User.scala

package ch2

import scala.collection.immutable.Queue

case class User(directFriends: Set[String] = Set.empty)
case class UserWorld(users: Map[String, User]) {
  def befriend(user1: String, user2: String): UserWorld = {
    val u1 = users(user1)
    val u2 = users(user2)

    UserWorld(users
        .updated(user1, User(u1.directFriends + user2))
        .updated(user2, User(u2.directFriends + user1))
    )
  }
  def isDirectFriendOf(user1: String, user2: String): Boolean =
    users(user1).directFriends.contains(user2)

  def isIndirectFriendOf(user1: String, user2: String): Boolean = {
    @annotation.tailrec
    def loop(visited: Set[String], queue: Queue[String]): Boolean = queue.headOption match {
      case None => false
      case Some(user2) => true
      case Some(user) => loop(visited + user, queue ++ users(user).directFriends)
    }
    loop(Set.empty, Queue(user1))
  }
}

object UserWorld extends App{
  val world = UserWorld(Map(
    "Abel" -> User(),
    "Bob" -> User(),
    "Carl" -> User(),
    "Danielle" -> User(),
    "Emily" -> User(),
  ))

  val world1 = world
    .befriend("Abel", "Bob")
    .befriend("Abel", "Carl")
    .befriend("Danielle", "Carl")
    .befriend("Emily", "Abel")

  println(world1.isDirectFriendOf("Bob", "Carl"))
  println(world1.isIndirectFriendOf("Bob", "Carl"))
  println(world1.isIndirectFriendOf("Bob", "Danielle"))
  println(world1.isIndirectFriendOf("Bob", "Emily"))

}