Solve Day21

README.md

@@ -25,6 +25,7 @@ Welcome to my Advent of Code[^aoc] Kotlin project. Here I will be sharing my sol
 | 18  | [Snailfish](src/Day18.kt)               |   🌟   |   🌟   |
 | 19  | [Beacon Scanner](src/Day19.kt)          |   🌟   |   🌟   |
 | 20  | [Trench Map](src/Day20.kt)              |   🌟   |   🌟   |
+| 21  | [Dirac Dice](src/Day21.kt)              |   🌟   |   🌟   |
 
 
 [^aoc]: [Advent of Code](https://adventofcode.com) – An annual event of Christmas-oriented programming challenges started December 2015.

src/Day21.kt

@@ -0,0 +1,44 @@
+fun main() {
+
+    fun part1(input: List<String>): Int {
+        val pos = Array(2) { input[it].substringAfterLast(" ").toInt() }
+        val score = Array(2) { 0 }
+        var (rolls, dice, i) = Triple(0, 1, 0)
+        fun roll() = dice.also { dice = dice % 100 + 1; rolls++ }
+        while (true) {
+            pos[i] = (pos[i] + roll() + roll() + roll() - 1) % 10 + 1
+            score[i] += pos[i]
+            if (score[i] >= 1000) return score[1 - i] * rolls
+            i = 1 - i
+        }
+    }
+
+    fun part2(input: List<String>): Long {
+        data class Universe(val p1: Int, val p2: Int, val s1: Int, val s2: Int)
+
+        val pos = Array(2) { input[it].substringAfterLast(" ").toInt() }
+        val dp = mutableMapOf<Universe, Pair<Long, Long>>()
+        fun solve(u: Universe): Pair<Long, Long> {
+            dp[u]?.let { return it }
+            if (u.s1 >= 21) return 1L to 0L
+            if (u.s2 >= 21) return 0L to 1L
+            var ans = 0L to 0L
+            for (d1 in 1..3) for (d2 in 1..3) for (d3 in 1..3) {
+                val newP1 = (u.p1 + d1 + d2 + d3 - 1) % 10 + 1
+                val newS1 = u.s1 + newP1
+                val (x, y) = solve(Universe(u.p2, newP1, u.s2, newS1))
+                ans = ans.first + y to ans.second + x
+            }
+            return ans.also { dp[u] = it }
+        }
+        return solve(Universe(pos[0], pos[1], 0, 0)).let { maxOf(it.first, it.second) }
+    }
+
+    val testInput = readInput("Day21_test")
+    checkEquals(part1(testInput), 739785)
+    checkEquals(part2(testInput), 444356092776315)
+
+    val input = readInput("Day21")
+    println(part1(input))
+    println(part2(input))
+}