Advent of Code 2023-9.scala

import java.io.FileInputStream
import scala.annotation.tailrec
import scala.io.StdIn


@tailrec
def buildAllDiffs(acc: Vector[Vector[Int]], values: Vector[Int]): Vector[Vector[Int]] = {
  if values.forall(_ == 0) then
    acc :+ values
  else
    val next = values.sliding(2).map {
      pair => pair(1) - pair(0)
    }.toVector
    buildAllDiffs(acc :+ values, next)
}

def solve9_1(values: Vector[Vector[Int]]): Int =
  values.map(values => {
    buildAllDiffs(Vector.empty, values).reverse.foldLeft(0) {
      case (acc, x) => acc + x.last
    }
  }).sum

def solve9_2(values: Vector[Vector[Int]]): Int =
  values.map(values => {
    buildAllDiffs(Vector.empty, values).reverse.foldLeft(0) {
      case (acc, x) => x.head - acc
    }
  }).sum


@main def solve9(): Unit =
  val in = new FileInputStream("example9-2.in")
  System.setIn(in)
  val inputs = Iterator.continually(StdIn.readLine())
    .takeWhile(line => null != line)
    .map(_.split(" ").map(_.toInt).toVector)
    .toVector

  println(solve9_1(inputs))
  println(solve9_2(inputs))