20questions.scala

package twentyquestions

import scala.io.StdIn


sealed trait Step
case class Question(text: String, yes: Step, no: Step) extends Step
case class Guess(text: String) extends Step


object Main {
  val initialModel =
    Question(
      text = "Is it an animal?",
      yes = Guess("an elephant"),
      no = Guess("a frying pan")
    )

  def runStep(step: Step): Step = step match {
    case Question(questionText, yes, no) =>
      StdIn.readLine(questionText + " ").toLowerCase match {
        case "yes" =>
          val newYesBranch = runStep(yes)
          Question(questionText, newYesBranch, no)
        case "no" =>
          val newNoBranch = runStep(no)
          Question(questionText, yes, newNoBranch)
      }
    case Guess(guessText)             =>
      StdIn.readLine("Is it " + guessText + "? ").toLowerCase match {
        case "yes" =>
          println("I won!")
          step
        case "no" =>
          val answer = StdIn.readLine("I give up. What were you thinking of? ")
          val newQuestionText =
            StdIn.readLine(
              "Enter a question that the answer is 'yes' for " + answer + " and 'no' for " + guessText + ". "
            )
          Question(text = newQuestionText, yes = Guess(answer), no = Guess(guessText))
      }
  }

  def main(args: Array[String]): Unit = {
    def loop(model: Step): Unit = {
      val result = runStep(model)
      StdIn.readLine("Do you want to play again? ").toLowerCase match {
        case "no" =>
        case "yes" => loop(result)
      }
    }
    loop(initialModel)
  }
}