2022 Day 13

2022/day13.swift

@@ -0,0 +1,107 @@
+import Foundation
+
+enum Packet {
+    case list([Packet])
+    case number(Int)
+
+    init(_ packets: Packet...) {
+        self =  .list(packets)
+    }
+}
+
+extension Scanner {
+    func packet() -> Packet? {
+        if let list = list() {
+            return list
+        }
+        
+        if let int = scanInt() {
+            return .number(int)
+        }
+        
+        return nil
+    }
+    
+    func list() -> Packet? {
+        guard scanString("[") != nil else {
+            return nil
+        }
+        
+        var result: [Packet] = []
+        while let packet = packet() {
+            result.append(packet)
+            _ = scanString(",")
+        }
+        guard scanString("]") != nil else {
+            fatalError("Invalid packet")        
+        }
+        return .list(result)
+    }
+}
+
+extension Packet: Comparable {
+    static func ==(lhs: Packet, rhs: Packet) -> Bool {
+        switch (lhs, rhs) {
+            case (.number(let a), .number(let b)): return a == b
+            case (.list(let a), .list(let b)): return a == b
+            default: return false
+        }
+    }
+    
+    static func <(lhs: Packet, rhs: Packet) -> Bool {
+        switch (lhs, rhs) {
+        case let (.number(a), .number(b)): return a < b
+        case (.list, .number): return lhs < .list([rhs])
+        case (.number, .list): return .list([lhs]) < rhs
+        case let (.list(a), .list(b)): 
+            var indexA = a.startIndex
+            var indexB = b.startIndex
+            while indexA < a.endIndex, indexB < b.endIndex {
+                if a[indexA] < b[indexB] {
+                    return true
+                }
+                
+                if b[indexB] < a[indexA] {
+                    return false
+                }
+                
+                indexA += 1
+                indexB += 1
+            }
+            return indexA == a.endIndex && indexB != b.endIndex
+        }
+    }
+}
+
+let input = try String(contentsOf: URL(fileURLWithPath: "day13.input"))
+let scanner = Scanner(string: input)
+
+var index = 1
+var sum = 0
+
+let marker1 = Packet(Packet(.number(2)))
+let marker2 = Packet(Packet(.number(6)))
+
+var all: [Packet] = [
+    marker1, marker2,
+]
+
+while !scanner.isAtEnd, let a = scanner.packet(), let b = scanner.packet() {
+    all.append(a)
+    all.append(b)
+    
+    if a < b {
+        sum += index
+    }
+    
+    index += 1
+}
+
+print("Part 1:", sum)
+
+all.sort { $0 < $1 }
+
+let index1 = all.firstIndex(of: marker1)!
+let index2 = all.firstIndex(of: marker2)!
+
+print("Part 2:", (index1 + 1) * (index2 + 1))