random donations

x.scala

import scala.util.Random

// http://www.decisionsciencenews.com/2017/06/19/counterintuitive-problem-everyone-room-keeps-giving-dollars-random-others-youll-never-guess-happens-next/

case class Person(var balance: Int) {
  def receive(amount: Int): Unit = {
    balance = balance + 1
  }

  def give(amount: Int, other: Person): Unit = {
    if (balance > 0) {
      other.receive(amount)
      balance = balance - 1
    }
  }

  override def toString: String = s"£$balance"
}

class World(population: Int, iterations: Int, startingBalance: Int, giftAmount: Int) {
  val people: List[Person] = (1 to population).map(_ => Person(startingBalance)).toList

  def run(): Unit = {
    for {
      i <- (1 to iterations)
      person <- people
      others = people diff List(person)
      other = others(Random.nextInt(others.size))
    } person.give(giftAmount, other)
  }

  override def toString: String = people.sortBy(_.balance).toString
}

object Main extends App {
  val world = new World(
    population = 100,
    iterations = 10000,
    startingBalance = 100,
    giftAmount = 1
  )

  world.run()

  println(world)
}