(def barber (agent 0))
(def customer_queue (ref 0))

(defn output [& args]
    (println (System/currentTimeMillis) (clojure.string/join " " args)))

(defn cut_hair [the_barber queue customer]
    (dosync
        (commute queue dec)
    )
    (output customer "Bzzzzz…")
    (Thread/sleep 20)
    (output customer "…zzzzzzz")
    (inc the_barber)
)

(defn enqueue_customer [the_barber queue customer]
    (dosync
        (if (< @queue 3)
            (do
                (commute queue inc) ; We *should* be okay using commut because we deref in the IF, which should commit all previous edits. We don't care about the value right now.
                (output customer "sat in queue")
                (send-off the_barber cut_hair queue customer) ; At some point, the barber will cut their hair
            )
            (output customer " walked away"); Else they walk away
        )
    )
)

(def min_interval 10)
(def max_count 100)
(def duration (* max_count  min_interval))
(output "Spawn customers")
(future
    (doseq [customer (range max_count)]  ; Max number of customers is if they arrive at shortest interval
        (let [delay (+ min_interval (rand-int 21))]      ; Note: (rand-int 21) to get 10 to 30 inclusive
            (Thread/sleep delay)
            (output customer "arrived after " delay "ms")
            (future (enqueue_customer barber customer_queue customer))
        )
    )
)
(output "Awaiting completion")
(Thread/sleep duration)
(println @barber "customers got haircuts in" (/ duration 1000) "seconds")
(shutdown-agents)