Solve 2025 day 7 part 2

Author: sim642

src/main/scala/eu/sim642/adventofcode2025/Day7.scala

@@ -5,6 +5,8 @@ import eu.sim642.adventofcodelib.GridImplicits.*
 import eu.sim642.adventofcodelib.graph.{BFS, GraphTraversal, UnitNeighbors}
 import eu.sim642.adventofcodelib.pos.Pos
 
+import scala.annotation.tailrec
+
 object Day7 {
 
   private val cellOffsets = Map(
@@ -27,11 +29,36 @@ object Day7 {
       .count(pos => grid(pos) == '^')
   }
 
+  def countTimelines(grid: Grid[Char]): Long = {
+
+    @tailrec
+    def helper(y: Int, xs: Map[Int, Long]): Long = {
+      if (y >= grid.size)
+        xs.values.sum
+      else {
+        val newXs =
+          (for {
+            (x, count) <- xs.view
+            pos = Pos(x, y)
+            offset <- cellOffsets(grid(pos))
+            newPos = pos + offset
+          } yield newPos.x -> count).groupMapReduce(_._1)(_._2)(_ + _)
+
+        helper(y + 1, newXs)
+      }
+    }
+
+    helper(0, Map(grid.posOf('S').x -> 1))
+  }
+
   def parseGrid(input: String): Grid[Char] = input.linesIterator.map(_.toVector).toVector
 
   lazy val input: String = scala.io.Source.fromInputStream(getClass.getResourceAsStream("day7.txt")).mkString.trim
 
   def main(args: Array[String]): Unit = {
     println(countBeamSplits(parseGrid(input)))
+    println(countTimelines(parseGrid(input)))
+
+    // part 2: 2012790981 (Int overflowed in countTimelines)
   }
 }

src/test/scala/eu/sim642/adventofcode2025/Day7Test.scala

@@ -30,4 +30,12 @@ class Day7Test extends AnyFunSuite {
   test("Part 1 input answer") {
     assert(countBeamSplits(parseGrid(input)) == 1587)
   }
+
+  test("Part 2 examples") {
+    assert(countTimelines(parseGrid(exampleInput)) == 40)
+  }
+
+  test("Part 2 input answer") {
+    assert(countTimelines(parseGrid(input)) == 5748679033029L)
+  }
 }