feat(aoc-2024): add Day 6 (part 2) (#34)

README.md

@@ -21,7 +21,7 @@ swift test
 | 03 | [Mull It Over](https://adventofcode.com/2024/day/3)       | ⭐️ | ⭐️ | [Day 03](Sources/AOC2024/Day03.swift) |
 | 04 | [Ceres Search](https://adventofcode.com/2024/day/4)       | ⭐️ | ⭐️ | [Day 04](Sources/AOC2024/Day04.swift) |
 | 05 | [Print Queue](https://adventofcode.com/2024/day/5)        | ⭐️ | ⭐️ | [Day 05](Sources/AOC2024/Day05.swift) |
-| 06 | [Guard Gallivant](https://adventofcode.com/2024/day/6)    | ⭐️ |    | [Day 06](Sources/AOC2024/Day06.swift) |
+| 06 | [Guard Gallivant](https://adventofcode.com/2024/day/6)    | ⭐️ | ⭐️ | [Day 06](Sources/AOC2024/Day06.swift) |
 | 09 | [Disk Fragmenter](https://adventofcode.com/2024/day/9)    | ⭐️ |    | [Day 09](Sources/AOC2024/Day09.swift) |
 | 10 | [Hoof It](https://adventofcode.com/2024/day/10)           | ⭐️ | ⭐️ | [Day 10](Sources/AOC2024/Day10.swift) |
 | 11 | [Plutonian Pebbles](https://adventofcode.com/2024/day/11) | ⭐️ | ⭐️ | [Day 11](Sources/AOC2024/Day11.swift) |

Sources/AOC2024/Day06.swift

@@ -8,13 +8,46 @@ struct Day06: Day {
 
     func part1() throws -> Int {
         let board = input().lines.map(\.characters)
+        let startPosition = board.findFirst(element: "^")!
 
-        var position = board.findFirst(element: "^")!
-        var direction = Direction.up
-        var path: Set<Position> = []
+        return patrol(startPosition, .up, board).count
+    }
+
+    func part2() throws -> Int {
+        let board = input().lines.map(\.characters)
+        let startPosition = board.findFirst(element: "^")!
+
+        return patrol(startPosition, .up, board)
+            .count { isLoop(startPosition, .up, $0, board) }
+    }
+
+    private func isLoop(_ startPosition: Position, _ startDirection: Direction, _ obstacle: Position, _ board: [[Character]]) -> Bool {
+        var route: Set<Tuple<Position, Direction>> = []
+        var position = startPosition
+        var direction = startDirection
 
         while board[position] != nil {
-            path.insert(position)
+            route.insert(.init(position, direction))
+
+            if board[position.offset(direction)] == "#" || position.offset(direction) == obstacle {
+                direction = direction.turn()
+            } else if !route.contains(.init(position.offset(direction), direction)) {
+                position = position.offset(direction)
+            } else {
+                return true
+            }
+        }
+
+        return false
+    }
+
+    private func patrol(_ startPosition: Position, _ startDirection: Direction, _ board: [[Character]]) -> Set<Position> {
+        var route: Set<Position> = []
+        var position = startPosition
+        var direction = startDirection
+
+        while board[position] != nil {
+            route.insert(position)
 
             if board[position.offset(direction)] == "#" {
                 direction = direction.turn()
@@ -23,11 +56,7 @@ struct Day06: Day {
             position = position.offset(direction)
         }
 
-        return path.count
-    }
-
-    func part2() throws -> Int {
-        -1
+        return route
     }
 }
 

Tests/AOCTests/AOC2024Tests.swift

@@ -136,6 +136,7 @@ struct AOC2024Tests {
         ......#...
         """
         #expect(try sut.part1() == 41, "Part 1")
+        #expect(try sut.part2() == 6, "Part 2")
     }
 
     @Test