diff --git a/2021/21.js b/2021/21.js
new file mode 100644
index 0000000..e9ab557
--- /dev/null
+++ b/2021/21.js
@@ -0,0 +1,135 @@
+// turn: 0-1; 1 bit (0 = player 1; 1 = player 2)
+// pos1: 1-10; 4 bits
+// pos2: 1-10; 4 bits
+// score1: 0-1009; 10 bits
+// score2: 0-1009; 10 bits
+const sizeS = 10
+const sizeP = 4
+const sizeT = 1
+
+const maskS = (1 << sizeS) - 1
+const maskP = (1 << sizeP) - 1
+const maskT = (1 << sizeT) - 1
+
+const offsetS2 = 0
+const offsetS1 = offsetS2 + sizeS
+const offsetP2 = offsetS1 + sizeS
+const offsetP1 = offsetP2 + sizeP
+const offsetT = offsetP1 + sizeP
+
+function encodeState(turn, pos1, pos2, score1, score2) {
+    return (score2 << offsetS2) |
+        (score1 << offsetS1) |
+        (pos2 << offsetP2) |
+        (pos1 << offsetP1) |
+        (turn << offsetT)
+}
+
+function getScore(state, player) {
+    return state >> (player ? offsetS2 : offsetS1) & maskS
+}
+
+function advState(state, roll, win) {
+    let turn = state >> offsetT
+
+    let offsetPos = turn ? offsetP2 : offsetP1
+    let offsetScore = turn ? offsetS2 : offsetS1
+
+    let pos = (state >> offsetPos) & maskP
+    let score = (state >> offsetScore) & maskS
+
+    pos = (((pos + roll) - 1) % 10) + 1
+    score += pos
+
+    if (score >= win) {
+        return turn
+    }
+
+    state &= ~(maskP << offsetPos)
+    state |= pos << offsetPos
+
+    state &= ~(maskS << offsetScore)
+    state |= score << offsetScore
+
+    state ^= 1 << offsetT
+
+    return state
+}
+
+const rollCombos = [1, 2, 3].cartProduct([1, 2, 3]).cartProduct([1, 2, 3]).map((e) => e.flat().sum())
+
+function day21(input, part2) {
+    let lines = input.split("\n")
+
+    let pos1 = +lines[0].split(" ").last
+    let pos2 = +lines[1].split(" ").last
+
+    let state = encodeState(0, pos1, pos2, 0, 0)
+
+    if (!part2) {
+        const win = 1000
+
+        let die = 0
+        let rolls = 0
+
+        while (getScore(state, 0) < win && getScore(state, 1) < win) {
+            let roll = ++die
+            die %= 100
+            roll += ++die
+            die %= 100
+            roll += ++die
+            die %= 100
+
+            rolls += 3
+
+            newState = advState(state, roll, win)
+
+            if (newState == 0 || newState == 1) {
+                return getScore(state, 1 - newState) * rolls
+            }
+
+            state = newState
+        }
+    } else {
+        let universes = {}
+        universes[state] = 1
+
+        let wins = [0, 0]
+
+        while (true) {
+            let universesLeft = false
+
+            for (let state in universes) {
+                if (!universes[state]) {
+                    continue
+                }
+
+                universesLeft = true
+
+                state = +state
+
+                for (let roll of rollCombos) {
+                    let newState = advState(state, roll, 21)
+
+                    if (newState == 0 || newState == 1) {
+                        wins[newState] += universes[state]
+                    } else {
+                        universes[newState] = (universes[newState] || 0) + universes[state]
+                    }
+                }
+
+                delete universes[state]
+            }
+
+            if (!universesLeft) {
+                break
+            }
+        }
+
+        return wins.max()
+    }
+}
+
+if (typeof window == "undefined") {
+    module.exports = day21
+}
diff --git a/out.js b/out.js
index ea865de..a4de305 100644
--- a/out.js
+++ b/out.js
@@ -1244,7 +1244,7 @@ if (typeof window == "undefined" && process.argv[2] == "test") {
 
 	const year = "2021"
 
-	for (let i = +process.argv[3] || 1; i <= 20; i++) {
+	for (let i = +process.argv[3] || 1; i <= 21; i++) {
 		const func = require(`./${year}/${i}.js`)
 		const input = fs.readFileSync(`./${year}/inputs/${i}`, "utf8")
 		const answers = fs.readFileSync(`./${year}/answers/${i}`, "utf8").split("\n-----\n")
diff --git a/test.js b/test.js
index a215a04..0c8effa 100644
--- a/test.js
+++ b/test.js
@@ -3,7 +3,7 @@ if (typeof window == "undefined" && process.argv[2] == "test") {
 
 	const year = "2021"
 
-	for (let i = +process.argv[3] || 1; i <= 20; i++) {
+	for (let i = +process.argv[3] || 1; i <= 21; i++) {
 		const func = require(`./${year}/${i}.js`)
 		const input = fs.readFileSync(`./${year}/inputs/${i}`, "utf8")
 		const answers = fs.readFileSync(`./${year}/answers/${i}`, "utf8").split("\n-----\n")