Solve 2023 day 4 part 2

src/main/scala/eu/sim642/adventofcode2023/Day4.scala

@@ -1,10 +1,13 @@
 package eu.sim642.adventofcode2023
 
+import scala.annotation.tailrec
+
 object Day4 {
 
   case class Card(winning: Set[Int], have: Set[Int]) {
+    def haveWinning: Int = (winning intersect have).size
+
     def points: Int = {
-      val haveWinning = (winning intersect have).size
       if (haveWinning == 0)
         0
       else
@@ -14,6 +17,22 @@ object Day4 {
 
   def sumPoints(cards: Seq[Card]): Int = cards.map(_.points).sum
 
+  def countWonCards(cards: Seq[Card]): Int = {
+
+    @tailrec
+    def helper(cardCounts: Seq[(Card, Int)], acc: Int): Int = cardCounts match {
+      case Seq() => acc
+      case (card, count) +: newCardCounts =>
+        val newCardCounts2 = (0 until card.haveWinning).foldLeft(newCardCounts)({ (newCardCounts, i) =>
+          val (newCard, newCount) = newCardCounts(i)
+          newCardCounts.updated(i, (newCard, newCount + count))
+        })
+        helper(newCardCounts2, acc + count)
+    }
+
+    helper(cards.map(_ -> 1), 0)
+  }
+
 
   def parseCard(s: String): Card = s match {
     case s"Card $i: $winning | $have" =>
@@ -29,5 +48,6 @@ object Day4 {
 
   def main(args: Array[String]): Unit = {
     println(sumPoints(parseCards(input)))
+    println(countWonCards(parseCards(input)))
   }
 }

src/test/scala/eu/sim642/adventofcode2023/Day4Test.scala

@@ -20,4 +20,12 @@ class Day4Test extends AnyFunSuite {
   test("Part 1 input answer") {
     assert(sumPoints(parseCards(input)) == 23235)
   }
+
+  test("Part 2 examples") {
+    assert(countWonCards(parseCards(exampleInput)) == 30)
+  }
+
+  test("Part 2 input answer") {
+    assert(countWonCards(parseCards(input)) == 5920640)
+  }
 }