2015 - Day 18 - Like a GIF For Your Yard

2015/Sources/AdventOfCode.swift

@@ -19,6 +19,7 @@ let allChallenges: [any AdventDay] = [
  Day15(),
  Day16(),
  Day17(),
+ Day18(),
 ]
 
 @main

2015/Sources/Day18.swift

@@ -0,0 +1,95 @@
+import Foundation
+
+struct Day18: AdventDay {
+ let data: String
+ let steps: Int
+ let width: Int
+ let height: Int
+
+ private var xRange: Range<Int> { 0..<width }
+ private var yRange: Range<Int> { 0..<height }
+ private var coordinates: [(Int, Int)] {
+ yRange.flatMap { y in
+ xRange.map { x in (x, y)
+ }
+ }
+ }
+ private var corners: [(Int, Int)] {
+ [
+ (0, 0),
+ (0, height - 1),
+ (width - 1, 0),
+ (width - 1, height - 1),
+ ]
+ }
+
+ private var grid: [[Bool]] {
+ data.split(separator: /\n/).map {
+ $0.map {
+ $0 == "#"
+ }
+ }
+ }
+
+ init(data: String) {
+ self.init(data: data, steps: 100, width: 100, height: 100)
+ }
+
+ init(data: String, steps: Int, width: Int, height: Int) {
+ self.data = data
+ self.steps = steps
+ self.width = width
+ self.height = height
+ }
+
+ private func solve(_ initialState: [[Bool]], coordinates: [(Int, Int)]) -> Int {
+ var state = initialState
+ var nextState = state
+
+ for _ in 0..<steps {
+ for (x, y) in coordinates {
+ let isOn = state[y][x]
+ let neighbours = neighbours(x, y)
+
+ switch neighbours.count(where: { x, y in state[y][x] }) {
+ case 2 where isOn:
+ nextState[y][x] = true
+ case 3:
+ nextState[y][x] = true
+ default:
+ nextState[y][x] = false
+ }
+ }
+
+ state = nextState
+ }
+
+ return state.reduce(0) { count, row in
+ count + row.count { $0 }
+ }
+ }
+
+ func part1() -> Int {
+ solve(grid, coordinates: coordinates)
+ }
+
+ func part2() -> Int {
+ var state = grid
+ for (x, y) in corners {
+ state[y][x] = true
+ }
+ return solve(state, coordinates: coordinates.filter({ x, y in !isCorner(x, y) }))
+ }
+
+ private func isCorner(_ x: Int, _ y: Int) -> Bool {
+ corners.contains(where: { $0 == (x, y) })
+ }
+
+ private func neighbours(_ x: Int, _ y: Int) -> [(Int, Int)] {
+ Array(stride(from: y - 1, through: y + 1, by: 1)).flatMap { r in
+ Array(stride(from: x - 1, through: x + 1, by: 1)).map { c in
+ (c, r)
+ }.filter { (c, r) in (c, r) != (x, y) && xRange ~= c && yRange ~= r }
+ }
+ }
+}

2015/Tests/Day18Tests.swift

@@ -0,0 +1,29 @@
+import Testing
+
+@testable import AoC2015
+
+struct Day18Tests {
+ private let testData = """
+ .#.#.#
+ ...##.
+ #....#
+ ..#...
+ #.#..#
+ ####..
+ """.replacingOccurrences(of: " ", with: "")
+
+ @Test func testPart1() {
+ #expect(Day18(data: testData, steps: 1, width: 6, height: 6).part1() == 11)
+ #expect(Day18(data: testData, steps: 2, width: 6, height: 6).part1() == 8)
+ #expect(Day18(data: testData, steps: 3, width: 6, height: 6).part1() == 4)
+ #expect(Day18(data: testData, steps: 4, width: 6, height: 6).part1() == 4)
+ }
+
+ @Test func testPart2() {
+ #expect(Day18(data: testData, steps: 1, width: 6, height: 6).part2() == 18)
+ #expect(Day18(data: testData, steps: 2, width: 6, height: 6).part2() == 18)
+ #expect(Day18(data: testData, steps: 3, width: 6, height: 6).part2() == 18)
+ #expect(Day18(data: testData, steps: 4, width: 6, height: 6).part2() == 14)
+ #expect(Day18(data: testData, steps: 5, width: 6, height: 6).part2() == 17)
+ }
+}