diff --git a/aoc2021/input/day21.txt b/aoc2021/input/day21.txt
new file mode 100644
index 0000000..94d668d
--- /dev/null
+++ b/aoc2021/input/day21.txt
@@ -0,0 +1,2 @@
+Player 1 starting position: 4
+Player 2 starting position: 2
diff --git a/aoc2021/input/day21_provided.txt b/aoc2021/input/day21_provided.txt
new file mode 100644
index 0000000..3f69194
--- /dev/null
+++ b/aoc2021/input/day21_provided.txt
@@ -0,0 +1,2 @@
+Player 1 starting position: 4
+Player 2 starting position: 8
diff --git a/aoc2021/src/day21.rs b/aoc2021/src/day21.rs
new file mode 100644
index 0000000..a926b65
--- /dev/null
+++ b/aoc2021/src/day21.rs
@@ -0,0 +1,109 @@
+use std::fmt::Write;
+use std::iter;
+use std::ops::RangeInclusive;
+
+use anyhow::{Context, Result};
+
+const INPUT: &str = include_str!("../input/day21.txt");
+
+pub fn run() -> Result<String> {
+    let mut res = String::with_capacity(128);
+
+    writeln!(res, "part 1: {}", part1(INPUT)?)?;
+
+    Ok(res)
+}
+
+fn part1(input: &str) -> Result<usize> {
+    let mut lines = input.lines();
+    let mut player1_pos: PlayerPos = lines
+        .next()
+        .and_then(|line| line.trim().strip_prefix("Player 1 starting position: "))
+        .and_then(|pos| pos.parse().ok())
+        .map(PlayerPos::new)
+        .context("couldn't find player 1 pos")?;
+    let mut player2_pos: PlayerPos = lines
+        .next()
+        .and_then(|line| line.trim().strip_prefix("Player 2 starting position: "))
+        .and_then(|pos| pos.parse().ok())
+        .map(PlayerPos::new)
+        .context("couldn't find player 2 pos")?;
+
+    let mut player1_score = 0;
+    let mut player2_score = 0;
+    let mut dice = DeterministicDice::new();
+
+    while player2_score < 1000 {
+        let mv = dice.next_3_sum();
+
+        player1_pos.advance_by(mv);
+        player1_score += player1_pos.pos();
+
+        std::mem::swap(&mut player1_pos, &mut player2_pos);
+        std::mem::swap(&mut player1_score, &mut player2_score);
+    }
+
+    let loser_score = player1_score;
+
+    Ok(loser_score * dice.rolls())
+}
+
+struct PlayerPos(usize);
+
+impl PlayerPos {
+    fn new(pos: usize) -> Self {
+        debug_assert!((1..=10).contains(&pos));
+
+        // represented from 0 to 9 for modulo ease of use
+        Self(pos - 1)
+    }
+
+    fn advance_by(&mut self, mv: usize) {
+        self.0 = (self.0 + mv) % 10
+    }
+
+    fn pos(&self) -> usize {
+        self.0 + 1
+    }
+}
+
+struct DeterministicDice {
+    iter: iter::Cycle<RangeInclusive<usize>>,
+    rolls: usize,
+}
+
+impl DeterministicDice {
+    fn new() -> Self {
+        Self {
+            iter: (1..=100).cycle(),
+            rolls: 0,
+        }
+    }
+
+    fn next_3_sum(&mut self) -> usize {
+        self.rolls += 3;
+
+        self.iter.next().unwrap() + self.iter.next().unwrap() + self.iter.next().unwrap()
+    }
+
+    fn rolls(&self) -> usize {
+        self.rolls
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const PROVIDED: &str = include_str!("../input/day21_provided.txt");
+
+    #[test]
+    fn part1_provided() {
+        assert_eq!(part1(PROVIDED).unwrap(), 739785);
+    }
+
+    #[test]
+    fn part1_real() {
+        assert_eq!(part1(INPUT).unwrap(), 908595);
+    }
+}
diff --git a/aoc2021/src/lib.rs b/aoc2021/src/lib.rs
index cf569f2..7e99714 100644
--- a/aoc2021/src/lib.rs
+++ b/aoc2021/src/lib.rs
@@ -17,3 +17,5 @@ pub mod day14;
 pub mod day15;
 pub mod day16;
 pub mod day17;
+
+pub mod day21;
diff --git a/aoc2021/src/main.rs b/aoc2021/src/main.rs
index 7dc5fe7..6ba207f 100644
--- a/aoc2021/src/main.rs
+++ b/aoc2021/src/main.rs
@@ -20,6 +20,8 @@ use aoc2021::day15;
 use aoc2021::day16;
 use aoc2021::day17;
 
+use aoc2021::day21;
+
 fn main() -> Result<()> {
     let days: &[DayFunc] = &[
         day01::run,
@@ -39,6 +41,7 @@ fn main() -> Result<()> {
         day15::run,
         day16::run,
         day17::run,
+        day21::run,
     ];
 
     aoc::run(days)