Advent of Code 2021 in Kotlin - Day 4


The Day 4 problem describes the simulation of Bingo game. While the rules of the game are widely known and the simulation of the game is not so hard, it turns out that the main point of the solution is to efficiently parse and represent the given data, to come up with some nice solution for the problem. Let’s begin then with some useful Kotlin concepts and helper functions 😊.

Helper functions

We define two main helpers that seems to be useful in the future tasks as they solve some general problems in pretty efficient way.

Transpose List<List<T>>

Flipping rows with columns of 2D array of list requires equal sizes of each row of data. We need to check that before processing the collection and then the transpose of List<List<T>> can be written in Kotlin in one simple line of code with no extra performance overhead - just generate new collection of collections with lambdas that use the original collection in pretty straightforward way - flipping $x$ and $y$ axes of values from matrix (i.e. List<List<T>>).

fun <T> List<List<T>>.transpose(): List<List<T>> {
    val n = map { it.size }.toSet().singleOrNull()
        ?: throw IllegalArgumentException("Invalid data to transpose: $this")
    return List(n) { y -> List(size) { x -> this[x][y] } }

Group data separated by value

This transformation is usually required when the data is separated with empty lines and single group of lines should be processed together. In today’s task we’re given a list of strings that represents the board games, but every board is represented with multiple lines and separated with empty line from other boards.

We can try to implement such functionality with Sequence<V> builder that is later collected to List<V> in single call. We use it because it allows to yield the result only at certain moments - in our case when the description of the board finishes, and we have accumulated the description of the last board, we yield our current result.

Notice that we use some var to keep the current value of accumulated value that is later yield after some transformation. It’s worth mentioning here how the variables caught by lambda scopes works in Kotlin as it’s quite different from other languages - when we deal with mutable var it remains mutable in the captured scope of the lambda and the assignments executed in scope of the lambda are visible outside. It’s really useful technique when we define some nested functions and don’t want to pass its state in the variable - we can just define it before function definitions and use later as it’d be given as function’s parameter.

What I’ve learned when writing this helper function is the restriction for the yield function calls that have to be defined directly inside the SequenceScope<T>, so we cannot define some helper function inside the sequence { } builder and use yield in it. One may ask then, why using forEach is then allowed here if it defines some lambda function too? The answer is somehow surprising but didactic - this function is defined with inline modifier, so it’s translated to direct call of the code of the for loop (as stated in its definition). Remember then, that if you find some unexpected pattern from top level perspective in your bytecode, it’s probably caused by inlining a few functions' calls definitions from standard library 😉.

fun <U, V> List<U>.groupDividedBy(
    separator: U,
    transform: (List<U>) -> V
): List<V> = sequence {
    var curr = mutableListOf<U>()
    forEach {
        if (it == separator && curr.isNotEmpty()) yield(transform(curr))
        if (it == separator) curr = mutableListOf()
        else curr += it
    if (curr.isNotEmpty()) yield(transform(curr))


We present some quite general solution that is not the post performant but seems to be one of the most readable ones seen today. That’s because we define some general function simulateSelectingFirst for a game that allows to simply define the strategy for selecting winning board during the simulation.

Additionally, parsing the data lines has become really readable with created helper functions. We use them also for transposing the data of the board to be able to check for bingo easily. The cost of checking for bingo is proportional to size of the board but the boards are tiny. If we would like to implement this in more efficient way, we should count marked values for each row and each column and check, if it’s big enough for bingo.


object Day4 : AdventDay() {
  override fun solve() {
    val lines = reads<String>() ?: return
    val game = Game(lines.extractOrder(), lines.extractBoards())

    game.simulateSelectingFirst { board -> board.wins() }
      ?.let { (b, v) -> b.unmarkedValues().sum() * v }.printIt()

    val leftBoards = game.boards.toMutableSet()
    game.simulateSelectingFirst { board ->
      if (board.wins()) leftBoards -= board
    }?.let { (b, v) -> b.unmarkedValues().sum() * v }.printIt()

  private fun List<String>.extractOrder() =
    firstOrNull()?.split(",")?.map { it.value<Int>() }
      ?: throw IllegalArgumentException("No order defined in data: $this")

  private fun List<String>.extractBoards() =
    drop(1).groupSeparatedBy("") { it.toBoard<Int>() }

private class Game<V>(val order: List<V>, val boards: List<Board<V>>) {
  fun simulateSelectingFirst(strategy: (Board<V>) -> Boolean): Pair<Board<V>, V>? {
    for (v in order) {
      boards.forEach { it.mark(v) }
      boards.firstOrNull(strategy)?.let { return Pair(it, v) }
    return null

private inline fun <reified V> List<String>.toBoard() = map { line ->
    .filter { it.isNotBlank() }
    .mapTo(mutableListOf()) { it.value<V>() }
}.let { Board(it) }

private class Board<V>(private val values: List<List<V>>) {
  private val transposedValues = values.transpose()
  private val markedValues = mutableSetOf<V>()
  private val allValues = values.flatten()

  fun mark(value: V) = markedValues.add(value)
  fun wins() = values.rowWins() || transposedValues.rowWins()
  fun unmarkedValues() = allValues - markedValues

  private fun List<List<V>>.rowWins() =
    any { row -> row.all { it in markedValues } }

Extra notes

Let’s see how the Board<V> is represented in the solution. It contains a copy of values in transposed order to check for the bingo as described above. What’s more interesting , it contains some extension function fun List<List<V>>.rowWins() that has access to the field of the class. In this way we define the function that has to be called in the context of specified object (in this case this can only be Board<V> context but the same context applies in DSL design in Kotlin). It’s a gorgeous way of expressing some intentions for functions when it can have a few contexts (so almost a few this receivers). You can read more about it in KEEP 259 discussion to see what cool features are going to be introduced to Kotlin in some future releases 🙃.

Student of Computer Science

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