aoc2023/Day9.scala

package aoc.day9

import aoc._

object Parser extends CommonParser:
  val nums = rep1(long)

val lns = lines.toSeq
val inputs = lns.map(Parser.parse(Parser.nums, _).get).toSeq

// part1

def calculate(add: Int => (Long, Seq[Long]) => Long)(ns: Seq[Long]) =
  @scala.annotation.tailrec
  def loop(ns: Seq[Long], depth: Int, sum: Long): Long =
    if ns.forall(_ == 0) then sum
    else
      val diff = ns.tail.lazyZip(ns).map(_ - _)
      loop(diff, depth + 1, add(depth)(sum, ns))
  loop(ns, 0, 0)

def part1 =
  println(inputs.map(calculate(_ => (sum, ns) => sum + ns.last)).sum)

// part 2

def part2 =
  println(
    inputs
      .map(
        calculate(depth => (sum, ns) => sum + ns.head * (if depth % 2 == 0 then 1 else -1))
      )
      .sum
  )

@main def Day9(part: Int) = part match
  case 1 => part1
  case 2 => part2