Day 8

Package.resolved

@@ -5,8 +5,8 @@
       "kind" : "remoteSourceControl",
       "location" : "https://github.com/apple/swift-algorithms",
       "state" : {
-        "revision" : "b14b7f4c528c942f121c8b860b9410b2bf57825e",
-        "version" : "1.0.0"
+        "revision" : "f6919dfc309e7f1b56224378b11e28bab5bccc42",
+        "version" : "1.2.0"
       }
     },
     {

Sources/AdventOfCode2023/AdventOfCode2023.swift

@@ -9,6 +9,7 @@ public struct AdventOfCode2023: Year {
         Day5(),
         Day6(),
         Day7(),
+        Day8(),
         // {DAYS}
     ]
 

Sources/AdventOfCode2023/Day8/Day8.swift

@@ -0,0 +1,81 @@
+import Algorithms
+import Foundation
+import Utilities
+
+public struct Day8: Day {
+    public func part1Solution(for input: Input) -> Int {
+        let parts = input.words(by: "\n\n")
+
+        let instructions = parts[0].characters
+
+        let map = parts[1]
+            .lines
+            .map { line in
+                line.words(by: CharacterSet.alphanumerics.inverted)
+                    .map(\.raw)
+            }
+            .keyed { $0[0] }
+            .mapValues { (left: $0[1], right: $0[2]) }
+
+        var index = 0
+        var count = 0
+        var location = "AAA"
+        let end = "ZZZ"
+
+        while location != end {
+            let instruction = instructions[index]
+            if instruction == "L" {
+                location = map[location]!.left
+            } else if instruction == "R" {
+                location = map[location]!.right
+            } else {
+                fatalError("impossible")
+            }
+
+            index = (index + 1) % instructions.count
+            count += 1
+        }
+
+        return count
+    }
+
+    public func part2Solution(for input: Input) -> Int {
+        let parts = input.words(by: "\n\n")
+
+        let instructions = parts[0].characters
+
+        let map = parts[1]
+            .lines
+            .map { line in
+                line.words(by: CharacterSet.alphanumerics.inverted)
+                    .map(\.raw)
+            }
+            .keyed { $0[0] }
+            .mapValues { (left: $0[1], right: $0[2]) }
+
+        return map.keys
+            .filter { $0.hasSuffix("A") }
+            .map { start in
+                var index = 0
+                var count = 0
+                var location = start
+
+                while !location.hasSuffix("Z") {
+                    let instruction = instructions[index]
+                    if instruction == "L" {
+                        location = map[location]!.left
+                    } else if instruction == "R" {
+                        location = map[location]!.right
+                    } else {
+                        fatalError("impossible")
+                    }
+
+                    index = (index + 1) % instructions.count
+                    count += 1
+                }
+
+                return count
+            }
+            .reduce(1) { $0.lcm(with: $1) }
+    }
+}