diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 404cfc8..cdc0b41 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -103,3 +103,8 @@ day-20:
   stage: build
   script:
     - cd day20; cargo run --release ./input
+
+day-21:
+  stage: build
+  script:
+    - cd day21; cargo run --release ./input
diff --git a/day21/Cargo.toml b/day21/Cargo.toml
new file mode 100644
index 0000000..ea8752e
--- /dev/null
+++ b/day21/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "day21"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/day21/input b/day21/input
new file mode 100644
index 0000000..20724a0
--- /dev/null
+++ b/day21/input
@@ -0,0 +1,2 @@
+Player 1 starting position: 1
+Player 2 starting position: 2
diff --git a/day21/src/main.rs b/day21/src/main.rs
new file mode 100644
index 0000000..dca58e2
--- /dev/null
+++ b/day21/src/main.rs
@@ -0,0 +1,100 @@
+use std::{fs, env, collections::HashMap};
+
+struct Dice {
+    n: u32,
+    i: u32
+}
+
+impl Dice {
+    fn new() -> Dice { Dice { n: 0, i: 0 } }
+
+    fn get_nb_draw(&self) -> u32 { return self.n; }
+
+    fn get_next(&mut self) -> u32 {
+        self.n += 1;
+        self.i = (self.i + 1) % 100;
+        if self.i == 0 { self.i = 100; }
+        return self.i;
+    }
+}
+
+fn read_input(path: &str) -> String {
+    return fs::read_to_string(path).expect("Cannot read file.");
+}
+
+fn parse_input(s: &str) -> (u8, u8) {
+    let p = s.lines()
+        .map(|l| l.split_once(": ").unwrap().1.parse::<u8>().unwrap())
+        .collect::<Vec<u8>>();
+    return (p[0], p[1]);
+}
+
+fn play_classic(mut v: (u8, u8)) -> (u32, u32) {
+    let mut d = Dice::new();
+    let mut s = (0, 0);
+    let mut p = true;
+    while s.0 < 1000 && s.1 < 1000 {
+        let a = d.get_next() + d.get_next() + d.get_next();
+        let mut x = (if p { v.0 } else { v.1 } as u32 + a) % 10;
+        if x == 0 { x = 10; }
+        if p {
+            v.0 = x as u8;
+            s.0 += x;
+            if s.0 >= 1000 { return (d.get_nb_draw(), s.1); }
+        } else {
+            v.1 = x as u8;
+            s.1 += x;
+            if s.1 >= 1000 { return (d.get_nb_draw(), s.0); }
+        }
+        p = !p;
+    }
+    return (d.get_nb_draw(), std::cmp::min(s.0, s.1));
+}
+
+fn play_quantum(v: (u8, u8), s: (u8, u8), p: bool, c: &mut HashMap<((u8, u8), (u8, u8), bool), (u128, u128)>) {
+    if s.0 >= 21 {
+        c.insert((v, s, p), (1, 0));
+        return;
+    }
+    if s.1 >= 21 {
+        c.insert((v, s, p), (0, 1));
+        return;
+    }
+    for i in 1..=3 {
+        for j in 1..=3 {
+            for k in 1..=3 {
+                let mut a = v;
+                let mut b = s;
+                if p {
+                    a.0 = (a.0 + i + j + k) % 10;
+                    if a.0 == 0 { a.0 = 10; }
+                    b.0 += a.0;
+                } else {
+                    a.1 = (a.1 + i + j + k) % 10;
+                    if a.1 == 0 { a.1 = 10; }
+                    b.1 += a.1;
+                }
+                if !c.contains_key(&(a, b, !p)) { play_quantum(a, b, !p, c); }
+                let r = *c.get(&(a, b, !p)).unwrap();
+                let x = *c.get(&(v, s, p)).unwrap_or(&(0, 0));
+                c.insert((v, s, p), (r.0 + x.0, r.1 + x.1));
+            }
+        }
+    }
+}
+
+fn main() {
+    let args: Vec<String> = env::args().collect();
+    for arg in args.iter().skip(1) {
+        let input = read_input(&arg);
+        let vec_in = parse_input(&input);
+        let c = play_classic(vec_in);
+        let mut w = HashMap::new();
+        play_quantum(vec_in, (0, 0), true, &mut w);
+        let w = w.get(&(vec_in, (0, 0), true)).unwrap();
+        println!("[{}]", &arg);
+        println!("\t[Part 1] => Answer is '{}'.", c.0 * c.1);
+        println!("\t[Part 2] => Answer is '{}'.", std::cmp::max(w.0, w.1));
+    }
+}
+