AoC/2021/day20.swift

@main
struct Day20: Puzzle {
    func run() {
        var image = Image(input)
        for i in 0..<50 {
            image = image.run(map: map)
            if i == 1 {
                print("Part 1:", image.countLitPixels())
            }
        }
        print("Part 2:", image.countLitPixels())
    }
}

struct Image {
    var width: Int
    var height: Int
    var pixels: [Bool]
    var border: Bool

    init(_ string: String) {
        pixels = []
        width = 0
        height = 1
        border = false
        for char in string {
            if char.isNewline {
                width = 0
                height += 1
                continue
            }

            pixels.append(char == "#")
            width += 1
        }
    }

    init(width: Int, height: Int, border: Bool) {
        self.width = width
        self.height = height
        self.border = border
        self.pixels = Array(repeating: false, count: width * height)
    }

    subscript(x: Int, y: Int) -> Bool {
        get {
            guard 0..<width ~= x && 0..<height ~= y else {
                return border
            }

            return pixels[x + width * y]
        }

        set {
            precondition(0..<width ~= x && 0..<height ~= y)
            pixels[x + width * y] = newValue
        }
    }

    var borderIndex: Int { border ? 7 : 0 }

    func lookupIndex(x: Int, y: Int) -> Int {
        var index = 0
        for i in -1...1 {
            for j in -1...1 {
                index = index << 1 | (self[x + j, y + i] ? 1 : 0)
            }
        }
        return index
    }

    func run(map: [Bool]) -> Image {
        var outputImage = Image(width: width + 2, height: height + 2, border: map[borderIndex])
        for y in -1...height {
            for x in -1...width {
                outputImage[x + 1, y + 1] = map[lookupIndex(x: x, y: y)]
            }
        }
        return outputImage
    }

    func countLitPixels() -> Int {
        pixels.lazy.filter { $0 }.count
    }
}

let map = "#####.#.###.###.#.#.####.#####.####.#..####.##.####...##......#.##.##.#.#..#.##.###.#.#.#.##.#..#.####.#.#.##.#..........###.##...#.#...#.#.###..#...##.####.##...###.....##..##..##.#......#.#.##.#.#.##.#.......###.##.#.###.##..##.......##....#.##....#..###.######.##...##.##...#.#.##...##.##.#....##.####..#..#..##.##.#.#..#....#######.###.##...#.####..#.#.#.##...##..##.#.#.#.##.#.......###..######..###..##......###..###.#.#.#.......#.....##.#.##..#.##.#.##.####..#.##....##.#.#..#.####.##.#.#.##...#..##.####."
    .map { $0 == "#" }

let input = """
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"""