Aoc2021_Day06.scala

package aoc2021

object Day06:
  type InputType = Seq[Int]

  @main def runDay06(): Unit =
    val testData = time("testReady", () => ready(testInput))
    time("testPart1", () => part1(testData)) // should be 5934
    time("testPart2", () => part2(testData)) // should be 26984457539

    val data = time("ready", () => ready(input))
    time("part1", () => part1(data))
    time("part2", () => part2(data))

  def ready(input: String): InputType =
    input
      .linesIterator
      .flatMap(_.split(","))
      .map(_.toInt)
      .toIndexedSeq

  def part1(data: InputType): Long =
    // naive solution
    Range(0, 80).foldLeft(data)((fishes, i) => {
      val next = fishes.map(timer => if (timer == 0) 6 else timer - 1)
      next ++ Seq.fill(fishes.count(n => n == 0))(8)
    }).length

  def part2(data: InputType): Long =
    val initial = data.foldLeft(Vector.fill(9)(0L))((l, n) => l.updated(n, l(n) + 1))
    Range(0, 256).foldLeft(initial)((counters, n) => {
      val rotated = counters.tail :+ counters.head
      rotated.updated(6, rotated(6) + counters(0))
    }).sum

  private val testInput =
    """3,4,3,1,2""".stripMargin

  private val input =
    """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""".stripMargin