Day 15

src/day15.scala

@@ -0,0 +1,46 @@
+package aoc
+package day15
+
+val dayNumber = "15"
+
+@main def part1: Unit =
+  println(part1(loadInput(dayNumber)))
+
+@main def part2: Unit =
+  println(part2(loadInput(dayNumber)))
+
+def part1(input: String): String =
+  input
+    .split(',')
+    .map(_.foldLeft(0)((acc, c) => ((acc + c) * 17) % 256))
+    .sum
+    .toString
+
+def part2(input: String): String =
+  val hash: String => Int = _.foldLeft(0)((acc, c) => ((acc + c) * 17) % 256)
+  val steps = input
+    .split(',')
+    .map {
+      case s"$label=$value" => (label, hash(label), Some(value.toInt))
+      case s"$label-"       => (label, hash(label), None)
+    }
+  val boxes = Map.empty[Int, List[(String, Int)]]
+  steps
+    .foldLeft(boxes) {
+      case (boxes, (label, number, Some(value))) =>
+        boxes.updatedWith(number) {
+          case Some(lenses) =>
+            val i = lenses.indexWhere(_._1 == label)
+            if i >= 0 then Some(lenses.updated(i, (label, value)))
+            else Some(lenses.appended((label, value)))
+          case None => Some(List((label, value)))
+        }
+      case (boxes, (label, number, None)) =>
+        boxes.updatedWith(number)(_.map(_.filterNot(_._1 == label)))
+
+    }
+    .flatMap((boxNumber, lenses) =>
+      lenses.zipWithIndex.map((s, i) => s._2 * (i + 1) * (boxNumber + 1))
+    )
+    .sum
+    .toString