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2020/day24/main.swift

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+import Foundation
+
+let input = loadData(day: 24)
+let scanner = Scanner(string: input)
+scanner.charactersToBeSkipped = nil
+
+enum Direction: String, CaseIterable {
+    case east = "e"
+    case southEast = "se"
+    case southWest = "sw"
+    case west = "w"
+    case northWest = "nw"
+    case northEast = "ne"
+}
+
+extension Scanner {
+    func readDirection() -> Direction? {
+        for dir in Direction.allCases {
+            if string(dir.rawValue) {
+                return dir
+            }
+        }
+        return nil
+    }
+}
+
+struct Coord: Hashable {
+    var south: Int
+    var east: Int
+
+    static let zero = Coord(south: 0, east: 0)
+
+    mutating func move(_ direction: Direction) {
+        switch direction {
+        case .east:
+            east += 1
+        case .southEast:
+            south += 1
+
+        case  .southWest:
+            south += 1
+            east -= 1
+
+        case .west:
+            east -= 1
+
+        case .northWest:
+            south -= 1
+
+        case .northEast:
+            south -= 1
+            east += 1
+        }
+    }
+
+    func neighbor(_ direction: Direction) -> Coord {
+        var result = self
+        result.move(direction)
+        return result
+    }
+
+    var neighbors: [Coord] {
+        Direction.allCases.map(neighbor)
+    }
+}
+
+func readCoordinate() -> Coord {
+    var current = Coord.zero
+    while let direction = scanner.readDirection() {
+        current.move(direction)
+    }
+    precondition(scanner.string("\n"))
+    return current
+}
+
+var floor: [Coord: Bool] = [:]
+var northWestCorner = Coord.zero
+var southEastCorner = Coord.zero
+
+
+func expandRange(toInclude coord: Coord) {
+    northWestCorner.east = min(northWestCorner.east, coord.east)
+    northWestCorner.south = min(northWestCorner.south, coord.south)
+
+    southEastCorner.east = max(southEastCorner.east, coord.east)
+    southEastCorner.south = max(southEastCorner.south, coord.south)
+}
+
+while !scanner.isAtEnd {
+    let coord = readCoordinate()
+    expandRange(toInclude: coord)
+
+    floor[coord, default: false].toggle()
+}
+
+print("part 1:", floor.values.lazy.filter { $0 }.count)
+
+func step() -> [Coord: Bool] {
+    var result = floor
+
+    for east in (northWestCorner.east - 1)...(southEastCorner.east + 1) {
+        for south in (northWestCorner.south - 1)...(southEastCorner.south + 1) {
+            let coord = Coord(south: south, east: east)
+
+            let isBlack = floor[coord, default: false]
+
+            let blackNeighbors = coord.neighbors
+                .lazy
+                .map { floor[$0, default: false] }
+                .filter { $0 }
+                .count
+
+            if isBlack && (blackNeighbors == 0 || blackNeighbors > 2) {
+                result[coord] = false
+            } else if !isBlack && blackNeighbors == 2 {
+                result[coord] = true
+                expandRange(toInclude: coord)
+            }
+        }
+    }
+
+    return result
+}
+
+for _ in 0..<100 {
+    floor = step()
+}
+
+print("part 2:", floor.values.lazy.filter { $0 }.count)