day 21

.aocrunner.json

@@ -299,16 +299,16 @@
     },
     {
       "part1": {
-        "solved": false,
+        "solved": true,
-        "result": null,
+        "result": "1002474",
         "attempts": [],
-        "time": null
+        "time": 0.08
       },
       "part2": {
-        "solved": false,
+        "solved": true,
-        "result": null,
+        "result": "919758187195363",
         "attempts": [],
-        "time": null
+        "time": 147.4
       }
     },
     {

README.md

@@ -31,7 +31,7 @@
 [![Day](https://badgen.net/badge/18/%E2%98%85%E2%98%85/green)](src/day18)
 ![Day](https://badgen.net/badge/19/%E2%98%86%E2%98%86/gray)
 ![Day](https://badgen.net/badge/20/%E2%98%86%E2%98%86/gray)
-![Day](https://badgen.net/badge/21/%E2%98%86%E2%98%86/gray)
+[![Day](https://badgen.net/badge/21/%E2%98%85%E2%98%85/green)](src/day21)
 ![Day](https://badgen.net/badge/22/%E2%98%86%E2%98%86/gray)
 ![Day](https://badgen.net/badge/23/%E2%98%86%E2%98%86/gray)
 ![Day](https://badgen.net/badge/24/%E2%98%86%E2%98%86/gray)
@@ -188,9 +188,9 @@ Both parts: 430.38ms
 
 ```
 Day 18
-Time part 1: 38.77ms
+Time part 1: 35.41ms
-Time part 2: 147.3ms
+Time part 2: 268.36ms
-Both parts: 186.07000000000002ms
+Both parts: 303.77ms
 ```
 
 ```
@@ -209,9 +209,9 @@ Both parts: -
 
 ```
 Day 21
-Time part 1: -
+Time part 1: 0.08ms
-Time part 2: -
+Time part 2: 147.4ms
-Both parts: -
+Both parts: 147.48000000000002ms
 ```
 
 ```
@@ -243,8 +243,8 @@ Both parts: -
 ```
 
 ```
-Total stars: 36/50
+Total stars: 38/50
-Total time: 2519.45ms
+Total time: 2784.6299999999997ms
 ```
 
 <!--/RESULTS-->

src/day21/README.md

@@ -0,0 +1,9 @@
+# 🎄 Advent of Code 2021 - day 21 🎄
+
+## Info
+
+Task description: [link](https://adventofcode.com/2021/day/21)
+
+## Notes
+
+...

src/day21/index.js

@@ -0,0 +1,85 @@
+import run from "aocrunner"
+
+const parseInput = (rawInput) => rawInput.match(/(?<=: )\d+/g).map(x=>+x)
+
+const part1 = (rawInput) => {
+  const [aStart, bStart] = parseInput(rawInput)
+  let a = aStart, b = bStart, aScore = 0, bScore = 0, isATurn = true
+  let die = 0, numRolls = 0
+  function roll() {
+    numRolls++
+    return (die = die % 100 + 1)
+  }
+  while (aScore < 1000 && bScore < 1000) {
+    const rolls = [roll(),roll(),roll()]
+    if (isATurn) {
+      a += rolls[0] + rolls[1] + rolls[2]
+      a = (a - 1) % 10 + 1
+      aScore += a
+    } else {
+      b += rolls[0] + rolls[1] + rolls[2]
+      b = (b - 1) % 10 + 1
+      bScore += b
+    }
+    isATurn = !isATurn
+  }
+  return Math.min(aScore, bScore) * numRolls
+}
+
+const frequencies = []
+for (let i = 1; i <= 3; i++)
+  for (let j = 1; j <= 3; j++)
+    for (let k = 1; k <= 3; k++) {
+      frequencies[i + j + k] = (frequencies[i + j + k] ?? 0) + 1
+    }
+
+const part2 = (rawInput) => {
+  const [aStart, bStart] = parseInput(rawInput)
+  const memo = []
+  const winThreshold = 21
+  function countWins(aScore, bScore, aPos, bPos, isATurn) {
+    let existing = memo[[aScore, bScore, aPos, bPos, isATurn]]
+    if (existing !== undefined) return existing
+    if (aScore >= winThreshold) {
+      memo[[aScore, bScore, aPos, bPos, isATurn]] = [1, 0]
+      return [1, 0]
+    }
+    if (bScore >= winThreshold) {
+      memo[[aScore, bScore, aPos, bPos, isATurn]] = [0, 1]
+      return [0, 1]
+    }
+    let totalWins = [0, 0]
+    for (let diceSum = 3; diceSum <= 9; diceSum++) {
+      let thisWin
+      if (isATurn) {
+        const nextAPos = (aPos - 1 + diceSum) % 10 + 1
+        thisWin = countWins(aScore + nextAPos, bScore, nextAPos, bPos, !isATurn)
+      } else {
+        const nextBPos = (bPos - 1 + diceSum) % 10 + 1
+        thisWin = countWins(aScore, bScore + nextBPos, aPos, nextBPos, !isATurn)
+      }
+      totalWins = [totalWins[0] + frequencies[diceSum] * thisWin[0], totalWins[1] + frequencies[diceSum] * thisWin[1]]
+    }
+    memo[[aScore, bScore, aPos, bPos, isATurn]] = totalWins
+    return totalWins
+  }
+  return Math.max(...countWins(0, 0, aStart, bStart, true))
+}
+
+run({
+  part1: {
+    tests: [
+      { input: `Player 1 starting position: 4
+Player 2 starting position: 8`, expected: 739785 },
+    ],
+    solution: part1,
+  },
+  part2: {
+    tests: [
+      { input: `Player 1 starting position: 4
+Player 2 starting position: 8`, expected: 444356092776315 },
+    ],
+    solution: part2,
+  },
+  trimTestInputs: true,
+})