Year 2017 Day 25

Author: maneatingape

README.md

@@ -172,3 +172,4 @@ The minimal SBT project provides:
 | 22 | [Sporifica Virus](https://adventofcode.com/2017/day/22) | [Source](src/main/scala/AdventOfCode2017/Day22.scala) |
 | 23 | [Coprocessor Conflagration](https://adventofcode.com/2017/day/23) | [Source](src/main/scala/AdventOfCode2017/Day23.scala) |
 | 24 | [Electromagnetic Moat](https://adventofcode.com/2017/day/24) | [Source](src/main/scala/AdventOfCode2017/Day24.scala) |
+| 25 | [The Halting Problem](https://adventofcode.com/2017/day/25) | [Source](src/main/scala/AdventOfCode2017/Day25.scala) |

src/main/resources/AdventOfCode2017/Day25.txt

@@ -0,0 +1,62 @@
+Begin in state A.
+Perform a diagnostic checksum after 12667664 steps.
+
+In state A:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state B.
+  If the current value is 1:
+    - Write the value 0.
+    - Move one slot to the left.
+    - Continue with state C.
+
+In state B:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the left.
+    - Continue with state A.
+  If the current value is 1:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state D.
+
+In state C:
+  If the current value is 0:
+    - Write the value 0.
+    - Move one slot to the left.
+    - Continue with state B.
+  If the current value is 1:
+    - Write the value 0.
+    - Move one slot to the left.
+    - Continue with state E.
+
+In state D:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state A.
+  If the current value is 1:
+    - Write the value 0.
+    - Move one slot to the right.
+    - Continue with state B.
+
+In state E:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the left.
+    - Continue with state F.
+  If the current value is 1:
+    - Write the value 1.
+    - Move one slot to the left.
+    - Continue with state C.
+
+In state F:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state D.
+  If the current value is 1:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state A.

src/main/scala/AdventOfCode2017/Day25.scala

@@ -0,0 +1,35 @@
+package AdventOfCode2017
+
+object Day25:
+  case class Rule(write: Boolean, step: Int, next: String)
+  case class Turing(tape: Set[Int], position: Int, state: String)
+
+  def parse(input: Seq[String]): (String, Int, Map[(String, Boolean), Rule]) =
+    val trimmed = input.map(_.init.trim.split(" "))
+    val start = trimmed(0)(3)
+    val total = trimmed(1)(5).toInt
+    val rules = trimmed.drop(2).grouped(10).map { group =>
+      val state = group(1)(2)
+      Seq((state, false) -> parseRule(group.drop(3)), (state, true) -> parseRule(group.drop(7)))
+    }
+    (start, total, rules.flatten.toMap)
+
+  def parseRule(block: Seq[Array[String]]): Rule =
+    val write = block(0)(4) == "1"
+    val step = if block(1)(6) == "right" then 1 else -1
+    val next = block(2)(4)
+    Rule(write, step, next)
+
+  def step(rules: Map[(String, Boolean), Rule])(turing: Turing): Turing =
+    val rule = rules(turing.state -> turing.tape.contains(turing.position))
+    val nextTape = if rule.write then turing.tape + turing.position else turing.tape - turing.position
+    Turing(nextTape, turing.position + rule.step, rule.next)
+
+  def part1(input: Seq[String]): Int =
+    val (start, total, rules) = parse(input)
+    val initial = Turing(Set(), 0, start)
+    Iterator.iterate(initial)(step(rules)).drop(total).next().tape.size
+
+  def main(args: Array[String]): Unit =
+    val data = io.Source.fromResource("AdventOfCode2017/Day25.txt").getLines().toSeq
+    println(part1(data))

src/test/scala/AdventOfCode2017/Day25Suite.scala

@@ -0,0 +1,32 @@
+package AdventOfCode2017
+
+import org.scalatest.funsuite.AnyFunSuite
+
+class Day25Suite extends AnyFunSuite:
+  val sample = Seq(
+    "Begin in state A.",
+    "Perform a diagnostic checksum after 6 steps.",
+    "",
+    "In state A:",
+    "  If the current value is 0:",
+    "    - Write the value 1.",
+    "    - Move one slot to the right.",
+    "    - Continue with state B.",
+    "  If the current value is 1:",
+    "    - Write the value 0.",
+    "    - Move one slot to the left.",
+    "    - Continue with state B.",
+    "",
+    "In state B:",
+    "  If the current value is 0:",
+    "    - Write the value 1.",
+    "    - Move one slot to the left.",
+    "    - Continue with state A.",
+    "If the current value is 1:",
+    "    - Write the value 1.",
+    "    - Move one slot to the right.",
+    "    - Continue with state A.")
+
+  test("Part 1 should handle sample input correctly") {
+    assert(Day25.part1(sample) == 3)
+  }