Day 13

aoc/src/Day13.scala

@@ -0,0 +1,74 @@
+import scala.io.Source
+
+object Day13 extends App:
+
+  val input = Source
+    .fromURL(getClass.getResource("day13Input.txt"))
+    .mkString
+    .split('\n')
+    .toList
+
+  enum Packet:
+    case Digit(v: Int)
+    case DList(v: List[Packet])
+
+  object Packet:
+
+    private def parseArray(input: String): DList =
+      DList(
+        input
+          .split("((?<=[,\\[\\]])|(?=[,\\[\\]]))")
+          .foldLeft((List[Packet](), "")) {
+            case ((packets, acc), next) if acc == "" && next == "," => (packets, acc)
+            case ((packets, acc), next) =>
+              parse(acc + next)
+                .map(p => (packets.appended(p), ""))
+                .getOrElse((packets, acc + next))
+
+          }._1
+      )
+
+    def parse(input: String): Option[Packet] =
+      input.toList match
+        case _ :: _ if input.forall(_.isDigit) => Some(Digit(input.toInt))
+        case List('[', _*) :+ ']' if input.count(_ == '[') == input.count(_ == ']') =>
+          Some(parseArray(input.tail.dropRight(1)))
+        case _ => None
+
+    val compare: (Packet, Packet) => Int =
+      case (Digit(left), Digit(right)) => left - right
+      case (DList(left), Digit(right)) => compare(DList(left), DList(List(Digit(right))))
+      case (Digit(left), DList(right)) => compare(DList(List(Digit(left))), DList(right))
+      case (DList(Nil), DList(Nil))    => 0
+      case (DList(Nil), DList(_ :: _)) => -1
+      case (DList(_ :: _), DList(Nil)) => 1
+      case (DList(l :: ls), DList(r :: rs)) =>
+        val headCmp = compare(l, r)
+        if headCmp != 0 then headCmp else compare(DList(ls), DList(rs))
+  end Packet
+
+  val part1 = input
+    .filter(_.nonEmpty)
+    .flatMap(Packet.parse)
+    .grouped(2)
+    .map(it => Packet.compare(it.head, it(1)))
+    .zipWithIndex
+    .filter(_._1 < 0)
+    .map(_._2 + 1)
+    .sum
+  println("Part 1:")
+  println(part1)
+
+  val divider1 = Packet.parse("[[2]]").get
+  val divider2 = Packet.parse("[[6]]").get
+
+  val sortedPackets = input
+    .filter(_.nonEmpty)
+    .flatMap(Packet.parse)
+    .prependedAll(List(divider1, divider2))
+    .sortWith(Packet.compare(_, _) < 1)
+
+  val part2 = (sortedPackets.indexOf(divider1) + 1) * (sortedPackets.indexOf(divider2) + 1)
+  println("Part 2:")
+  println(part2)
+end Day13