Day25.scala

package sungkmi.aoc2020.day25

def loopSize(subjectNumber: Int, pubKey: Int): Int =
  @annotation.tailrec
  def loop(v: Int, i: Int): Int =
    if v == pubKey then i else
      val v1 = ((v.toLong * subjectNumber) % 20201227).toInt
      loop(v1, i + 1)
  loop(1, 0)

def encryptionKey(sub1: Int, sub2: Int): Int =
  val loopsize1 = loopSize(7, sub1)
  println(s"===> Loopsize #1: $loopsize1")

  (0 until loopsize1).foldLeft(1){ (v, i) =>
    ((v.toLong * sub2) % 20201227).toInt
  }

def solve1(s: String): Int = 
  val Array(sub1, sub2) = s.split("\n").map(_.toInt)
  encryptionKey(sub1, sub2)

def solve2(s: String): Int = ???

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

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

lazy val input: String = """11349501
5107328
"""

Day25Test.scala

package sungkmi.aoc2020.day25

class Day25Test extends munit.FunSuite {

  test("day25 loopSize") {
    assertEquals(loopSize(7, 5764801), 8)
    assertEquals(loopSize(7, 17807724), 11)
  }
  test("day25 encryptionKey") {
    assertEquals(encryptionKey(5764801, 17807724), 14897079)
  }
}