aoc2023_day09.scala

aoc2023_day09.scala

import utils.loadInput

import scala.annotation.tailrec

@main def main: Unit =
  val testInput = """0 3 6 9 12 15
                    |1 3 6 10 15 21
                    |10 13 16 21 30 45
                    |""".stripMargin

  val input = loadInput("day09.txt")

  assert(part1(testInput) == 114)
  assert(part1(input) == 1921197370)
  assert(part2(testInput) == 2)
  assert(part2(input) == 1124)

def part1(testInput: String): Int = {
  testInput.split("\n").foldLeft(0) { (acc, input) =>
    val numbers = parseInput(input)
    acc + calcLast1(numbers)
  }
}

def part2(testInput: String): Int = {
  testInput.split("\n").foldLeft(0) { (acc, input) =>
    val numbers = parseInput(input)
    acc + calcLast2(numbers)
  }
}


def calcLast1(numbers: List[Int]): Int = calc(numbers, _.sum, items => items.last)
def calcLast2(numbers: List[Int]): Int = calc(numbers, _.reduce((left, right) => right - left), items => items.head)

def calc(numbers: List[Int], lastOp: List[Int] => Int, toAccItem: List[Int] => Int): Int =
  @tailrec
  def untilZero(numbers: List[Int], lasts: List[Int]): Int =
    val next: List[Int] = numbers.sliding(2).map(pair => pair.last - pair.head).toList
    if (next.forall(_ == 0)) {
      lastOp(lasts.reverse)
    } else {
      untilZero(next, lasts :+ toAccItem(next))
    }
  untilZero(numbers, List(toAccItem(numbers)))

def parseInput(testInput: String): List[Int] = {
  testInput.split(" ").toList.map(_.toInt)
}