Solve 2025 day 11 part 2

Author: sim642

src/main/scala/eu/sim642/adventofcode2025/Day11.scala

@@ -6,8 +6,7 @@ object Day11 {
 
   type Device = String
 
-  def countPaths(devices: Map[Device, Seq[Device]], from: Device = "you", to: Device = "out"): Int = { // TODO: Long?
-
+  def countPaths(devices: Map[Device, Seq[Device]], from: Device = "you", to: Device = "out"): Int = {
     val memo = mutable.Map.empty[Device, Int]
 
     def helper(device: Device): Int = {
@@ -22,6 +21,23 @@ object Day11 {
     helper(from)
   }
 
+  def countPaths2(devices: Map[Device, Seq[Device]], from: Device = "svr", to: Device = "out"): Long = {
+    val vias = Set("dac", "fft")
+
+    val memo = mutable.Map.empty[Device, Map[Set[Device], Long]]
+
+    def helper(device: Device): Map[Set[Device], Long] = {
+      memo.getOrElseUpdate(device, {
+        if (device == to)
+          Map(Set.empty -> 1)
+        else
+          devices(device).flatMap(helper).groupMapReduce(_._1)(_._2)(_ + _).map((k, v) => (k.union(vias.intersect(Set(device)))) -> v)
+      })
+    }
+
+    helper(from)(vias)
+  }
+
   def parseDevice(s: String): (Device, Seq[Device]) = s match {
     case s"$key: $values" => key -> values.split(" ").toSeq
   }
@@ -32,5 +48,6 @@ object Day11 {
 
   def main(args: Array[String]): Unit = {
     println(countPaths(parseDevices(input)))
+    println(countPaths2(parseDevices(input)))
   }
 }

src/test/scala/eu/sim642/adventofcode2025/Day11Test.scala

@@ -17,11 +17,34 @@ class Day11Test extends AnyFunSuite {
       |hhh: ccc fff iii
       |iii: out""".stripMargin
 
+  val exampleInput2 =
+    """svr: aaa bbb
+      |aaa: fft
+      |fft: ccc
+      |bbb: tty
+      |tty: ccc
+      |ccc: ddd eee
+      |ddd: hub
+      |hub: fff
+      |eee: dac
+      |dac: fff
+      |fff: ggg hhh
+      |ggg: out
+      |hhh: out""".stripMargin
+
   test("Part 1 examples") {
     assert(countPaths(parseDevices(exampleInput)) == 5)
   }
 
   test("Part 1 input answer") {
     assert(countPaths(parseDevices(input)) == 643)
   }
+
+  test("Part 2 examples") {
+    assert(countPaths2(parseDevices(exampleInput2)) == 2)
+  }
+
+  test("Part 2 input answer") {
+    assert(countPaths2(parseDevices(input)) == 417190406827152L)
+  }
 }