Kotlin solution for https://adventofcode.com/2021/day/20

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 rowIndex(i: Int, rows: Int, cols: Int) = i / cols

fun colIndex(i: Int, rows: Int, cols: Int) = i % cols

fun index(r: Int, c: Int, rows: Int, cols: Int) = r * cols + c

fun process(image: ByteArray, rules: ByteArray, rows: Int, cols: Int, outsidePixelValue: Byte) : ByteArray {
	val output = ByteArray((rows + 2) * (cols + 2))

	for (r in -1 .. rows) {
		for (c in -1 .. cols) {
			var ruleIndex = 0

			for (rc in r - 1 .. r + 1) {
				for (cc in c - 1 .. c + 1) {
					ruleIndex = (ruleIndex shl 1) + if (rc !in 0 until rows || cc !in 0 until cols) outsidePixelValue else image[index(rc, cc, rows, cols)]
				}
			}

			output[index(r + 1, c + 1, rows + 2, cols + 2)] = rules[ruleIndex]
		}
	}

	return output
}

fun main() {
	val iterations = 50
	val byteOne = 1.toByte()

	val rules = readString().map { if (it == '.') 0 else byteOne }.toByteArray()
	var input = mutableListOf<String>()

	readString() // empty line

	var nextLine = readString()
	nextLine@ while (nextLine.isNotEmpty()) {
		input.add(nextLine)

		nextLine = readString()
	}

	var rows = input.size
	var cols = input[0].length
	var image = ByteArray(rows * cols) { if (input[rowIndex(it, rows, cols)][colIndex(it, rows, cols)] == '.') 0 else 1 }

	for (iteration in 0 until iterations) {
		image = process(image, rules, rows, cols, (rules[0].toInt() and (iteration % 2)).toByte())
		rows += 2
		cols += 2
	}

	for (r in 0 until rows) {
		println(image.slice(r * cols until (r + 1) * cols).map { if (it == byteOne) '#' else '.' }.joinToString(""))
	}

	println(image.filter { it == byteOne }.size)
}

/*

..#.#..#####.#.#.#.###.##.....###.##.#..###.####..#####..#....#..#..##..###..######.###...####..#..#####..##..#.#####...##.#.#..#.##..#.#......#.###.######.###.####...#.##.##..#..#..#####.....#.#....###..#.##......#.....#..#..#..##..#...##.######.####.####.#.#...#.......#..#.#.#...####.##.#......#..#...##.#.##..#...##.#.##..###.#......#.#.......#.#.#.####.###.##...#.....####.#..#..#.##.#....##..#.####....##...##..#...#......#.#.......#.......##..####..#...#.#.#...##..#.#..###..#####........#..####......#..#

#..#.
#....
##..#
..#..
..###


 */