;; Erlang runtime — scheduler, process records, mailbox queue.
;; Phase 3 foundation. spawn/send/receive build on these primitives.
;;
;; Scheduler is a single global dict in `er-scheduler` holding:
;;   :next-pid    INT    — counter for fresh pid allocation
;;   :processes   DICT   — pid-key (string) -> process record
;;   :runnable    QUEUE  — FIFO of pids ready to run
;;   :current     PID    — pid currently executing, or nil
;;
;; A pid value is tagged: {:tag "pid" :id INT}. Pids compare by id.
;;
;; Process record fields:
;;   :pid             — this process's pid
;;   :mailbox         — queue of received messages (arrival order)
;;   :state           — "runnable" | "running" | "waiting" | "exiting" | "dead"
;;   :continuation    — saved k (for receive suspension); nil otherwise
;;   :receive-pats    — patterns the process is blocked on; nil otherwise
;;   :trap-exit       — bool
;;   :links           — list of pids
;;   :monitors        — list of {:ref :pid}
;;   :env             — Erlang env at the last yield
;;   :exit-reason     — nil until the process exits
;;
;; Queue — amortised-O(1) FIFO with head-pointer + slab-compact:
;;   {:items (list...) :head-idx INT}

;; ── queue ────────────────────────────────────────────────────────
(define er-q-new (fn () {:head-idx 0 :items (list)}))

(define er-q-push! (fn (q x) (append! (get q :items) x)))

(define
  er-q-pop!
  (fn
    (q)
    (let
      ((h (get q :head-idx)) (items (get q :items)))
      (if
        (>= h (len items))
        nil
        (let
          ((x (nth items h)))
          (dict-set! q :head-idx (+ h 1))
          (er-q-compact! q)
          x)))))

(define
  er-q-peek
  (fn
    (q)
    (let
      ((h (get q :head-idx)) (items (get q :items)))
      (if (>= h (len items)) nil (nth items h)))))

(define
  er-q-len
  (fn (q) (- (len (get q :items)) (get q :head-idx))))

(define er-q-empty? (fn (q) (= (er-q-len q) 0)))

;; Compact the backing list when the head pointer gets large so the
;; queue doesn't grow without bound. Threshold chosen to amortise the
;; O(n) copy — pops are still amortised O(1).
(define
  er-q-compact!
  (fn
    (q)
    (let
      ((h (get q :head-idx)) (items (get q :items)))
      (when
        (> h 128)
        (let
          ((new (list)))
          (for-each
            (fn (i) (append! new (nth items i)))
            (range h (len items)))
          (dict-set! q :items new)
          (dict-set! q :head-idx 0))))))

(define
  er-q-to-list
  (fn
    (q)
    (let
      ((h (get q :head-idx)) (items (get q :items)) (out (list)))
      (for-each
        (fn (i) (append! out (nth items i)))
        (range h (len items)))
      out)))

;; Read the i'th entry (relative to head) without popping.
(define
  er-q-nth
  (fn (q i) (nth (get q :items) (+ (get q :head-idx) i))))

;; Remove entry at logical index i, shift tail in.
(define
  er-q-delete-at!
  (fn
    (q i)
    (let
      ((h (get q :head-idx)) (items (get q :items)) (new (list)))
      (for-each
        (fn
          (j)
          (when (not (= j (+ h i))) (append! new (nth items j))))
        (range h (len items)))
      (dict-set! q :items new)
      (dict-set! q :head-idx 0))))

;; ── pids ─────────────────────────────────────────────────────────
(define er-mk-pid (fn (id) {:id id :tag "pid"}))
(define er-pid? (fn (v) (er-is-tagged? v "pid")))
(define er-pid-id (fn (pid) (get pid :id)))
(define er-pid-key (fn (pid) (str "p" (er-pid-id pid))))
(define
  er-pid-equal?
  (fn (a b) (and (er-pid? a) (er-pid? b) (= (er-pid-id a) (er-pid-id b)))))

;; ── scheduler state ──────────────────────────────────────────────
(define er-scheduler (list nil))

(define
  er-sched-init!
  (fn
    ()
    (set-nth!
      er-scheduler
      0
      {:next-pid 0
        :current nil
        :processes {}
        :runnable (er-q-new)})))

(define er-sched (fn () (nth er-scheduler 0)))

(define
  er-pid-new!
  (fn
    ()
    (let
      ((s (er-sched)))
      (let
        ((n (get s :next-pid)))
        (dict-set! s :next-pid (+ n 1))
        (er-mk-pid n)))))

(define
  er-sched-runnable
  (fn () (get (er-sched) :runnable)))

(define
  er-sched-processes
  (fn () (get (er-sched) :processes)))

(define
  er-sched-enqueue!
  (fn (pid) (er-q-push! (er-sched-runnable) pid)))

(define
  er-sched-next-runnable!
  (fn () (er-q-pop! (er-sched-runnable))))

(define
  er-sched-runnable-count
  (fn () (er-q-len (er-sched-runnable))))

(define
  er-sched-set-current!
  (fn (pid) (dict-set! (er-sched) :current pid)))

(define er-sched-current-pid (fn () (get (er-sched) :current)))

(define
  er-sched-process-count
  (fn () (len (keys (er-sched-processes)))))

;; ── process records ──────────────────────────────────────────────
(define
  er-proc-new!
  (fn
    (env)
    (let
      ((pid (er-pid-new!)))
      (let
        ((proc
          {:pid pid
            :env env
            :links (list)
            :mailbox (er-q-new)
            :state "runnable"
            :monitors (list)
            :continuation nil
            :receive-pats nil
            :trap-exit false
            :exit-reason nil}))
        (dict-set! (er-sched-processes) (er-pid-key pid) proc)
        (er-sched-enqueue! pid)
        proc))))

(define
  er-proc-get
  (fn (pid) (get (er-sched-processes) (er-pid-key pid))))

(define
  er-proc-exists?
  (fn (pid) (dict-has? (er-sched-processes) (er-pid-key pid))))

(define
  er-proc-field
  (fn (pid field) (get (er-proc-get pid) field)))

(define
  er-proc-set!
  (fn
    (pid field val)
    (let
      ((p (er-proc-get pid)))
      (if
        (= p nil)
        (error (str "Erlang: no such process " (er-pid-key pid)))
        (dict-set! p field val)))))

(define
  er-proc-mailbox-push!
  (fn (pid msg) (er-q-push! (er-proc-field pid :mailbox) msg)))

(define
  er-proc-mailbox-size
  (fn (pid) (er-q-len (er-proc-field pid :mailbox))))

;; ── process BIFs ────────────────────────────────────────────────
(define
  er-bif-is-pid
  (fn (vs) (er-bool (er-pid? (er-bif-arg1 vs "is_pid")))))

(define
  er-bif-self
  (fn
    (vs)
    (if
      (not (= (len vs) 0))
      (error "Erlang: self/0: arity")
      (let
        ((pid (er-sched-current-pid)))
        (if
          (= pid nil)
          (error "Erlang: self/0: no current process")
          pid)))))

(define
  er-bif-spawn
  (fn
    (vs)
    (cond
      (= (len vs) 1) (er-spawn-fun (nth vs 0))
      (= (len vs) 3) (error
        "Erlang: spawn/3: module-based spawn deferred to Phase 5 (modules)")
      :else (error "Erlang: spawn: wrong arity"))))

(define
  er-spawn-fun
  (fn
    (fv)
    (if
      (not (er-fun? fv))
      (error "Erlang: spawn/1: not a fun")
      (let
        ((proc (er-proc-new! (er-env-new))))
        (dict-set! proc :initial-fun fv)
        (get proc :pid)))))

;; ── scheduler loop ──────────────────────────────────────────────
;; Each process's entry runs inside a `reset`; `receive` uses `shift`
;; to suspend (saving a continuation on the proc record). When a `!`
;; delivers a message to a waiting process we re-enqueue it — the
;; scheduler step invokes the saved continuation, which retries the
;; receive against the updated mailbox.
(define er-suspend-marker {:tag "er-suspend-marker"})

(define
  er-suspended?
  (fn
    (v)
    (and
      (= (type-of v) "dict")
      (= (get v :tag) "er-suspend-marker"))))

(define
  er-sched-run-all!
  (fn
    ()
    (let
      ((pid (er-sched-next-runnable!)))
      (when
        (not (= pid nil))
        (er-sched-step! pid)
        (er-sched-run-all!)))))

(define
  er-sched-step!
  (fn
    (pid)
    (er-sched-set-current! pid)
    (er-proc-set! pid :state "running")
    (let
      ((prev-k (er-proc-field pid :continuation))
        (result-ref (list nil)))
      (if
        (= prev-k nil)
        (set-nth!
          result-ref
          0
          (reset (er-apply-fun (er-proc-field pid :initial-fun) (list))))
        (do
          (er-proc-set! pid :continuation nil)
          (set-nth! result-ref 0 (prev-k nil))))
      (let
        ((r (nth result-ref 0)))
        (cond
          (er-suspended? r) nil
          :else (do
            (er-proc-set! pid :state "dead")
            (er-proc-set! pid :exit-reason (er-mk-atom "normal"))
            (er-proc-set! pid :exit-result r)
            (er-proc-set! pid :continuation nil)))))
    (er-sched-set-current! nil)))