Day 09

src/day09.scala

@@ -0,0 +1,85 @@
+package aoc
+package day09
+
+import scala.annotation.tailrec
+
+val dayNumber = "09"
+
+@main def part1: Unit =
+  println(part1(loadInput(dayNumber)))
+
+@main def part2: Unit =
+  println(part2(loadInput(dayNumber)))
+
+def part1(input: String): String =
+  val blocks = input
+    .map(_.asDigit)
+    .grouped(2)
+    .zipWithIndex
+    .flatMap {
+      case (IndexedSeq(fileSize), fileId) => List.fill(fileSize)(Some(fileId))
+      case (IndexedSeq(fileSize, freeSpace), fileId) =>
+        List.fill(fileSize)(Some(fileId)) ++ List.fill(freeSpace)(None)
+    }
+    .toVector
+
+  @tailrec def compact(blocks: Vector[Option[Int]], acc: List[Int]): List[Int] =
+    if blocks.isEmpty then acc.reverse
+    else
+      blocks.head match
+        case Some(fileId) => compact(blocks.tail, fileId :: acc)
+        case None         => compact(blocks.last +: blocks.tail.init, acc)
+
+  compact(blocks, Nil).zipWithIndex
+    .map(_.toLong * _)
+    .sum
+    .toString
+
+def part2(input: String): String =
+  enum Block:
+    val length: Int
+    case File(fileId: Int, override val length: Int)
+    case Empty(override val length: Int)
+
+  val blocks = input
+    .map(_.asDigit)
+    .grouped(2)
+    .zipWithIndex
+    .flatMap {
+      case (IndexedSeq(fileSize), fileId) =>
+        List(Block.File((fileId), fileSize))
+      case (IndexedSeq(fileSize, freeSpace), fileId) =>
+        List(Block.File(fileId, fileSize), Block.Empty(freeSpace))
+    }
+    .toVector
+
+  @tailrec def compact(blocks: Vector[Block], acc: List[Block]): List[Block] =
+    blocks.lastOption match
+      case Some(last: Block.File) =>
+        blocks.indexWhere {
+          case Block.Empty(length) => last.length <= length
+          case _: Block.File       => false
+        } match
+          case -1 => compact(blocks.init, last :: acc)
+          case idx =>
+            val newBlocks = List(
+              Block.File(last.fileId, last.length),
+              Block.Empty(blocks(idx).length - last.length)
+            )
+            compact(
+              blocks.take(idx) ++ newBlocks ++ blocks.drop(idx + 1).init,
+              Block.Empty(last.length) :: acc
+            )
+
+      case Some(last: Block.Empty) => compact(blocks.init, last :: acc)
+      case None                    => acc
+
+  compact(blocks, Nil)
+    .flatMap {
+      case Block.File(fileId, length) => List.fill(length)(fileId.toLong)
+      case Block.Empty(length)        => List.fill(length)(0L)
+    }
+    .zipWithIndex
+    .map(_ * _)
+    .sum
+    .toString