#lang racket
;;; Example 6c - Selective Accepts with Process Check

(require (planet williams/simulation/simulation))

;;; process (lock)
;;; entry (lock) -> void?
;;; entry (unlock) -> void?
(define-process (lock)
  (let ((process #f)
        (locked? #f))
    (let loop ()
        ((when (not locked?)
           (accept caller (lock)
             (set! process caller)))
         (set! locked? #t))
        ((accept caller (unlock)
           (unless (eq? caller process)
             (error 'unlock
                    "process does not have the lock"
         (set! process #f)
         (set! locked? #f)))

;;; process (p1 i a-lock)
;;;   i : exact-nonnegative-integer?
;;;   a-lock : process?
;;; This process uses the lock to protect its critical region, which just does a
;;; random wait in this case.
(define-process (p1 i a-lock)
  (printf "~a: process p1(~a) started.~n"
          (current-simulation-time) i)
  (call a-lock (lock))
  (printf "~a: process p1(~a) acquired lock.~n"
          (current-simulation-time) i)
  (wait (random-flat 0.0 10.0))
  (printf "~a: process p1(~a) releasing lock.~n"
          (current-simulation-time) i)
  (call a-lock (unlock)))

;;; (main n) -> void?
;;;   n : exact-nonnegative-integer?
;;; Creates the lock process and schedules n p1 processes that use the lock.
(define (main n)
    (let ((a-lock (schedule #:now (lock))))
      (for ((i (in-range n)))
        (schedule #:at (random-flat 0.0 10.0) (p1 i a-lock)))

;;; Test with 10 p1 processes.
(main 10)