Advent of Code 09.scala

import java.io.FileInputStream
import scala.io.StdIn

@main def solve9() =

  def appendNumberToMap(numberCount: Map[BigInt, Int], number: BigInt): Map[BigInt, Int] = {
    numberCount.updated(number, numberCount.getOrElse(number, 0) + 1)
  }

  def removeNumberToMap(numberCount: Map[BigInt, Int], number: BigInt): Map[BigInt, Int] = {
    val updatedCount = numberCount.getOrElse(number, 0) - 1
    if 0 >= updatedCount then
      numberCount.removed(number)
    else
      numberCount.updated(number, updatedCount)
  }
  
  def solve1(numbers: Vector[BigInt], previousNumber: Int): Option[BigInt] = {
    def _solve(idx: Int, numberCount: Map[BigInt, Int]): Option[BigInt] = {
      if idx >= numbers.size then
        None
      else
        val number = numbers(idx)
        val isValid = numberCount.exists { x =>
          0 != numberCount.getOrElse(number - x._1, 0)
        }
        if isValid then
          val nextNumberCount = appendNumberToMap(numberCount, number)
          if 0 <= idx - previousNumber then
            _solve(idx + 1, removeNumberToMap(nextNumberCount, numbers(idx - previousNumber)))
          else
            _solve(idx + 1, nextNumberCount)
        else
          Some(number)
    }
    _solve(previousNumber, numbers.take(previousNumber).foldLeft(Map())(appendNumberToMap))
  }

  def solve2(numbers: Vector[BigInt], target: BigInt): Option[BigInt] = {
    def _solve(leftIdx: Int, rightIdx: Int, sum: BigInt): Option[BigInt] = {
      if sum == target then
        val answers = numbers.drop(leftIdx).take(rightIdx - leftIdx)
        Some(answers.min + answers.max)
      else if sum < target && rightIdx < numbers.size then
        _solve(leftIdx, rightIdx + 1, sum + numbers(rightIdx))
      else if sum > target && leftIdx < numbers.size then
        _solve(leftIdx + 1, rightIdx, sum - numbers(leftIdx))
      else
        None
    }
    _solve(0, 0, 0)
  }
  
  val in = new FileInputStream("example9-1.in")
  System.setIn(in)

  val inputs = Iterator.continually(StdIn.readLine()).takeWhile(_ != null).toVector
  val numbers = inputs.map { line => BigInt(line) }
  val answer1 = solve1(numbers, 25)
  println(answer1)
  val answer2 = solve2(numbers, answer1.getOrElse(0))
  println(answer2)