Kotlin solution for https://adventofcode.com/2021/day/21

submarine.kt

private fun readChar(): Char = readString()[0]
private fun readChars(): CharArray = readString().toCharArray()
private fun readInt(): Int = readLine()!!.toInt()
private fun readInts(): List<Int> = readLine()!!.trim().split(" ").map(String::toInt)
private fun readLong(): Long = readLine()!!.toLong()
private fun readLongs(): List<Long> = readLine()!!.trim().split(" ").map(String::toLong)
private fun readString(): String = readLine()!!.trim()
private fun readStrings(): List<String> = readLine()!!.trim().split(" ")

fun roll(die: Int, dieSides: Int) = Pair(die + (die % dieSides) + (die + 1) % dieSides + 2, (die + 2) % dieSides + 1)

fun main() {
	var rolls = IntArray(10)
	rolls[0] = 1
	for (i in 1..3) {
		val newRolls = IntArray(10)

		for ((score, count) in rolls.withIndex()) {
			if (count == 0) continue
			for (roll in 1..3) {
				newRolls[score + roll] += count
			}
		}

		rolls = newRolls
	}

	val boardSize = 10
	val dieSides = 100
	val winningScore = 21

	val startPosition1 = readString().split(": ")[1].toInt()
	val startPosition2 = readString().split(": ")[1].toInt()

	var state = Array(winningScore + boardSize) { Array(winningScore + boardSize) { Array(boardSize + 1) { LongArray(boardSize + 1) } } }
	state[0][0][startPosition1][startPosition2] = 1

	var done = false

	var wins1 = 0L
	var wins2 = 0L

	var newState : Array<Array<Array<LongArray>>>
	while (!done) {
		newState = Array(winningScore + boardSize) { Array(winningScore + boardSize) { Array(boardSize + 1) { LongArray(boardSize + 1) } } }

		done = true
		for (score1 in 0 until winningScore) {
			for (score2 in 0 until winningScore) {
				for (pos1 in 1..boardSize) {
					for (pos2 in 1..boardSize) {
						val stateCount = state[score1][score2][pos1][pos2]
						if (stateCount == 0L) continue
						done = false

						for ((roll, count) in rolls.withIndex()) {
							val newPos1 = (pos1 - 1 + roll) % boardSize + 1
							val newScore1 = score1 + newPos1

							if (newScore1 >= 21) wins1 += stateCount * count
							else newState[newScore1][score2][newPos1][pos2] += stateCount * count
						}
					}
				}
			}
		}
		state = newState
		if (done) break

		newState = Array(31) { Array(31) { Array(11) { LongArray(11) } } }
		for (score1 in 0..20) {
			for (score2 in 0..20) {
				for (pos1 in 1..10) {
					for (pos2 in 1..10) {
						val stateCount = state[score1][score2][pos1][pos2]
						if (stateCount == 0L) continue
						done = false

						for ((roll, count) in rolls.withIndex()) {
							val newPos2 = (pos2 - 1 + roll) % boardSize + 1
							val newScore2 = score2 + newPos2

							if (newScore2 >= 21) wins2 += stateCount * count
							else newState[score1][newScore2][pos1][newPos2] += stateCount * count
						}
					}
				}
			}
		}
		state = newState
	}

	println(wins1.coerceAtLeast(wins2))
}

/*

Player 1 starting position: 4
Player 2 starting position: 8

 */