Advent of Code 2021-6.scala

import java.io.FileInputStream
import scala.annotation.tailrec
import scala.io.StdIn

def solve6(inputs: Vector[Int], remainDays: Int): BigInt =
  val maxLanternfishDay = 9
  val lanternfish = inputs.foldLeft(Vector.fill[BigInt](maxLanternfishDay)(0)) { (acc ,x) =>
    acc.updated(x, acc(x) + 1)
  }
  @tailrec
  def simulateDay(lanternfish: Vector[BigInt], remainDays: Int): BigInt =
    remainDays match {
      case 0 =>
        lanternfish.sum
      case _ =>
        val nextLanternfish = lanternfish.tail :+ lanternfish.head
        simulateDay(nextLanternfish.updated(6, nextLanternfish(6) + lanternfish.head), remainDays - 1)
    }
  end simulateDay
  simulateDay(lanternfish, remainDays)
end solve6

def solve6_1(inputs: Vector[Int]): BigInt =
  solve6(inputs, 80)

def solve6_2(inputs: Vector[Int]): BigInt =
  solve6(inputs, 256)

@main def solve6(): Unit =
  val in = new FileInputStream("example6-2.in")
  System.setIn(in)
  val inputs = StdIn.readLine().split(",").map(_.toInt).toVector
  println(solve6_1(inputs)) // 5934 387413
  println(solve6_2(inputs)) // 26984457539 1738377086345