Advent of Code 2021 in Kotlin - Day 21


The Day 21 problem starts with a pretty simple and obvious part of not random dice game, while in second part we have to think deeper how to efficiently simulate multiple paths of quantum game that may appear. Let’s see then how we can deal with both parts by working with model built with immutable objects.


We simulate the game with its immutable representation as DiceGame that holds points for both players and the order of players to move. We added to it some helper methods looser and winner which are designed to answer the question “Was the last moved player a looser/winner?” as we use them only after player moves. We’re able to transform such a game with some dice value by creating a new instance of game with points of player updated and players switched. As it is an immutable data, we used fold once again as in multiple previous problems to simulate state change of such object.

In the second part, we have to count the worlds, in which players win. As the number of possible values of dice is limited, we write them to QUANTUM_DICE_SPLITS for better readability of our intention. The numbers on right represent on how many ways we have get every sum from left, when the possible sums are listed in comment.

Then, the simulation of quantum game have to count the occurrences of each game instead of keeping them in some collection. For example, instead of having a list with 42 the same games, we just remember the game and its associated value that equals 42. We can update these values accordingly by multiplying the last count by the number of worlds splits, in which current dice value appear by simply writing

updated[nextGame] = updated[nextGame] + splits * playing[game]

as we’ve used the DefaultMap<DiceGame, Long> once again for simpler problem representation.


object Day21 : AdventDay() {
  override fun solve() {
    val data = reads<String>() ?: return
    val (p1, p2) = { it.toPlayer() }

    simulateGame(p1, p2)?.let { (p, idx) -> p.points * idx }.printIt()
    simulateQuantumGame(p1, p2).maxOf { it.value }.printIt()

private fun simulateGame(p1: Player, p2: Player): Pair<Player, Int>? {
  generateDiceNumbers().foldIndexed(DiceGame(p1, p2, toPoints = 1000)) { idx, game, dice ->
    game.move(dice).apply { looser()?.let { return Pair(it, 3 * (idx + 1)) } }
  return null

private val QUANTUM_DICE_SPLITS = listOf(
  3 to 1, // 1+1+1
  4 to 3, // 1+1+2, 1+2+1, 2+1+1,
  5 to 6, // 2+2+1, 2+1+2, 1+2+2, 1+1+3, 1+3+1, 3+1+1
  6 to 7, // 1+2+3, 1+3+2, 2+1+3, 2+3+1, 3+1+2, 3+2+1, 2+2+2
  7 to 6, // 2+2+3, 2+3+2, 3+2+2, 3+3+1, 3+1+3, 1+3+3
  8 to 3, // 3+3+2, 3+2+3, 2+3+3
  9 to 1, // 3+3+3

private fun simulateQuantumGame(p1: Player, p2: Player): Map<Int, Long> {
  val playing = mapOf(DiceGame(p1, p2, toPoints = 21) to 1L).toDefaultMap(0)
  val winCount = DefaultMap<Int, Long>(0L)

  while (playing.isNotEmpty()) {
    val updated = DefaultMap<DiceGame, Long>(0)
    for (game in playing.keys) {
      for ((dice, splits) in QUANTUM_DICE_SPLITS) {
        val nextGame = game.move(dice)
        when (val winner = nextGame.winner()) {
          null -> updated[nextGame] = updated[nextGame] + splits * playing[game]
          else -> winCount[winner.idx] = winCount[winner.idx] + splits * playing[game]
    playing.also { it.clear() }.also { it.putAll(updated) }
  return winCount

private fun generateDiceNumbers() = generateSequence(0) { it + 1 }
  .map { it % 100 + 1 }.windowed(size = 3, step = 3) { it.sum() }

private fun String.toPlayer() = removePrefix("Player ").run {
  val idx = takeWhile { it.isDigit() }.toInt()
  val position = dropWhile { it.isDigit() }.removePrefix(" starting position: ").toInt()
  Player(idx, position, points = 0)

private data class Player(val idx: Int, val position: Int, val points: Long) {
  fun move(rolled: Int): Player = ((position - 1 + rolled) % 10 + 1).let { copy(position = it, points = it + points) }

private data class DiceGame(val now: Player, val last: Player, val toPoints: Long) {
  fun move(rolled: Int): DiceGame = copy(now = last, last = now.move(rolled))
  fun looser(): Player? = if (last.points >= toPoints) now else null
  fun winner(): Player? = if (last.points >= toPoints) last else null

Extra notes

The process of generation next values of dice in the first part of problem could have been expressed in really handy way, with the usage of sequences and windowed function of them. It’s not the most performant approach to this problem as we could define closed formula for these numbers, but as it was the first part of the problem, single line solution with just

generateSequence(0) { it + 1 }.map { it % 100 + 1 }.windowed(size = 3, step = 3) { it.sum() }

was in my opinion the best fit here.

Notice, that in the solution of second part, we decided to use some local updated map, which is used to store the current state of the playing games instead of modifying the playing map. We need to remember that in case of Java collections (which are used in Kotlin), we cannot modify the collection when iterating over its values. The ConcurrentModificationException is thrown, if we would try to do so. There are of course different approaches to solve such problems, e.g. we can copy the collection to iterate over copy and modify the original one or just iterate over original and modify it after iteration. The second approach seemed to give more clear result in this solution and was fast enough, so we decided to apply it to our solution.

Student of Computer Science

My interests include robotics (mainly with Arduino), mobile development for Android (love Kotlin) and Java SE/EE applications development.