Year 2015 Day 23

Author: maneatingape

README.md

@@ -232,3 +232,4 @@ The minimal SBT project provides:
 | 20 | [Infinite Elves and Infinite Houses](https://adventofcode.com/2015/day/20) | [Source](src/main/scala/AdventOfCode2015/Day20.scala) |
 | 21 | [RPG Simulator 20XX](https://adventofcode.com/2015/day/21) | [Source](src/main/scala/AdventOfCode2015/Day21.scala) |
 | 22 | [Wizard Simulator 20XX](https://adventofcode.com/2015/day/22) | [Source](src/main/scala/AdventOfCode2015/Day22.scala) |
+| 23 | [Opening the Turing Lock](https://adventofcode.com/2015/day/23) | [Source](src/main/scala/AdventOfCode2015/Day23.scala) |

src/main/resources/AdventOfCode2015/Day23.txt

@@ -0,0 +1,48 @@
+jio a, +22
+inc a
+tpl a
+tpl a
+tpl a
+inc a
+tpl a
+inc a
+tpl a
+inc a
+inc a
+tpl a
+inc a
+inc a
+tpl a
+inc a
+inc a
+tpl a
+inc a
+inc a
+tpl a
+jmp +19
+tpl a
+tpl a
+tpl a
+tpl a
+inc a
+inc a
+tpl a
+inc a
+tpl a
+inc a
+inc a
+tpl a
+inc a
+inc a
+tpl a
+inc a
+tpl a
+tpl a
+jio a, +8
+inc b
+jie a, +4
+tpl a
+inc a
+jmp +2
+hlf a
+jmp -7

src/main/scala/AdventOfCode2015/Day23.scala

@@ -0,0 +1,39 @@
+package AdventOfCode2015
+
+object Day23:
+  sealed trait State
+  case object Running extends State
+  case object Halted extends State
+
+  case class Cpu(instructions: Seq[String], registers: Map[String, Int] = Map(), ip: Int = 0, state: State = Running):
+    def next: Cpu = copy(ip = ip + 1)
+    def read(key: String): Int = key.toIntOption.getOrElse(registers.getOrElse(key, 0))
+    def write(key: String, value: Int): Cpu = next.copy(registers = registers.updated(key, value))
+    def step: Cpu =
+      if !instructions.indices.contains(ip) then return copy(state = Halted)
+      val (op, src, dest) = instructions(ip).split("[, ]+") match
+        case Array(op, src, dest) => (op, src, dest)
+        case Array(op, dest) => (op, "", dest)
+      op match
+        case "hlf" => write(dest, read(dest) / 2)
+        case "tpl" => write(dest, read(dest) * 3)
+        case "inc" => write(dest, read(dest) + 1)
+        case "jmp" => copy(ip = ip + read(dest))
+        case "jie" => if read(src) % 2 == 0 then copy(ip = ip + read(dest)) else next
+        case "jio" => if read(src) == 1 then copy(ip = ip + read(dest)) else next
+  end Cpu
+
+  // Reverse engineering the code shows that it calculates the length of the
+  // 3n + 1 sequence (https://en.wikipedia.org/wiki/Collatz_conjecture)
+  // for one of two different numbers chosen depending on whether a is 0 or 1.
+  def run(input: Seq[String], a: Int): Int =
+    Iterator.iterate(Cpu(input, Map("a" -> a)))(_.step).dropWhile(_.state == Running).next().registers("b")
+
+  def part1(input: Seq[String]): Int = run(input, 0)
+
+  def part2(input: Seq[String]): Int = run(input, 1)
+
+  def main(args: Array[String]): Unit =
+    val data = io.Source.fromResource("AdventOfCode2015/Day23.txt").getLines().toSeq
+    println(part1(data))
+    println(part2(data))

src/test/scala/AdventOfCode2015/Day23Suite.scala

@@ -0,0 +1,6 @@
+package AdventOfCode2015
+
+import org.scalatest.funsuite.AnyFunSuite
+
+class Day23Suite extends AnyFunSuite
+// No unit tests possible