Day20.kt

package day20

import java.nio.file.Files
import java.nio.file.Path


class Enhancer(val code: String) {
    private fun getPixel(r: Int, c: Int, data: List<String>) = data.getOrNull(r)?.getOrNull(c) ?: defaultChar
    private var defaultChar = '.'

    fun enhance(input: List<String>): List<String> {
        val rows = input.size
        val cols = input[0].length

        fun getValue(r: Int,c: Int): Char {
            val pixels = (r-1..r+1).flatMap { rr -> (c-1..c+1).map { cc -> getPixel(rr,cc, input) } }
            val idx  = pixels
                .map { p -> if(p == '#') 1 else 0 }
                .fold(0){acc, next -> (acc shl 1) + next }
            return code[idx]
        }

        val newData =  (-1..rows).map { r ->
            (-1..cols).map { c ->
                getValue(r, c)
            }.joinToString("")
        }
        defaultChar = if(defaultChar == '.') code[0] else code[code.lastIndex]
        return newData
    }
}

fun enhance(enhancer: Enhancer, data: List<String>, times: Int): List<String> {
    var current = data
    (1..times).forEach {
        current = enhancer.enhance(current)
    }
    return current
}

fun main(args: Array<String>) {
    val input = Files.readAllLines(Path.of(args[0]))
    val data = input.drop(1).filter { it.isNotEmpty() }
    val enhancer = Enhancer(input.first()!!)
    val res = enhance(enhancer, data, 50)
    println("${res.size} x ${res[0].length}")
    println(res.sumOf { s -> s.count { it == '#' } })
}