Solve 2023 day 15 part 2

src/main/scala/eu/sim642/adventofcode2023/Day15.scala

@@ -6,12 +6,54 @@ object Day15 {
 
   def sumStepHashes(steps: Seq[String]): Int = steps.map(hash).sum
 
+  case class HashMap(boxes: Vector[Vector[(String, Int)]]) {
+    def +(keyValue: (String, Int)): HashMap = {
+      val (key -> value) = keyValue
+      val boxI = hash(key)
+      val box = boxes(boxI)
+      val keyI = box.indexWhere(_._1 == key)
+      val newBox =
+        if (keyI < 0)
+          box :+ keyValue
+        else
+          box.updated(keyI, keyValue)
+      HashMap(boxes.updated(boxI, newBox))
+    }
+
+    def -(key: String): HashMap = {
+      val boxI = hash(key)
+      HashMap(boxes.updated(boxI, boxes(boxI).filterNot(_._1 == key)))
+    }
+
+    def focusingPower: Int = {
+      (for {
+        (box, boxI) <- boxes.zipWithIndex
+        ((_, focalLength), slotI) <- box.zipWithIndex
+      } yield (boxI + 1) * (slotI + 1) * focalLength).sum
+    }
+  }
+
+  object HashMap {
+    def empty: HashMap = HashMap(Vector.fill(256)(Vector.empty))
+  }
+
+  def installLenses(steps: Seq[String]): HashMap = {
+    steps.foldLeft(HashMap.empty)({
+      case (acc, s"$key=$value") => acc + (key -> value.toInt)
+      case (acc, s"$key-") => acc - key
+      case (_, _) => ???
+    })
+  }
+
+  def focusingPower(steps: Seq[String]): Int = installLenses(steps).focusingPower
+
 
   def parseSteps(input: String): Seq[String] = input.split(',').toSeq
 
   lazy val input: String = scala.io.Source.fromInputStream(getClass.getResourceAsStream("day15.txt")).mkString.trim
 
   def main(args: Array[String]): Unit = {
     println(sumStepHashes(parseSteps(input)))
+    println(focusingPower(parseSteps(input)))
   }
 }

src/test/scala/eu/sim642/adventofcode2023/Day15Test.scala

@@ -19,4 +19,12 @@ class Day15Test extends AnyFunSuite {
   test("Part 1 input answer") {
     assert(sumStepHashes(parseSteps(input)) == 507666)
   }
+
+  test("Part 2 examples") {
+    assert(focusingPower(parseSteps(exampleInput)) == 145)
+  }
+
+  test("Part 2 input answer") {
+    assert(focusingPower(parseSteps(input)) == 233537)
+  }
 }