Switched from string fiddling to actual character pairs

Package.swift

@@ -22,7 +22,8 @@ let package = Package(
             .executable(name: "10", targets: ["10"]),
             .executable(name: "11", targets: ["11"]),
             .executable(name: "12", targets: ["12"]),
-            .executable(name: "13", targets: ["13"])
+            .executable(name: "13", targets: ["13"]),
+            .executable(name: "14", targets: ["14"])
         ],
     dependencies: [
         // Dependencies declare other packages that this package depends on.
@@ -106,6 +107,10 @@ let package = Package(
         .executableTarget(
             name: "13",
             dependencies: [ .targetItem(name: "Runner", condition: nil) ]
+        ),
+        .executableTarget(
+            name: "14",
+            dependencies: [ .targetItem(name: "Runner", condition: nil) ]
         )
     ]
 )

Sources/14/14.swift

@@ -0,0 +1,83 @@
+//
+//  File.swift
+//
+//
+//  Created by Max Nuding on 14.12.21.
+//
+
+import Foundation
+import Runner
+
+struct Pair: Hashable {
+    let first: String.Element
+    let second: String.Element
+    
+    init(_ str: String) {
+        first = str.first!
+        second = str.last!
+    }
+    
+    init(first: String.Element, second: String.Element) {
+        self.first = first
+        self.second = second
+    }
+}
+
+class Day14: Runnable {
+    let inputPath: String
+    
+    required init(inputPath: String) {
+        self.inputPath = inputPath
+    }
+    
+    public func run() {
+        let input = try! String(contentsOfFile: inputPath)
+        let parts = input
+            .trimmingCharacters(in: .newlines)
+            .components(separatedBy: "\n\n")
+        let startingPolymer = parts.first!
+        let replacements = parts.last!
+            .components(separatedBy: .newlines)
+            .map {$0.components(separatedBy: " -> ")}
+            .reduce(into: [Pair:String.Element](), {
+                $0[Pair($1.first!)] = $1.last!.first!
+            })
+        
+        var pairCounts = [Pair:Int]()
+        for i in startingPolymer.indices {
+            let next = startingPolymer.index(i, offsetBy: 1)
+            guard next != startingPolymer.endIndex else {
+                continue
+            }
+            pairCounts[Pair(first: startingPolymer[i], second: startingPolymer[next]), default: 0] += 1
+        }
+        var charCount = startingPolymer.reduce(into: [String.Element:Int](), {
+            $0[$1, default: 0] += 1
+        })
+        
+        for step in 1...40 {
+            let tmpPairCounts = pairCounts
+            
+            for pairCount in tmpPairCounts {
+                let pair = pairCount.key
+                let replacement = replacements[pair]!
+                pairCounts[pair]! -= pairCount.value
+                pairCounts[Pair(first: pair.first, second: replacement), default: 0] += pairCount.value
+                pairCounts[Pair(first: replacement, second: pair.second), default: 0] += pairCount.value
+                charCount[replacement, default: 0] += pairCount.value
+            }
+            if step == 10 {
+                printResult(charCount: charCount)
+            }
+        }
+        printResult(charCount: charCount)
+    }
+    
+    func printResult(charCount: [String.Element:Int]) {
+        // Sort instead of min/max so we need to only go through the dict once
+        let sorted = charCount.sorted(by: {$0.value > $1.value })
+        let maxC = sorted.first!
+        let minC = sorted.last!
+        print(maxC.value - minC.value)
+    }
+}

Sources/14/main.swift

@@ -0,0 +1,12 @@
+//
+//  File.swift
+//  
+//
+//  Created by Max Nuding on 14.12.21.
+//
+
+import Foundation
+import Runner
+
+//Runner(target: Day14.self, day: "14", isTest: true).run()
+Runner(target: Day14.self, day: "14", isTest: false).run()

Sources/Runner/Resources/input/14.txt

@@ -0,0 +1,102 @@
+FNFPPNKPPHSOKFFHOFOC
+
+VS -> B
+SV -> C
+PP -> N
+NS -> N
+BC -> N
+PB -> F
+BK -> P
+NV -> V
+KF -> C
+KS -> C
+PV -> N
+NF -> S
+PK -> F
+SC -> F
+KN -> K
+PN -> K
+OH -> F
+PS -> P
+FN -> O
+OP -> B
+FO -> C
+HS -> F
+VO -> C
+OS -> B
+PF -> V
+SB -> V
+KO -> O
+SK -> N
+KB -> F
+KH -> C
+CC -> B
+CS -> C
+OF -> C
+FS -> B
+FP -> H
+VN -> O
+NB -> N
+BS -> H
+PC -> H
+OO -> F
+BF -> O
+HC -> P
+BH -> S
+NP -> P
+FB -> C
+CB -> H
+BO -> C
+NN -> V
+SF -> N
+FC -> F
+KK -> C
+CN -> N
+BV -> F
+FK -> C
+CF -> F
+VV -> B
+VF -> S
+CK -> C
+OV -> P
+NC -> N
+SS -> F
+NK -> V
+HN -> O
+ON -> P
+FH -> O
+OB -> H
+SH -> H
+NH -> V
+FF -> B
+HP -> B
+PO -> P
+HB -> H
+CH -> N
+SN -> P
+HK -> P
+FV -> H
+SO -> O
+VH -> V
+BP -> V
+CV -> P
+KP -> K
+VB -> N
+HV -> K
+SP -> N
+HO -> P
+CP -> H
+VC -> N
+CO -> S
+BN -> H
+NO -> B
+HF -> O
+VP -> K
+KV -> H
+KC -> F
+HH -> C
+BB -> K
+VK -> P
+OK -> C
+OC -> C
+PH -> H

Sources/Runner/Resources/input/14_test.txt

@@ -0,0 +1,18 @@
+NNCB
+
+CH -> B
+HH -> N
+CB -> H
+NH -> C
+HB -> C
+HC -> B
+HN -> C
+NN -> C
+BH -> H
+NC -> B
+NB -> B
+BN -> B
+BB -> N
+BC -> B
+CC -> N
+CN -> C