Day 9

aoc/src/Day9.scala

@@ -0,0 +1,44 @@
+import scala.io.Source
+import scala.collection.mutable
+
+object Day9 extends App:
+
+  val input = Source
+    .fromURL(getClass.getResource("day9Input.txt"))
+    .mkString
+    .split('\n')
+    .toList
+    .map(_.split(" "))
+    .flatMap { case Array(d, moves) => Array.fill(moves.toInt)(d) }
+    .map {
+      case "R" => (1, 0)
+      case "L" => (-1, 0)
+      case "D" => (0, 1)
+      case "U" => (0, -1)
+    }
+
+  def followPrevious(headPos: (Int, Int), tailPos: (Int, Int)): (Int, Int) =
+    val offsetX = headPos._1 - tailPos._1
+    val offsetY = headPos._2 - tailPos._2
+    if Math.abs(offsetX) <= 1 && Math.abs(offsetY) <= 1 then tailPos
+    else
+      val xMove = if offsetX > 0 then 1 else -1
+      val yMove = if offsetY > 0 then 1 else -1
+      (
+        if Math.abs(offsetX) > 0 then tailPos._1 + xMove else tailPos._1,
+        if Math.abs(offsetY) > 0 then tailPos._2 + yMove else tailPos._2
+      )
+
+  def computeRopePositions(ropeLength: Int): List[List[(Int, Int)]] =
+    input.scanLeft(List.fill(ropeLength)((0, 0))) { (rope, change) =>
+      rope.tail.scanLeft((rope.head._1 + change._1, rope.head._2 + change._2))(
+        (node, previousPos) => followPrevious(node, previousPos)
+      )
+    }
+
+  val part1 = computeRopePositions(2).map(_.last).distinct.length
+  println(s"Part 1: $part1")
+
+  val part2 = computeRopePositions(10).map(_.last).distinct.length
+  println(s"Part 2: $part2")
+end Day9