Day3.scala

def findMax(xs: List[String], index: Int): Char = {
  xs.groupBy(line => line(index)).maxBy(map => map._2.length)._1
}

def lifeSupportRating(report: List[String]): Int = {
  val oxygenPred: (Char, Char) => Boolean = (left, right) => left == right
  val co2Pred: (Char, Char) => Boolean = (left, right) => left != right

  @tailrec
  def filterUntil(ratings: List[String], index: Int)(pred: (Char, Char) => Boolean): String = {
    if (ratings.length == 1) ratings.head
    else {
      val maxForFiltered = findMax(ratings, index)
      val cleaned = ratings.filter(line => pred(line(index), maxForFiltered))
      filterUntil(cleaned, index + 1)(pred)
    }
  }

  List(oxygenPred, co2Pred)
    .map(filterUntil(report, 0)(_))
    .map(Integer.parseInt(_, 2))
    .product
}