day 21

src/day21/README.md

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

src/day21/index.js

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+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,
+})