Day 23 - Part 1

day23/main.swift

@@ -0,0 +1,46 @@
+var stack = [9, 5, 2, 3, 1, 6, 4, 8, 7]
+
+extension Array {
+    mutating func rotateLeft(mid: Index) {
+        let slice = self[0..<mid]
+        removeSubrange(0..<mid)
+        append(contentsOf: slice)
+    }
+}
+
+func round() {
+    let picked = stack[1 ... 3]
+    stack.removeSubrange(1 ...  3)
+
+    var destination = stack[0]
+    repeat {
+        destination -= 1
+        if destination < 1 {
+            destination = 9
+        }
+    } while picked.contains(destination)
+
+    let destinationIndex = stack.firstIndex(of: destination)! + 1
+    stack.insert(contentsOf: picked, at: destinationIndex)
+
+    stack.rotateLeft(mid: 1)
+}
+
+func printStack() {
+    for (offset, value) in stack.enumerated() {
+        print(offset == 0 ? "(\(value))" : "\(value)", terminator: " ")
+    }
+    print()
+}
+
+for i in 0..<100 {
+    print("\(i + 1)) ", terminator: "")
+    printStack()
+    round()
+}
+print("Final: ", terminator: "")
+printStack()
+
+let oneIndex = stack.firstIndex(of: 1)!
+stack.rotateLeft(mid: (oneIndex + 1) % stack.count)
+print(stack.dropLast().map {"\($0)"}.joined())