Advent of Code 15.scala

import java.io.FileInputStream
import scala.io.StdIn

@main def solve15() =
  def solve1(turn: Int, history: Vector[Int]): Int = {
    val result = history.init.lastIndexOf(history.last) match {
      case -1 =>
        0
      case idx =>
        turn - idx - 2
    }
    if turn == 2020 then
      result
    else
      solve1(turn + 1, history :+ result)
  }

  def solve2(numbers: Vector[Int]): Int = {
    def _solve2(turn: Int, history: Map[Int, Int], remains: Vector[Int], lastNumber: Int): Int = {
      if turn == 30000000 + 1 then
        lastNumber
      else
        val nextHistory = history.updated(lastNumber, turn - 1)
        val nextTurn = turn + 1
        (remains.headOption, history.get(lastNumber)) match {
          case (Some(number), _) =>
            _solve2(nextTurn, nextHistory, remains.tail, number)
          case (_, Some(idx)) =>
            _solve2(nextTurn, nextHistory, remains, turn - idx - 1)
          case _ =>
            _solve2(nextTurn, nextHistory, remains, 0)
        }
    }
    _solve2(2, Map(), numbers.tail, numbers.head)
  }

  val in = new FileInputStream("example15-1.in")
  System.setIn(in)

  val inputs = Iterator.continually(StdIn.readLine())
  val numbers = inputs.next().split(",").toVector.map { _.toInt }
  val answer1 = solve1(numbers.length + 1, numbers)
  println(answer1)
  val answer2 = solve2(numbers)
  println(answer2)