Advent of Code Day 8.swift

import Foundation

enum Pixel: Int {
  case black = 0
  case white = 1
  case transparent = 2
}

let width = 25
let height = 6
let size = width * height

let input = try
  String(contentsOfFile: "data.txt")
  .trimmingCharacters(in: .whitespacesAndNewlines)

input
  .enumerated()
  .reduce(into: Array(repeating: [Pixel](), count: input.count / size)) { partial, current in
    let pixel = Pixel(rawValue: Int(String(current.1))!)!
    partial[current.0 / size].append(pixel)
  }
  .reduce(into: Array(repeating: Pixel.transparent, count: size)) { partial, layer in
    for (idx, pixel) in layer.enumerated() {
      if pixel != .transparent && (partial[idx] == .transparent) {
        partial[idx] = pixel
      }
    }
  }
  .enumerated()
  .forEach {
    if $0.0 != 0 && $0.0 % width == 0 { print("") }
    print($0.1 == .white ? "█" : " ", terminator: "")
  }

Readable.swift

import Foundation

enum Pixel: Int {
  case black = 0
  case white = 1
  case transparent = 2
}

func makeLayers(_ input: String, size: Int) -> [[Pixel]] {
  var layers = Array(repeating: [Pixel](), count: input.count / size)
  for (i, char) in input.enumerated() {
    let pixel = Pixel(rawValue: Int(String(char))!)!
    layers[i / size].append(pixel)
  }
  return layers
}

func flatten(_ layers: [[Pixel]], size: Int) -> [Pixel] {
  var image = Array(repeating: Pixel.transparent, count: size)
  layers.forEach { layer in
    for (i, pixel) in layer.enumerated() {
      if pixel != .transparent && (image[i] == .transparent) {
        image[i] = pixel
      }
    }
  }
  return image
}

func printImage(_ pixels: [Pixel], width: Int) {
  for (i, pixel) in pixels.enumerated() {
    if i != 0 && i % width == 0 { print("") }
    print(pixel == .white ? "█" : " ", terminator: "")
  }
}

let width = 25
let height = 6
let size = width * height

let input = try
  String(contentsOfFile: "data.txt")
  .trimmingCharacters(in: .whitespacesAndNewlines)

let layers = makeLayers(input, size: size)
let image = flatten(layers, size: size)
printImage(image, width: width)