day 20 part 1

Sources/Day20/Day20.swift

@@ -5,18 +5,103 @@
 //
 
 import AoCTools
+import Collections
 
 final class Day20: AOCDay {
-    let title = ""
+    let title = "Race Condition"
+
+    let grid: [[Character]]
+    let walls: Set<Point>
+    let start: Point
+    let end: Point
 
     init(input: String) {
+        var points = input.lines.map { Array($0) }
+        var walls = Set<Point>()
+        var start: Point?
+        var end: Point?
+        for y in 0..<points.count {
+            for x in 0..<points[y].count {
+                let p = Point(x, y)
+                if points[p] == "S" {
+                    start = p
+                } else if points[p] == "E" {
+                    end = p
+                } else if points[p] == "#" {
+                    walls.insert(p)
+                }
+            }
+        }
+        points[start!] = "."
+        points[end!] = "."
+        self.grid = points
+        self.walls = walls
+        self.start = start!
+        self.end = end!
     }
 
     func part1() -> Int {
-        0
+        part1(minTimeSaved: 100)
+    }
+
+    func part1(minTimeSaved: Int) -> Int {
+        let raceTrack = RaceTrack(grid: grid)
+        let regularTime = raceTrack.shortestTime(from: start, to: end)
+
+        var cheatCandidates = [Point]()
+        for wall in walls {
+            if grid[safe: wall + .left] == "." && grid[safe: wall + .right] == "." {
+                cheatCandidates.append(wall)
+            }
+            if grid[safe: wall + .up] == "." && grid[safe: wall + .down] == "." {
+                cheatCandidates.append(wall)
+            }
+        }
+
+        var timesSaved = [Int: Int]()
+        for cheat in cheatCandidates {
+            var grid = grid
+            grid[cheat] = "."
+            let cheatTrack = RaceTrack(grid: grid)
+            let cheatTime = cheatTrack.shortestTime(from: start, to: end)
+            timesSaved[regularTime - cheatTime, default: 0] += 1
+        }
+
+        return timesSaved.filter { $0.key >= minTimeSaved }.reduce(0) { $0 + $1.value }
     }
 
     func part2() -> Int {
         0
     }
 }
+
+struct RaceTrack {
+    let grid: [[Character]]
+
+    func shortestTime(from start: Point, to end: Point) -> Int {
+        var queue = Deque([(start, 0)])
+        var visited = Set([start])
+
+        while let (next, len) = queue.popFirst() {
+            if next == end {
+                return len
+            }
+            visited.insert(next)
+            for n in neighbors(of: next) {
+                if !visited.contains(n) {
+                    queue.append((n, len + 1))
+                }
+            }
+        }
+        fatalError()
+    }
+
+    func neighbors(of point: Point) -> [Point] {
+        var n = [Point]()
+        for dir in Direction.orthogonal {
+            let p = point + dir
+            if grid[p] == "." { n.append(p) }
+        }
+        return n
+    }
+}

Tests/Day20Tests.swift

@@ -6,20 +6,35 @@ import Testing
 @testable import AdventOfCode
 
 fileprivate let testInput = """
+###############
+#...#...#.....#
+#.#.#.#.#.###.#
+#S#...#.#.#...#
+#######.#.#.###
+#######.#.#...#
+#######.#.###.#
+###..E#...#...#
+###.#######.###
+#...###...#...#
+#.#####.#.###.#
+#.#...#.#.#...#
+#.#.#.#.#.#.###
+#...#...#...###
+###############
 """
 
 @Suite("Day 20 Tests")
 struct Day20Tests {
     @MainActor @Test("Day 20 Part 1")
     func testDay20_part1() {
         let day = Day20(input: testInput)
-        #expect(day.part1() == 0)
+        #expect(day.part1(minTimeSaved: 0) == 44)
     }
 
     @MainActor @Test("Day 20 Part 1 Solution")
     func testDay20_part1_solution() {
         let day = Day20(input: Day20.input)
-        #expect(day.part1() == 0)
+        #expect(day.part1() == 1327)
     }
 
     @MainActor @Test("Day 20 Part 2")