Advent of Code 2021 in Kotlin - Day 1


We start with Day 1 problem for which the solution is based on the template from the last year Advent of Code. Let’s begin to see some cool features of modern language - Kotlin 😎.


We solve the problem in pretty straightforward way - we analyze the input data in windows of different sizes. In the first part it’s enough to analyze the windows of size = 2 and count how many of them contains increase.

In the second part, we add extra step before counting the difference - we need to calculate the sums of windows of size = 3. Then, the solution is the same as in the first part.


object Day1 : AdventDay() {
  override fun solve() {
    val depths = reads<Int>() ?: return

  private fun Sequence<Int>.countIncreases() = windowed(size = 2)
    .count { (prev, curr) -> curr > prev }

  private fun Sequence<Int>.countSumIncreases(size: Int = 3) = windowed(size)
    .map { it.sum() }

Extra notes

Let’s see that we used Sequence<T> when solving the problem. It’s worth recalling that multiple operations on items in iterables should be implemented with usage of sequences because only in this way we can use the functional programming style and not cause the quadratic complexity of our solutions.

Also in countIncreases we could use the zipWithNext function, but it’s not required because of the feature of lists in Kotlin standard library. I want to recall that they can be destructured with componentN() functions as Pair<K, V> and that’s what we do in (prev, curr) -> curr > prev lambda definition. It’s worth mentioning that these componentN() functions can be defined also in our classes, so keep this in your mind when designing some Kotlin library API that could benefit from using these constructs 😉.

Student of Computer Science

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