    Five,
+    Six,
+    One,
+    Two,
+    Three,
+    Zero,
+    A,
+}
+
+impl NumpadState {
+    fn get_col(&self) -> i32 {
+        match self {
+            NumpadState::Seven | NumpadState::Four | NumpadState::One => 0,
+            NumpadState::Eight | NumpadState::Five | NumpadState::Two | NumpadState::Zero => 1,
+            NumpadState::Nine | NumpadState::Six | NumpadState::Three | NumpadState::A => 2,
+        }
+    }
+
+    fn get_row(&self) -> i32 {
+        match self {
+            NumpadState::Seven | NumpadState::Eight | NumpadState::Nine => 0,
+            NumpadState::Four | NumpadState::Five | NumpadState::Six => 1,
+            NumpadState::One | NumpadState::Two | NumpadState::Three => 2,
+            NumpadState::Zero | NumpadState::A => 3,
+        }
+    }
+
+    fn get_sequence(&self, goto: NumpadState) -> String {
+        let row = self.get_row();
+        let col = self.get_col();
+        let goto_row = goto.get_row();
+        let goto_col = goto.get_col();
+        if row == goto_row && col == goto_col {
+            return "A".to_string();
+        }
+        let row_diff = row.abs_diff(goto_row);
+        let col_diff = col.abs_diff(goto_col);
+        let mut sequence = String::new();
+        let row_seq = match goto_row.cmp(&row) {
+            std::cmp::Ordering::Less => vec!["^"; row_diff as usize].join(""),
+            std::cmp::Ordering::Equal => "".to_string(),
+            std::cmp::Ordering::Greater => vec!["v"; row_diff as usize].join(""),
+        };
+        let col_seq = match goto_col.cmp(&col) {
+            std::cmp::Ordering::Less => vec!["<"; col_diff as usize].join(""),
+            std::cmp::Ordering::Equal => "".to_string(),
+            std::cmp::Ordering::Greater => vec![">"; col_diff as usize].join(""),
+        };
+        if (goto_col < col && row < 3) || col == 0 {
+            sequence.push_str(&col_seq);
+            sequence.push_str(&row_seq);
+        } else {
+            sequence.push_str(&row_seq);
+            sequence.push_str(&col_seq);
+        }
+        sequence.push('A');
+        sequence
+    }
+}
+
+impl TryFrom<char> for NumpadState {
+    type Error = Box<dyn Error>;
+
+    fn try_from(value: char) -> Result<Self, Self::Error> {
+        match value {
+            'A' => Ok(NumpadState::A),
+            '0' => Ok(NumpadState::Zero),
+            '1' => Ok(NumpadState::One),
+            '2' => Ok(NumpadState::Two),
+            '3' => Ok(NumpadState::Three),
+            '4' => Ok(NumpadState::Four),
+            '5' => Ok(NumpadState::Five),
+            '6' => Ok(NumpadState::Six),
+            '7' => Ok(NumpadState::Seven),
+            '8' => Ok(NumpadState::Eight),
+            '9' => Ok(NumpadState::Nine),
+            _ => Err(Box::from("Unrecognised keystate")),
+        }
+    }
+}
+
+impl TryFrom<(usize, usize)> for NumpadState {
+    type Error = Box<dyn Error>;
+
+    fn try_from(value: (usize, usize)) -> Result<Self, Self::Error> {
+        match value {
+            (2, 3) => Ok(NumpadState::A),
+            (1, 3) => Ok(NumpadState::Zero),
+            (0, 2) => Ok(NumpadState::One),
+            (1, 2) => Ok(NumpadState::Two),
+            (2, 2) => Ok(NumpadState::Three),
+            (0, 1) => Ok(NumpadState::Four),
+            (1, 1) => Ok(NumpadState::Five),
+            (2, 1) => Ok(NumpadState::Six),
+            (0, 0) => Ok(NumpadState::Seven),
+            (1, 0) => Ok(NumpadState::Eight),
+            (2, 0) => Ok(NumpadState::Nine),
+            (0, 3) => Err(Box::from("Robot arm hovering over numpad gap")),
+            (x, y) => {
+                let err = format!("Unrecognised numpad key coord ({x}, {y})");
+                Err(err.into())
+            }
+        }
+    }
+}
+
+impl From<NumpadState> for char {
+    fn from(value: NumpadState) -> Self {
+        match value {
+            NumpadState::Seven => '7',
+            NumpadState::Eight => '8',
+            NumpadState::Nine => '9',
+            NumpadState::Four => '4',
+            NumpadState::Five => '5',
+            NumpadState::Six => '6',
+            NumpadState::One => '1',
+            NumpadState::Two => '2',
+            NumpadState::Three => '3',
+            NumpadState::Zero => '0',
+            NumpadState::A => 'A',
+        }
+    }
+}
+
+struct Numpad {
+    state: NumpadState,
+    sequence: String,
+}
+
+impl Numpad {
+    fn press_key(&mut self, goto: NumpadState) {
+        let sequence = self.state.get_sequence(goto);
+        self.state = goto;
+        self.sequence.push_str(&sequence);
+    }
+
+    #[allow(dead_code)]
+    fn revert_from(sequence: &str) -> String {
+        let mut state = NumpadState::A;
+        let mut col_idx = 2;
+        let mut row_idx = 3;
+        let mut reverted = String::new();
+        for movements in sequence.split_inclusive("A") {
+            for movement in movements.chars() {
+                match movement {
+                    '<' => {
+                        col_idx -= 1;
+                        state = NumpadState::try_from((col_idx, row_idx)).unwrap()
+                    }
+                    '>' => {
+                        col_idx += 1;
+                        state = NumpadState::try_from((col_idx, row_idx)).unwrap()
+                    }
+                    '^' => {
+                        row_idx -= 1;
+                        state = NumpadState::try_from((col_idx, row_idx)).unwrap()
+                    }
+                    'v' => {
+                        row_idx += 1;
+                        state = NumpadState::try_from((col_idx, row_idx)).unwrap()
+                    }
+                    'A' => {
+                        reverted.push(state.into());
+                    }
+                    _ => panic!("Impossible"),
+                }
+            }
+        }
+        reverted
+    }
+}
+
+impl From<&str> for Numpad {
+    fn from(value: &str) -> Self {
+        let mut numpad = Numpad {
+            state: NumpadState::A,
+            sequence: String::new(),
+        };
+        let keys = value
+            .chars()
+            .map(|numkey| NumpadState::try_from(numkey).unwrap())
+            .collect_vec();
+        for key in keys {
+            numpad.press_key(key);
+        }
+        numpad
+    }
+}
+
+#[derive(Debug, Clone, Copy)]
+enum KeypadState {
+    Up,
+    A,
+    Left,
+    Down,
+    Right,
+}
+
+impl KeypadState {
+    fn get_col(&self) -> i32 {
+        match self {
+            KeypadState::Left => 0,
+            KeypadState::Up | KeypadState::Down => 1,
+            KeypadState::A | KeypadState::Right => 2,
+        }
+    }
+
+    fn get_row(&self) -> i32 {
+        match self {
+            KeypadState::Up | KeypadState::A => 0,
+            KeypadState::Left | KeypadState::Down | KeypadState::Right => 1,
+        }
+    }
+
+    fn get_sequence(&self, goto: KeypadState) -> String {
+        let row = self.get_row();
+        let col = self.get_col();
+        let goto_row = goto.get_row();
+        let goto_col = goto.get_col();
+        if row == goto_row && col == goto_col {
+            return "A".to_string();
+        }
+        let row_diff = row.abs_diff(goto_row);
+        let col_diff = col.abs_diff(goto_col);
+        let mut sequence = String::new();
+        let row_seq = match goto_row.cmp(&row) {
+            std::cmp::Ordering::Less => vec!["^"; row_diff as usize].join(""),
+            std::cmp::Ordering::Equal => "".to_string(),
+            std::cmp::Ordering::Greater => vec!["v"; row_diff as usize].join(""),
+        };
+        let col_seq = match goto_col.cmp(&col) {
+            std::cmp::Ordering::Less => vec!["<"; col_diff as usize].join(""),
+            std::cmp::Ordering::Equal => "".to_string(),
+            std::cmp::Ordering::Greater => vec![">"; col_diff as usize].join(""),
+        };
+        if (goto_col < col && row == 1) || col == 0 {
+            sequence.push_str(&col_seq);
+            sequence.push_str(&row_seq);
+        } else {
+            sequence.push_str(&row_seq);
+            sequence.push_str(&col_seq);
+        }
+        sequence.push('A');
+        sequence
+    }
+}
+
+impl TryFrom<char> for KeypadState {
+    type Error = Box<dyn Error>;
+
+    fn try_from(value: char) -> Result<Self, Self::Error> {
+        match value {
+            'A' => Ok(KeypadState::A),
+            '<' => Ok(KeypadState::Left),
+            '>' => Ok(KeypadState::Right),
+            'v' => Ok(KeypadState::Down),
+            '^' => Ok(KeypadState::Up),
+            _ => Err(Box::from("Unrecognised keystate")),
+        }
+    }
+}
+
+impl TryFrom<(usize, usize)> for KeypadState {
+    type Error = Box<dyn Error>;
+
+    fn try_from(value: (usize, usize)) -> Result<Self, Self::Error> {
+        match value {
+            (2, 0) => Ok(KeypadState::A),
+            (0, 1) => Ok(KeypadState::Left),
+            (2, 1) => Ok(KeypadState::Right),
+            (1, 1) => Ok(KeypadState::Down),
+            (1, 0) => Ok(KeypadState::Up),
+            (0, 0) => Err(Box::from("Robot arm hovering over keypad gap")),
+            (x, y) => {
+                let err = format!("Unrecognised key coord ({x}, {y})");
+                Err(err.into())
+            }
+        }
+    }
+}
+
+impl From<KeypadState> for char {
+    fn from(value: KeypadState) -> Self {
+        match value {
+            KeypadState::A => 'A',
+            KeypadState::Up => '^',
+            KeypadState::Left => '<',
+            KeypadState::Down => 'v',
+            KeypadState::Right => '>',
+        }
+    }
+}
+
+struct Keypad {
+    state: KeypadState,
+    sequence: String,
+}
+
+impl Keypad {
+    fn press_key(&mut self, goto: KeypadState) {
+        let sequence = self.state.get_sequence(goto);
+        self.state = goto;
+        self.sequence.push_str(&sequence);
+    }
+
+    #[allow(dead_code)]
+    fn revert_from(sequence: &str) -> String {
+        let mut state = KeypadState::A;
+        let mut col_idx = 2;
+        let mut row_idx = 0;
+        let mut reverted = String::new();
+        for movements in sequence.split_inclusive("A") {
+            for movement in movements.chars() {
+                match movement {
+                    '<' => {
+                        col_idx -= 1;
+                        state = KeypadState::try_from((col_idx, row_idx)).unwrap()
+                    }
+                    '>' => {
+                        col_idx += 1;
+                        state = KeypadState::try_from((col_idx, row_idx)).unwrap()
+                    }
+                    '^' => {
+                        row_idx -= 1;
+                        state = KeypadState::try_from((col_idx, row_idx)).unwrap()
+                    }
+                    'v' => {
+                        row_idx += 1;
+                        state = KeypadState::try_from((col_idx, row_idx)).unwrap()
+                    }
+                    'A' => {
+                        reverted.push(state.into());
+                    }
+                    _ => panic!("Impossible"),
+                }
+            }
+        }
+        reverted
+    }
+}
+
+impl From<&str> for Keypad {
+    fn from(value: &str) -> Self {
+        let mut numpad = Keypad {
+            state: KeypadState::A,
+            sequence: String::new(),
+        };
+        let keys = value
+            .chars()
+            .map(|numkey| KeypadState::try_from(numkey).unwrap())
+            .collect_vec();
+        for key in keys {
+            numpad.press_key(key);
+        }
+        numpad
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use rayon::result;
+
+    use super::*;
+
+    const INPUT_1: &str = "029A";
+    const INPUT_2: &str = "980A";
+    const INPUT_3: &str = "179A";
+    const INPUT_4: &str = "456A";
+    const INPUT_5: &str = "379A";
+
+    //const INPUT_1_BACK: &str =
+    //    "<vA<AA>>^AvAA<^A>A<v<A>>^AvA^A<vA>^A<v<A>^A>AAvA^A<v<A>A>^AAAvA<^A>A";
+    //const INPUT_2_BACK: &str = "<v<A>>^AAAvA^A<vA<AA>>^AvAA<^A>A<v<A>A>^AAAvA<^A>A<vA>^A<A>A";
+    //const INPUT_3_BACK: &str =
+    //    "<v<A>>^A<vA<A>>^AAvAA<^A>A<v<A>>^AAvA^A<vA>^AA<A>A<v<A>A>^AAAvA<^A>A";
+    //const INPUT_4_BACK: &str = "<v<A>>^AA<vA<A>>^AAvAA<^A>A<vA>^A<A>A<vA>^A<A>A<v<A>A>^AAvA<^A>A";
+    const INPUT_5_BACK: &str = "<v<A>>^AvA^A<vA<AA>>^AAvA<^A>AAvA^A<vA>^AA<A>A<v<A>A>^AAAvA<^A>A";
+
+    #[test]
+    fn part1_full() {
+        let result = process_part1(&[INPUT_1, INPUT_2, INPUT_3, INPUT_4, INPUT_5].join("\n"));
+        assert_eq!(result, 126384);
+    }
+
+    #[test]
+    fn part1_1() {
+        let result = process_part1(INPUT_1);
+        assert_eq!(result, 29 * 68);
+    }
+
+    #[test]
+    fn part1_2() {
+        let result = process_part1(INPUT_2);
+        assert_eq!(result, 60 * 980);
+    }
+
+    #[test]
+    fn part1_3() {
+        let result = process_part1(INPUT_3);
+        assert_eq!(result, 68 * 179);
+    }
+
+    #[test]
+    fn part1_4() {
+        let result = process_part1(INPUT_4);
+        assert_eq!(result, 64 * 456);
+    }
+
+    #[test]
+    fn part1_5() {
+        let result = process_part1(INPUT_5);
+        assert_eq!(result, 64 * 379);
+    }
+
+    #[test]
+    fn part1_edge() {
+        let code = "1";
+        let numpad = Numpad::from(code);
+        let keypad_1 = Keypad::from(numpad.sequence.as_str());
+        assert_eq!(numpad.sequence, "^<<A");
+        assert_eq!(keypad_1.sequence, "<Av<AA>>^A");
+    }
+
+    //#[test]
+    //fn part1_1_back() {
+    //    let result = Keypad::revert_from(INPUT_1_BACK);
+    //    let result = Keypad::revert_from(&result);
+    //    let result = Numpad::revert_from(&result);
+    //    assert_eq!(result, "029A");
+    //}
+
+    //#[test]
+    //fn part1_2_back() {
+    //    let result = Keypad::revert_from(INPUT_2_BACK);
+    //    let result = Keypad::revert_from(&result);
+    //    let result = Numpad::revert_from(&result);
+    //    assert_eq!(result, "980A");
+    //}
+
+    //#[test]
+    //fn part1_3_back() {
+    //    let result = Keypad::revert_from(INPUT_3_BACK);
+    //    let result = Keypad::revert_from(&result);
+    //    let result = Numpad::revert_from(&result);
+    //    assert_eq!(result, "179A");
+    //}
+
+    //#[test]
+    //fn part1_4_back() {
+    //    let result = Keypad::revert_from(INPUT_4_BACK);
+    //    let result = Keypad::revert_from(&result);
+    //    let result = Numpad::revert_from(&result);
+    //    assert_eq!(result, "456A");
+    //}
+
+    #[test]
+    fn part1_5_back() {
+        let result = Keypad::revert_from(INPUT_5_BACK);
+        println!("back 1 {result}");
+        let result = Keypad::revert_from(&result);
+        println!("back 2 {result}");
+        let result = Numpad::revert_from(&result);
+        assert_eq!(result, "379A");
+    }
+
+    //#[test]
+    //fn part1_5_backnforth() {
+    //    let numpad = Numpad::from(INPUT_5);
+    //    let keypad_1 = Keypad::from(numpad.sequence.as_str());
+    //    let keypad_2 = Keypad::from(keypad_1.sequence.as_str());
+    //    let keypad_1_back = Keypad::revert_from(INPUT_5_BACK);
+    //    let numpad_back = Keypad::revert_from(&keypad_1_back);
+    //    let code = Numpad::revert_from(&numpad_back);
+    //    //assert_eq!(code, INPUT_5);
+    //    assert_eq!(numpad_back, numpad.sequence);
+    //    //assert_eq!(keypad_1_back, keypad_1.sequence);
+    //    //assert_eq!(INPUT_5_BACK, keypad_2.sequence);
+    //}
+
+    #[test]
+    fn part2() {
+        let result = process_part2(INPUT_1);
+        assert_eq!(result, 0);
+    }
+}
diff --git a/y2024/src/days/mod.rs b/y2024/src/days/mod.rs
index bf81c6c..4dcf084 100644
--- a/y2024/src/days/mod.rs
+++ b/y2024/src/days/mod.rs
@@ -29,3 +29,5 @@ pub mod d18;
 pub mod d19;
 
 pub mod d20;
+
+pub mod d21;