A ScalaSTM approach to the sleeping barber problem.

build.sbt

scalaVersion := "2.10.0"

libraryDependencies += "org.scala-stm" %% "scala-stm" % "0.7"

SleepingBarber.scala

/*
 * A ScalaSTM approach to the sleeping barber problem. Build and run with sbt and a
 * build.sbt file with these lines:
 * 
 * 
 * scalaVersion := "2.10.0"
 * 
 * libraryDependencies += "org.scala-stm" %% "scala-stm" % "0.7"
 * 
 * 
 * This is my first use of STM, so bugs and idiotic usage may abound.
 * 
 * Links:
 * 
 * http://www.bestinclass.dk/index.clj/2009/09/scala-vs-clojure-round-2-concurrency.html
 * http://scala-stm.org/
 * https://raw.github.com/paulp/sbt-extras/master/sbt
 * 
 */
object SleepingBarber extends App {
  import annotation.tailrec
  import util.Random._

  import concurrent._
  import ExecutionContext.Implicits._
  import stm._

  val queue = Ref(collection.immutable.Queue[Int]())
  val tally = Ref(0).single
  val SEATS = 3

  def debug(msg: String, customer: Int) { println("%-35s %d" format (msg, customer)) }

  def enterShop(customer: Int) {
    debug("(c) entering shop", customer)
    atomic { implicit t =>
      if (queue().size < SEATS)
        queue transform { _ :+ customer }
      else
        debug("(s) turning away customer", customer)
    }
  }

  @tailrec def cutHair() {
    atomic { implicit t =>
      queue.single.await(!_.isEmpty)
      debug("(b) cutting hair of customer", queue().head)
      queue transform { _.tail }
      tally += 1
    }
    Thread sleep (100 + nextInt(600))
    cutHair
  }

  future { cutHair }

  1 to 20 foreach { customer =>
    enterShop(customer)
    Thread sleep (100 + nextInt(200))
  }

  Thread sleep 2000

  println(s"(!) ${tally()} customers got haircuts today")
}