Sleeping 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))))

(defn open-for-business
  (do
    (dosync (ref-set shop-is-open true))
    (Thread/sleep (* 10 100))
    (dosync (ref-set shop-is-open false))))

; 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.
(while @shop-is-open
  (if (not (empty? @waiting-customers))
    (do
      (cut-hair (first @waiting-customers))
      (dosync (alter waiting-customers pop)))))

(while true
  (do
    (Thread/sleep (+ 10 (rand-int 21)))
    (greet-customer :customer)))

(open-for-business)
(println @serviced-customers)
(println @lost-customers)