d11

day11/d11p1.swift

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+import Foundation
+
+class Node : CustomStringConvertible {
+    var value: Int
+    var next: Node?
+    init(_ value: Int, next: Node? = nil) {
+        self.value = value
+        self.next = next
+    }
+    var description: String {
+        let nextStr = if let next { String(describing: next) } else { "nil" }
+        return "(\(value))->\(nextStr)"
+    }
+    func evolve() -> Int {
+        if value == 0 {
+            value = 1
+            return 1
+        }
+        // I don't have log10() unless I import Graphics, which is overkill...
+        let s = String(value)
+        if s.count.isMultiple(of: 2) {
+            let secondHalf = s.count/2 ..< s.count
+            let newNode = Node(Int(String(Array(s)[secondHalf]))!, next: next)
+            value = Int(String(Array(s)[0 ..< s.count/2]))!
+            next = newNode
+            return 2
+        }
+        value = value*2024
+        return 1
+    }
+}
+
+func readInput(_ filePath: String) throws -> Node {
+    let content = try String(contentsOfFile: filePath, encoding: .ascii)
+        .trimmingCharacters(in: .whitespacesAndNewlines)
+    let head: Node = Node(-1)
+    var current = head
+    content.split(separator: " ").forEach { i in
+        print(i)
+        let new = Node(Int(i)!)
+        current.next = new
+        current = new
+    }
+    return head.next!
+}
+
+func evolve(_ chain: Node) -> Int {
+    var next: Node? = chain
+    var count = 0
+    while let node = next {
+        next = node.next
+        count += node.evolve()
+    }
+    return count
+}
+
+let nums = try readInput(CommandLine.arguments[1])
+print(nums)
+for i in 1...25 {
+    let count = evolve(nums)
+    print("\(i): \(count)")  // - \(nums)")
+}

day11/d11p2.swift

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+import Foundation
+
+struct NumTimes : Hashable {
+    let num: Int
+    let times: Int
+}
+
+func readInput(_ filePath: String) throws -> [Int] {
+    return try String(contentsOfFile: filePath, encoding: .ascii)
+        .trimmingCharacters(in: .whitespacesAndNewlines)
+        .split(separator: " ").map { Int($0)! }
+}
+
+func evolve(_ num: Int) -> [Int] {
+    if num == 0 { return [1] }
+    let s = String(num)
+    if s.count.isMultiple(of: 2) {
+        return [
+            Int(String(Array(s)[s.count/2 ..< s.count]))!,
+            Int(String(Array(s)[0 ..< s.count/2]))!
+        ]
+    }
+    return [num * 2024]
+   
+}
+
+var cache: [NumTimes: Int] = [:]
+
+func count(_ num: Int, times: Int) -> Int {
+    if times == 0 { return 1 }
+    let key = NumTimes(num: num, times: times)
+    if let cached = cache[key] { return cached }
+    cache[key] = evolve(num).map { n in count(n, times: times-1) }.reduce(0, +)
+    return cache[key]!
+}
+
+let nums = try readInput(CommandLine.arguments[1])
+print(nums)
+print(nums.map { count($0, times: 75) }.reduce(0, +))

day11/input.txt

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+0 27 5409930 828979 4471 3 68524 170

day11/test.txt

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+125 17