#lang racket/base
;; Virtualized operating system, this time with presence.

(require racket/match)

;;---------------------------------------------------------------------------
;; Data definitions

;; A PID is an (arbitrary) VM-unique process identifier.
;; A SID is an (arbitrary) process-unique subscription identifier.

;; A QuasiQueue<X> is a list of Xs in *reversed* order.

(struct vm (processes ;; PID -> Process
	    topics ;; Topic -> Set<Flow>
	    flows ;; Flow -> Set<Route>
	    next-process-id ;; PID
	    pending-actions ;; QuasiQueue<(cons PID Action)>
	    ) #:transparent)

;; (route PID SID Handlers)
(struct route (process-id sid handlers) #:transparent)

(struct process (id ;; PID
		 state
		 interests ;; SID -> List<Handlers>
		 meta-interests ;; SID -> List<Handlers>
		 ) #:transparent)

(struct topic (role pattern virtual?) #:prefab)

;; A Flow is a Topic that comes from the intersection of two dual
;; topics.

;; PresenceHandler = Topic -> State -> Transition
;; AbsenceHandler = Topic * Reason -> State -> Transition
;; MessageHandler = Topic * Message -> State -> Transition
(struct handlers (topic presence absence message) #:transparent)

;; actions is a plain old List<Action>, not a QuasiQueue.
(struct transition (state actions) #:transparent)

;; Preactions
(struct add-role (sid handlers) #:prefab)
(struct delete-roles (sid) #:prefab)
(struct send-message (topic body) #:prefab)
(struct spawn (thunk) #:prefab)

;; An Action is either a Preaction or an (at-meta-level Preaction).
(struct at-meta-level (preaction) #:prefab)

;;---------------------------------------------------------------------------
;; Topics and roles

(define (topic-publisher pattern #:virtual? [virtual? #f])
  (topic 'publisher pattern virtual?))

(define (topic-subscriber pattern #:virtual? [virtual? #f])
  (topic 'subscriber pattern virtual?))

(define (co-roles r)
  (case r
    [(publisher) '(subscriber)]
    [(subscriber) '(publisher)]
    [else #f]))

(define (co-topics t)
  (for/list ([role (co-roles (topic-role t))])
    (struct-copy topic t [topic-role role])))

;;---------------------------------------------------------------------------

;; QuasiQueue<X>
(define empty-quasi-queue '())

;; X QuasiQueue<X> -> QuasiQueue<X>
(define (quasi-enqueue-one thing existing-quasi-queue)
  (cons thing existing-quasi-queue))

;; List<X> QuasiQueue<X> -> QuasiQueue<X>
(define (quasi-enqueue-many many-things-in-order existing-quasi-queue)
  (append (reverse many-things-in-order) existing-quasi-queue))

;; QuasiQueue<X> -> List<X>
(define (quasi-queue->list quasi-queue)
  (reverse quasi-queue))

;; List<X> -> QuasiQueue<X>
(define (list->quasi-queue xs)
  (reverse xs))

;;---------------------------------------------------------------------------

(define (make-vm boot)
  (vm (hash)
      (hash)
      (hash)
      0
      (list->quasi-queue (list (spawn boot)))))

(define (run-vm state)
  (let loop ((remaining-actions (quasi-queue->list (vm-pending-actions state)))
	     (state (struct-copy vm state [pending-actions empty-quasi-queue]))
	     (outbound-actions empty-quasi-queue))
    (match remaining-actions
      ['() (transition state (quasi-queue->list outbound-actions))]
      [(cons (cons pid action) rest)
       (if (at-meta-level? action)
	   (let-values (((state new-actions)
			 (perform-meta-action pid (at-meta-level-preaction action) state)))
	     (loop rest state (quasi-enqueue-many new-actions outbound-actions)))
	   (loop rest (perform-action pid action state) outbound-actions))])))

(define (run-user-code v)
  ;; TODO: use this hook to find all the bits of code that will need
  ;; with-handlers and crash compensation.
  v)

(define (perform-action pid action state)
  (match action
    [(add-role sid handlers) ...]
    [(delete-roles sid) ...]
    [(send-message topic body) ...]
    [(spawn thunk)
     (match-define (transition initial-state initial-actions) (run-user-code (thunk)))
     (define new-pid (vm-next-process-id state))
     (struct-copy vm (enqueue-actions state new-pid initial-actions)
		  [processes (hash-set (vm-processes state) new-pid (process new-pid
									     initial-state
									     (hash)
									     (hash)))]
		  [next-process-id (+ new-pid 1)])]))

(define (enqueue-actions state pid actions)
  (struct-copy vm state
	       [pending-actions (quasi-enqueue-many (for/list ([a actions]) (cons pid a))
						    (vm-pending-actions state))]))


-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

  (let* ((state (requeue-pollers state))
	 (state (run-runnables state))
	 (state (dispatch-messages state))
	 (meta-messages (reverse (vm-pending-meta-messages state)))
	 (meta-handlers (append-map extract-downward-meta-message-handlers (vm-suspensions state)))
	 (poller-k (and (should-poll? state) run-vm)) ;; only block if there's nothing left to do
	 (state (struct-copy vm state [pending-meta-messages (list)])))
    (kernel-mode-transition (suspension state poller-k meta-handlers '())
			    meta-messages
			    '()
			    '())))

(define (requeue-pollers state)
  (foldl (lambda (susp state)
	   (if (suspension-polling? susp)
	       (enqueue-runnable (lambda () ((suspension-k susp) (suspension-state susp))) state)
	       (enqueue-suspension susp state)))
	 (struct-copy vm state [suspensions '()])
	 (vm-suspensions state)))

(define (run-runnables state)
  (foldl (lambda (r state) (perform-transition (r) state))
	 (struct-copy vm state [pending-processes (list)])
	 (reverse (vm-pending-processes state))))

(define (dispatch-messages state)
  (foldl dispatch-message
	 (struct-copy vm state [pending-messages (list)])
	 (reverse (vm-pending-messages state))))

(define (extract-downward-meta-message-handlers susp)
  (for/list ([mmh (suspension-meta-message-handlers susp)])
    (message-handler (message-handler-pattern mmh) dispatch-meta-message)))

(define ((dispatch-meta-message message) state)
  (run-vm
   (foldl (match-suspension message
			    (vm-meta-pattern-predicate state)
			    suspension-meta-message-handlers)
	  (struct-copy vm state [suspensions '()])
	  (vm-suspensions state))))

(define (perform-transition transition state)
  (match transition
    [(kernel-mode-transition new-suspension
			     messages
			     meta-messages
			     new-processes)
     (let* ((state (foldl enqueue-message state messages))
	    (state (foldl enqueue-runnable state new-processes))
	    (state (enqueue-suspension new-suspension state))
	    (state (foldl enqueue-meta-message state meta-messages)))
       state)]
    [other
     (error 'vm "Processes must return a kernel-mode-transition struct; got ~v" other)]))

(define (enqueue-message message state)
  (struct-copy vm state [pending-messages (cons message (vm-pending-messages state))]))

(define (enqueue-runnable r state)
  (struct-copy vm state [pending-processes (cons r (vm-pending-processes state))]))

(define (enqueue-suspension susp state)
  (match susp
    [(suspension _ #f '() '())
     ;; dead process because no continuations offered
     state]
    [(suspension _ _ _ _)
     (struct-copy vm state [suspensions (cons susp (vm-suspensions state))])]))

(define (enqueue-meta-message message state)
  (struct-copy vm state [pending-meta-messages (cons message (vm-pending-meta-messages state))]))

(define (dispatch-message message state)
  (foldl (match-suspension message
			   (vm-pattern-predicate state)
			   suspension-message-handlers)
	 (struct-copy vm state [suspensions '()])
	 (vm-suspensions state)))

(define ((match-suspension message apply-pattern handlers-getter) susp state)
  (let search-handlers ((message-handlers (handlers-getter susp)))
    (cond
     [(null? message-handlers)
      ;; No handler matched this message. Put the suspension
      ;; back on the list for some future message.
      (enqueue-suspension susp state)]
     [(apply-pattern (message-handler-pattern (car message-handlers)) message)
      (define trapk (message-handler-k (car message-handlers)))
      (define interruptk (trapk message))
      (perform-transition (interruptk (suspension-state susp)) state)]
     [else
      (search-handlers (cdr message-handlers))])))

(define (suspension-polling? susp)
  (not (eq? (suspension-k susp) #f)))

(define (should-poll? state)
  (or (not (null? (vm-pending-processes state)))
      (not (null? (vm-pending-messages state)))
      (ormap suspension-polling? (vm-suspensions state))))

(define (nested-vm boot
		   #:pattern-predicate [pattern-predicate default-pattern-predicate]
		   #:meta-pattern-predicate [meta-pattern-predicate default-pattern-predicate])
  (lambda () (run-vm (make-vm boot
			      #:pattern-predicate pattern-predicate
			      #:meta-pattern-predicate meta-pattern-predicate))))

(define default-pattern-predicate
  (lambda (p m) (p m)))

;;---------------------------------------------------------------------------

(define (nested-vm-inert? susp)
  (match susp
    [(suspension (vm _ '() '() '() _ _) #f '() '())
     ;; Inert iff not waiting for any messages or metamessages, and
     ;; with no internal work left to do.
     #t]
    [_ #f]))

(struct ground-event-pattern (tag evt) #:transparent)
(struct ground-event-value (tag val) #:transparent)

(define (match-ground-event p m)
  (equal? (ground-event-pattern-tag p) (ground-event-value-tag m)))

(define (ground-vm boot
		   #:pattern-predicate [pattern-predicate default-pattern-predicate])
  (let loop ((transition (run-vm (make-vm boot
					  #:pattern-predicate pattern-predicate
					  #:meta-pattern-predicate match-ground-event))))
    (for-each (lambda (thunk) (thunk)) (kernel-mode-transition-messages transition))
    (when (not (nested-vm-inert? (kernel-mode-transition-suspension transition)))
      (match transition
	[(kernel-mode-transition (suspension new-state
					     polling-k
					     message-handlers
					     '())
				 _
				 '()
				 '())
	 (define inbound-messages
	   (map (match-lambda [(message-handler (ground-event-pattern tag evt) k)
			       (wrap-evt evt (lambda (v) (cons (ground-event-value tag v) k)))])
		message-handlers))
	 (match-define (cons inbound-value inbound-continuation)
	   (apply sync
		  (wrap-evt (if polling-k always-evt never-evt)
			    (lambda (v) (cons (ground-event-value 'idle (void))
					      (lambda (dummy) polling-k))))
		  inbound-messages))
	 (loop ((inbound-continuation inbound-value) new-state))]
	[_
	 (error 'ground-vm
		"Outermost VM may not spawn new siblings or send or receive metamessages")]))))