Kotlin solution for day 3 of advent of code 2020. https://adventofcode.com/2020/day/3

Task.kt

package day03

import java.io.File

@JvmInline
private value class Map(val map: List<List<Square>>) {

    fun countNumberOfTreesForStep(step: Coordinate): Int {
        return generateSequence(
            seed = Coordinate(0, 0),
            nextFunction = { coordinate: Coordinate ->
                val nextCoordinate = coordinate + step
                if (nextCoordinate.y > map.size) return@generateSequence null
                nextCoordinate
            }
        ).count { coordinate ->
            this.getSquareAtCoordinateOrNull(coordinate) == Square.Tree
        }
    }

    private fun getSquareAtCoordinateOrNull(coordinate: Coordinate): Square? {
        return map.getOrNull(coordinate.y)?.let { row ->
            row.getOrNull((coordinate.x % row.size))
        }
    }
}

private data class Coordinate(val x: Int, val y: Int) {
    operator fun plus(other: Coordinate): Coordinate {
        return Coordinate(x + other.x, y + other.y)
    }
}

private enum class Square {
    Empty,
    Tree;

    companion object {
        fun fromChar(char: Char): Square {
            return when (char) {
                '#' -> Tree
                '.' -> Empty
                else -> throw IllegalArgumentException()
            }
        }
    }
}

fun main() {
    val inputMap = File("src/main/kotlin/day03/input.txt")
        .readLines()
        .filter(String::isNotEmpty)
        .map { line: String ->
            line.map(Square::fromChar)
        }
        .let(::Map)

    // Part 1
    inputMap.countNumberOfTreesForStep(Coordinate(3, 1)).also(::println)

    // Part 2
    listOf(
        Coordinate(1, 1),
        Coordinate(3, 1),
        Coordinate(5, 1),
        Coordinate(7, 1),
        Coordinate(1, 2),
    ).map { coordinate: Coordinate ->
        inputMap.countNumberOfTreesForStep(coordinate)
    }.fold(1L) { accumulatedScore: Long, score: Int ->
        accumulatedScore * score
    }.also(::println)
}