Choice voting election

Election.scala

package domain

case class Choice(name: String) extends AnyVal

case class Ballot(topChoice: Choice, next: List[Choice]) {

  def eliminateChoice(choice: Choice): Option[Ballot] =
    if (topChoice == choice) {
      next match {
        case Nil => None
        case nextTop :: rest => Some(Ballot(nextTop, rest))
      }
    } else {
      Some(this.copy(next = next.filter(_ != choice)))
    }
}

object Ballot {
  def apply(choices: Choice*) = new Ballot(choices.head, choices.tail.toList)
}

object Election {

  @scala.annotation.tailrec
  def determineWinner(ballots: List[Ballot], roundNumber: Int): Choice = {
    val neededToWin = ballots.size / 2 + 1
    val roundResult = counts(ballots.map(_.topChoice))
    val (winningChoice, winncingCount) = roundResult.maxBy(_._2)
    println(s"Round #$roundNumber most votes ($winncingCount) goes to $winningChoice. All votes: $roundResult")
    if (winncingCount >= neededToWin) winningChoice
    else {
      val (eliminate, count) = roundResult.minBy(_._2)
      println(s"Not enough votes, needed $neededToWin. Eliminating $eliminate")
      val newBallots = ballots.flatMap(_.eliminateChoice(eliminate))
      determineWinner(newBallots, roundNumber + 1)
    }
  }

  private def counts[A](s: Seq[A]): Map[A, Int] = s.groupBy(identity).map { case (item, items) => item -> items.size }

}

Main.scala

import domain.{Ballot, Choice, Election}

object Main extends App {
  // TODO: IO.
}


object Test extends App {

  val `🍷` = Choice("Winery Tour")
  val `🎳` = Choice("Ten pin bowling")
  val `🎥` = Choice("Movie night")
  val `🖼` = Choice("Museum")

  val ballots = List(
    Ballot(`🍷`, `🎳`, `🖼`, `🎥`),
    Ballot(`🎳`, `🍷`, `🖼`),
    Ballot(`🎥`, `🍷`, `🎳`, `🖼`),
    Ballot(`🎥`, `🖼`, `🍷`, `🎳`),
    Ballot(`🖼`, `🍷`),
    Ballot(`🖼`, `🎳`),
    Ballot(`🎳`, `🍷`, `🎥`),
    Ballot(`🎥`, `🎳`, `🍷`, `🖼`),
  )

  Election.determineWinner(ballots, 1)
  /**
  Output was:
  
Round #1 most votes (3) goes to Choice(Movie night). All votes: Map(Choice(Museum) -> 2, Choice(Ten pin bowling) -> 2, Choice(Winery Tour) -> 1, Choice(Movie night) -> 3)
Not enough votes, needed 5. Eliminating Choice(Winery Tour)
Round #2 most votes (3) goes to Choice(Ten pin bowling). All votes: Map(Choice(Museum) -> 2, Choice(Ten pin bowling) -> 3, Choice(Movie night) -> 3)
Not enough votes, needed 5. Eliminating Choice(Museum)
Round #3 most votes (4) goes to Choice(Ten pin bowling). All votes: Map(Choice(Ten pin bowling) -> 4, Choice(Movie night) -> 3)

 */

}