Discussion on: Advent of Code 2019 Solution Megathread - Day 8: Space Image Format

[Neil Gall]

Felt like a Kotlin day today. Pretty simple compared to yesterday.

import java.io.File

typealias Pixel = Char

const val BLACK: Char = '0'
const val WHITE: Char = '1'
const val TRANSPARENT: Char = '2'

data class Size(val width: Int, val height: Int) {
    val pixels: Int = width * height
}

data class Layer(val pixels: List<Pixel>)

data class SpaceImageFormat(val size: Size, val layers: List<Layer>)

fun String.decode(size: Size): SpaceImageFormat =
    SpaceImageFormat(
        size = size,
        layers = trim().asIterable().chunked(size.pixels).map(::Layer)
    )

fun transparentLayer(size: Size): Layer =
    Layer(List<Pixel>(size.pixels) { TRANSPARENT })

fun Layer.countPixels(pixel: Pixel): Int =
    pixels.count { p -> p == pixel }

fun Pair<Pixel, Pixel>.merge() =
    if (first == TRANSPARENT) second else first

fun Layer.merge(next: Layer): Layer =
    Layer(pixels.zip(next.pixels).map { p -> p.merge() })

fun Layer.render(size: Size): String =
    pixels.map { p -> if (p == WHITE) '#' else ' ' }
        .chunked(size.width)
        .map { row -> row.joinToString("") }
        .joinToString("\n")

fun part1(image: SpaceImageFormat): Int {
    val layerWithLeastZeros = image.layers.minBy { layer -> layer.countPixels('0') }!!
    val ones = layerWithLeastZeros.countPixels('1')
    val twos = layerWithLeastZeros.countPixels('2')
    return ones * twos
}

fun part2(image: SpaceImageFormat): String =
    image.layers.fold(transparentLayer(image.size), Layer::merge)
        .render(image.size)

fun main() {
    val image = File("input.txt").readText().decode(Size(25, 6))
    println("Part 1... ${part1(image)}")
    println("Part 2...\n${part2(image)}")
}