Day 6

aoc/resources/day6Input.txt

@@ -0,0 +1 @@
+3,4,1,1,5,1,3,1,1,3,5,1,1,5,3,2,4,2,2,2,1,1,1,1,5,1,1,1,1,1,3,1,1,5,4,1,1,1,4,1,1,1,1,2,3,2,5,1,5,1,2,1,1,1,4,1,1,1,1,3,1,1,3,1,1,1,1,1,1,2,3,4,2,1,3,1,1,2,1,1,2,1,5,2,1,1,1,1,1,1,4,1,1,1,1,5,1,4,1,1,1,3,3,1,3,1,3,1,4,1,1,1,1,1,4,5,1,1,3,2,2,5,5,4,3,1,2,1,1,1,4,1,3,4,1,1,1,1,2,1,1,3,2,1,1,1,1,1,4,1,1,1,4,4,5,2,1,1,1,1,1,2,4,2,1,1,1,2,1,1,2,1,5,1,5,2,5,5,1,1,3,1,4,1,1,1,1,1,1,1,4,1,1,4,1,1,1,1,1,2,1,2,1,1,1,5,1,1,3,5,1,1,5,5,3,5,3,4,1,1,1,3,1,1,3,1,1,1,1,1,1,5,1,3,1,5,1,1,4,1,3,1,1,1,2,1,1,1,2,1,5,1,1,1,1,4,1,3,2,3,4,1,3,5,3,4,1,4,4,4,1,3,2,4,1,4,1,1,2,1,3,1,5,5,1,5,1,1,1,5,2,1,2,3,1,4,3,3,4,3

aoc/src/Day6.scala

@@ -0,0 +1,61 @@
+import scala.io.Source
+
+object Day6 extends App:
+
+  val input = Source
+    .fromURL(getClass.getResource("day6Input.txt"))
+    .mkString
+
+  final case class Lanternfish(timer: Int = 8):
+    def spendADay(): Seq[Lanternfish] =
+      if (timer == 0) Seq(Lanternfish(6), Lanternfish())
+      else Seq(Lanternfish(timer - 1))
+
+  val initialFishes = input.split(',').toList.map(t => Lanternfish(t.toInt))
+
+  val fishesAfter18Days =
+    (1 to 18).foldLeft(initialFishes)((fishes, _) =>
+      fishes.flatMap(_.spendADay())
+    )
+  println(s"Part 1: ${fishesAfter18Days.size}")
+
+  final case class SchoolOfLanternfish(fishes: Map[Lanternfish, Long]):
+    def spendADay(): SchoolOfLanternfish =
+      val reproducingFishes = fishes(Lanternfish(0))
+
+      SchoolOfLanternfish(
+        (1 to 6)
+          .appended((8))
+          .map(timer => (Lanternfish(timer - 1) -> fishes(Lanternfish(timer))))
+          .prepended(
+            (Lanternfish(6)) -> (fishes(Lanternfish(7)) + reproducingFishes)
+          )
+          .prepended(
+            (Lanternfish(8)) -> reproducingFishes
+          )
+          .toMap
+      )
+
+    def countFishes = fishes.values.sum
+
+  object SchoolOfLanternfish:
+    def apply(fishes: Seq[Lanternfish]): SchoolOfLanternfish =
+      val samples = (0 to 8).map(Lanternfish(_))
+
+      SchoolOfLanternfish(
+        samples.foldLeft(fishes.groupMapReduce(identity)(_ => 1L)(_ + _))(
+          (fishesMap, sample) =>
+            fishesMap.updatedWith(sample) {
+              case Some(count) => Some(count)
+              case None        => Some(0)
+            }
+        )
+      )
+
+  val initialSchool = SchoolOfLanternfish(initialFishes)
+
+  val fishesAfter256Days =
+    (1 to 256)
+      .foldLeft(initialSchool)((school, _) => school.spendADay())
+      .countFishes
+  println(s"Part 2: $fishesAfter256Days")