Day23.scala

package sungkmi.aoc2020.day23

case class CupCircle(
  cups: Vector[Int]
)

object CupCircle:
  def apply(cups: Vector[Int], current: Int): CupCircle =
    val (f, e) = cups.splitAt(cups.indexOf(current) + 1)
    CupCircle(e ++ f)
  def parse(s: String): CupCircle =
    val cups: Vector[Int] = s.map(_ - '0').toVector
    CupCircle(cups.tail :+ cups.head)

extension (c: CupCircle)
  def pickup: Vector[Int] = c.cups.take(3)
  def destination: Int =
    @annotation.tailrec
    def loop(n: Int): Int =
      if pickup contains n then loop(n - 1)
      else if n < 1 then loop(c.cups.size)
      else n
    loop(c.cups.last - 1)
  def move: CupCircle =
    val (pickup, remainder) = c.cups.splitAt(3)
//    println(s"===> $pickup / $remainder")
    val destIndex = remainder.indexOf(c.destination)
    if (destIndex == -1) then println(s"===> wrong dest: ${c.destination}")
    val (front1, end1) = remainder.splitAt(destIndex + 1)
//    println(s"===> $front1 / $end1")
    val cups1 = front1.tail ++ pickup ++ end1 :+ front1.head
//    println(s"===> $cups1")
    CupCircle(cups1)

def solve1(s: String): String =
  val cups1 = (1 to 100).foldLeft(CupCircle.parse(s))((cups, _) => cups.move)
  val indexOf1 = cups1.cups.indexOf(1)
  (cups1.cups ++ cups1.cups).drop(indexOf1 + 1).take(cups1.cups.size - 1).mkString

def solve2(s: String): BigInt =
  val CupCircle(cups0) = CupCircle.parse(s)
  val cups = CupCircle(cups0 ++ ((cups0.size + 1) to 1000000))
  val cups1: Vector[Int] = (1 to 10000000).foldLeft(cups)((cups, _) => cups.move).cups
  val indexOf1 = cups1.indexOf(1)
  val n1 = cups1((indexOf1 + 1) % 10000000)
  val n2 = cups1((indexOf1 + 2) % 10000000)
  BigInt(n1) * BigInt(n2)

@main def part1: Unit =
  val ans = solve1(input)
  println(ans)

@main def part2: Unit = 
  val ans = solve2(input)
  println(ans)

lazy val input: String = """487912365"""

Day23Test.scala

package sungkmi.aoc2020.day23

import scala.collection.immutable.Queue
class Day23Test extends munit.FunSuite {

  val testInput = """389125467"""

  val cups1 = CupCircle(Vector(3,8,9,1,2,5,4,6,7), 3)
  val cups2 = CupCircle(Vector(3,2,8,9,1,5,4,6,7), 2)
  val cups3 = CupCircle(Vector(3,2,5,4,6,7,8,9,1), 5)
  val cups4 = CupCircle(Vector(7,2,5,8,9,1,3,4,6), 8)
  val cups5 = CupCircle(Vector(3,2,5,8,4,6,7,9,1), 4)
  val cups6 = CupCircle(Vector(9,2,5,8,4,1,3,6,7), 1)
  val cups7 = CupCircle(Vector(7,2,5,8,4,1,9,3,6), 9)
  val cups8 = CupCircle(Vector(8,3,6,7,4,1,9,2,5), 2)
  val cups9 = CupCircle(Vector(7,4,1,5,8,3,9,2,6), 6)
  val cups10 = CupCircle(Vector(5,7,4,1,8,3,9,2,6), 5)
  val cupsF = CupCircle(Vector(5,8,3,7,4,1,9,2,6), 8)

  test("day23 parse") {
    assertEquals(CupCircle.parse(testInput), cups1)
  }

  test("day23 CupCircle.pickup") {
    assertEquals(cups1.pickup, Vector(8,9,1))
    assertEquals(cups2.pickup, Vector(8,9,1))
    assertEquals(cups3.pickup, Vector(4,6,7))
    assertEquals(cups4.pickup, Vector(9,1,3))
    assertEquals(cups5.pickup, Vector(6,7,9))
    assertEquals(cups6.pickup, Vector(3,6,7))
    assertEquals(cups7.pickup, Vector(3,6,7))
    assertEquals(cups8.pickup, Vector(5,8,3))
    assertEquals(cups9.pickup, Vector(7,4,1))
    assertEquals(cups10.pickup, Vector(7,4,1))
  }

  test("day23 CupCircle.destination") {
    assertEquals(cups1.destination, 2)
    assertEquals(cups2.destination, 7)
    assertEquals(cups3.destination, 3)
    assertEquals(cups4.destination, 7)
    assertEquals(cups5.destination, 3)
    assertEquals(cups6.destination, 9)
    assertEquals(cups7.destination, 8)
    assertEquals(cups8.destination, 1)
    assertEquals(cups9.destination, 5)
    assertEquals(cups10.destination, 3)
  }

  //test("day23 CupCircle.move") {
  //  assertEquals(cups2.move, cups3)
  //}
  
  test("day23 CupCircle.move") {
    assertEquals(cups1.move, cups2)
    assertEquals(cups2.move, cups3)
    assertEquals(cups3.move, cups4)
    assertEquals(cups4.move, cups5)
    assertEquals(cups5.move, cups6)
    assertEquals(cups6.move, cups7)
    assertEquals(cups7.move, cups8)
    assertEquals(cups8.move, cups9)
    assertEquals(cups9.move, cups10)
    assertEquals(cups10.move, cupsF)
  }

  test("day23 solve1") {
    assertEquals(solve1(testInput), "67384529")
  }

  //test("day23 solve2") {
  //  assertEquals(solve2("389125467"), BigInt("149245887792"))
  //}
}