Sleeping Barber...

barber.clj

; declare a total variable which will keep
; count of the number of haircuts
(def total (agent 0))
; declare a queue for customer/barber:producer/consumer behavior
(def seats (ref (clojure.lang.PersistentQueue/EMPTY)))

; it takes 20ms to give a haircut.
; when a haircut is done, we'll
; safely increment the total.
(defn cut-hair [thetotal]
    (dosync
        (if (peek @seats)
            (do
                ; alter must be called within a transaction
                ; alter <ref> <function> <args>
                (alter seats pop)
                (Thread/sleep 20)
                (inc thetotal)
            )
            thetotal
        )))

; if there are seats available in the lobby, sit down,
; otherwise, leave (which is a no-op)
(defn enter_lobby [customer]
    (dosync
        (if (< (count @seats) 3)
            ; conjoin returns a new collection with cusomter added
            (alter seats conj customer)
    )))

; this is the whole key to the program. adding a watch allows
; the barber to wake up when the reference to seats has been changed.
; the "send-to-barber" is a key, its only use is to remove the watch.
(add-watch seats :send-to-barber
    (fn [_ _ _ _] (send total cut-hair)))

; run a test for 10 seconds
(def start (System/currentTimeMillis))
(println (format "starting with %s" @total))
; while 10 seconds has not passed since the start time
(while
    (<= (- (System/currentTimeMillis) start) 10000)
    ; sleep at a random interval between 10 and 30
    (Thread/sleep (nth (range 10 30) (rand-int 20)))
    ; it doesn't really matter what we send here, as long as it's
    ; an agent. the agent represents the customer entering the lobby here.
    (send (agent :customer) enter_lobby)
    )
(println (format "finished with %s in %d seconds" @total (- (System/currentTimeMillis) start)))
(remove-watch seats :send-to-barber)