d20p1

day20/d20p1.swift

@@ -0,0 +1,89 @@
+import Foundation
+
+struct Coord : Hashable, CustomStringConvertible {
+    let (x, y): (Int, Int)
+    var description: String { return "(\(x), \(y))" }
+}
+
+struct Maze : CustomStringConvertible {
+    let (w, h): (Int, Int)
+    let walls: Set<Coord>
+    let (start, end): (Coord, Coord)
+    var description: String {
+        var s = Array(repeating: Array(repeating: " ", count: w), count: (h+1)/2)
+        walls.forEach { wall in
+            if wall.y.isMultiple(of: 2) {
+                s[wall.y/2][wall.x] = (s[wall.y/2][wall.x] == " ") ? "▀" : "█"
+            } else {
+                s[wall.y/2][wall.x] = (s[wall.y/2][wall.x] == " ") ? "▄" : "█"
+            }
+        }
+        return s.map { $0.joined() + "\n" }.joined() + "\(start), \(end)"
+    }
+    let adjacents = [(-1, 0), (1, 0), (0, -1), (0, 1)]
+
+    func valid(_ cell: Coord) -> Bool {
+        return cell.x >= 0 && cell.x < w && cell.y >= 0 && cell.y < h &&
+            !walls.contains(cell)
+    }
+
+    func neighbor(of cell: Coord, isNot excluded: Coord) -> Coord {
+        return adjacents
+            .map { dx, dy in Coord(x: cell.x + dx, y: cell.y + dy) }
+            .filter { newCell in valid(newCell) && newCell != excluded }[0]
+    }
+
+    func cheats(of cell: Coord) -> [Coord] {
+        return adjacents
+            .map { dx, dy in Coord(x: cell.x + dx*2, y: cell.y + dy*2) }
+            .filter(valid)
+    }
+
+    func cheats() -> [Int] {
+        var cost: [Coord: Int] = [start: 0]
+        var jumps: [(Coord, Coord)] = []
+        var cell = start
+        var prevCell = start
+        var currentCost = 0
+        while cell != end {
+            // list potential cheats from current cell
+            let newCheats = cheats(of: cell).filter { !cost.keys.contains($0) }
+            jumps.append(contentsOf: newCheats.map { dest in (cell, dest)} )
+            // add cost to table
+            let nextCell = neighbor(of: cell, isNot: prevCell)
+            currentCost += 1
+            cost[nextCell] = currentCost 
+            prevCell = cell
+            cell = nextCell
+        }
+        return jumps.map { from, to in cost[to]! - cost[from]! - 2 }
+    }
+}
+
+func readInput(_ filePath: String) throws -> Maze {
+    let content = try String(contentsOfFile: filePath, encoding: .ascii)
+    let lines = content.split(separator: "\n")
+    let (w, h) = (lines[0].count, lines.count)
+    var walls: Set<Coord> = []
+    var (start, end) = (Coord(x: 0, y: 0), Coord(x: w-1, y: h-1))
+    lines.enumerated().forEach { i, line in
+        line.enumerated().forEach { j, cell in
+            let coord = Coord(x: j, y: i)
+            if cell == "#" {
+                walls.insert(coord)
+            } else if cell == "S" {
+                start = coord
+            } else if cell == "E" {
+                end = coord
+            }
+        }
+    }
+    return Maze(w: w, h: h, walls: walls, start: start, end: end)
+}
+
+let maze = try readInput(CommandLine.arguments[1])
+print(maze)
+let cheats = maze.cheats()
+print(cheats.reduce(into: [:]) { counts, t in counts[t, default: 0] += 1 })
+print(cheats.filter { $0 >= 100 }.count)
+