Barber…

gistfile1.clj

(ns barber.core
  (:gen-class))

(def chair (ref []))
(def lost-customers (ref []))
(def shop-is-open (ref false))
(def serviced-customers (ref []))
(def waiting-customers (ref clojure.lang.PersistentQueue/EMPTY))

; If there are less than three customers waiting for a haircut, we can welcome
; them in and sit them down. If there are already three customers waiting, the
; barber has lost some business.
(defn greet-customer [customer]
  (dosync
    (if (< (count @waiting-customers) 3)
      (alter waiting-customers conj customer)
      (alter lost-customers conj customer))))

; Take a customer and cut his hair for 20 milliseconds. Once we’re done, we add
; him to our list of satisfied customers.
(defn cut-hair [customer]
  (do
    (Thread/sleep 20)
    (dosync (alter serviced-customers conj customer))))

; While the shop is open, continuously check the waiting room for waiting
; customers. If we find one, remove him from the waiting room and send him to
; have his hair cut.
(defn monitor-waiting-room []
  (while @shop-is-open
    (if (not (empty? @waiting-customers))
      (do
        (cut-hair (first @waiting-customers))
        (dosync (alter waiting-customers pop))))))

(defn send-customers-from-street []
  (while @shop-is-open
    (do
      (Thread/sleep (+ 10 (rand-int 21)))
      (greet-customer :customer))))

(defn open-for-business []
  (do
    (dosync (ref-set shop-is-open true))
    (monitor-waiting-room)
    (send-customers-from-street)
    (Thread/sleep (* 10 1000))
    (dosync (ref-set shop-is-open false))))

(open-for-business)
(println "serviced customers: " (count @serviced-customers))
(println "lost customers: " (count @lost-customers))