Day 11

aoc/src/dev/ctsk/aoc/Main.scala

@@ -12,7 +12,8 @@ val solvers = Map[Int, Solver](
   7 -> Day07,
   8 -> Day08,
   9 -> Day09,
-  10 -> Day10
+  10 -> Day10,
+  11 -> Day11
 )
 
 def runSolver(solver: Solver, input: os.Path): Unit =

aoc/src/dev/ctsk/aoc/days/Day11.scala

@@ -0,0 +1,51 @@
+package dev.ctsk.aoc.days
+
+import dev.ctsk.aoc._
+import scala.collection.mutable
+
+object Day11 extends Solver(11):
+  // upper and lower halves of number
+  def halves(n: Long): Option[(Long, Long)] =
+    val numDigits = (Math.log10(n) + 1).toLong
+    numDigits % 2 match
+      case 1 => None
+      case 0 => {
+        Some(
+          (
+            n % Math.pow(10, numDigits / 2).toLong,
+            n / Math.pow(10, numDigits / 2).toLong
+          )
+        )
+      }
+
+  def counter_compute(stones: Array[Long], depth: Long): Long =
+    var acc: mutable.Map[Long, Long] =
+      mutable.Map.from(stones.map((_, 1L)))
+
+    for (_ <- 0L until depth) {
+      val next = mutable.Map.empty[Long, Long].withDefaultValue(0L)
+      for ((stone, count) <- acc) {
+        if stone == 0 then next(1) += count
+        else
+          halves(stone.toLong) match
+            case Some((a, b)) =>
+              next(a) += count
+              next(b) += count
+            case None =>
+              next(stone * 2024) += count
+      }
+      acc = next
+    }
+
+    acc.values.sum
+
+  def run(input: os.ReadablePath): (Timings, Solution) =
+    val in = longs(os.read.lines(input).head).toArray
+
+    val p1 = timed { counter_compute(in, 25) }
+    val p2 = timed { counter_compute(in, 75) }
+
+    (
+      Timings(0, p1._1, p2._1),
+      Solution(Long.box(p1._2), Long.box(p2._2))
+    )

data/11.ans

@@ -0,0 +1,2 @@
+189167
+225253278506288

data/11.in

@@ -0,0 +1 @@
+28 4 3179 96938 0 6617406 490 816207