;; lib/host/otel.sx — OpenTelemetry in SX, P1: the span model + API.
;;
;; A TRACE is a COMPOSITION: a span is {:trace :span :parent :name :t0 :t1 :attrs
;; :events}, so a trace is a tree of spans (the same shape as an object's :body
;; composition). Later phases fold that tree into a waterfall (render-fold), OTLP
;; JSON (export-fold) and metrics (aggregate-fold). This phase gives us the model,
;; the dynamic parent stack that builds the tree, and a bounded in-memory ring
;; buffer — spans are cheap timed effects, NOT durable KV rows.

;; ── monotonic id + clock ─────────────────────────────────────────────
;; A simple process-monotonic counter gives collision-free ids without needing a
;; random source (Math.random/Date.now aren't available on this host).
(define otel/-id-counter 0)
(define otel/-next-id
  (fn (prefix)
    (begin
      (set! otel/-id-counter (+ otel/-id-counter 1))
      (str prefix otel/-id-counter))))
(define otel/gen-trace-id (fn () (otel/-next-id "trace-")))
(define otel/gen-span-id  (fn () (otel/-next-id "span-")))

;; now-ns — real epoch time in NANOSECONDS (the unit OTLP wants for
;; start/endTimeUnixNano). The OCaml host exposes `clock-milliseconds`
;; (Unix.gettimeofday, epoch ms); we scale by 1e6. Wall clocks can step
;; backwards (NTP), so we clamp against a high-water mark: now-ns never
;; decreases, keeping span durations non-negative even across a clock step.
(define otel/-last-ns 0)
(define otel/now-ns
  (fn ()
    (let ((raw (* (clock-milliseconds) 1000000)))
      (let ((t (if (> raw otel/-last-ns) raw otel/-last-ns)))
        (begin
          (set! otel/-last-ns t)
          t)))))

;; ── the dynamic parent stack ─────────────────────────────────────────
;; head = the innermost (current) span context {:span id :trace id}. Pushing on
;; with-span entry / popping on exit is what turns lexical nesting into parent
;; links, so a trace tree falls out of ordinary call nesting.
(define otel/-stack (list))
(define otel/-push! (fn (ctx) (set! otel/-stack (cons ctx otel/-stack))))
(define otel/-pop!  (fn () (set! otel/-stack (rest otel/-stack))))
(define otel/-top   (fn () (if (= (len otel/-stack) 0) nil (first otel/-stack))))

(define otel/current-span
  (fn () (let ((t (otel/-top))) (if t (get t :span) nil))))
(define otel/current-trace
  (fn () (let ((t (otel/-top))) (if t (get t :trace) nil))))

;; ── the bounded ring buffer ──────────────────────────────────────────
;; Oldest → newest. record! appends and drops from the front once over cap, so the
;; buffer stays O(cap) no matter how many spans flow through. NOT persisted.
(define otel/-cap 1000)
(define otel/-ring (list))
(define otel/set-cap! (fn (n) (begin (set! otel/-cap n) nil)))

(define otel/record!
  (fn (span)
    (begin
      (set! otel/-ring (append otel/-ring (list span)))
      (if (> (len otel/-ring) otel/-cap)
        (set! otel/-ring (slice otel/-ring (- (len otel/-ring) otel/-cap)))
        nil)
      span)))

(define otel/recent (fn () otel/-ring))

;; Clear the ring AND the parent stack — used between tests / requests.
(define otel/reset!
  (fn ()
    (begin
      (set! otel/-ring (list))
      (set! otel/-stack (list))
      nil)))

;; ── -timed: the shared timed-effect core ─────────────────────────────
;; Records a span around (thunk): a fresh span id, the trace inherited from the
;; enclosing span (or a new trace at the root), the enclosing span as :parent, and
;; t0/t1 straddling the call. Pushes its context so nested spans see it as parent,
;; pops after, then records the finished span. `finalize` maps the thunk's result
;; to EXTRA attrs merged onto `attrs` (e.g. an http.status only known post-call).
;; Returns the thunk's value unchanged.
(define otel/-timed
  (fn (name attrs finalize thunk)
    (let ((parent-ctx (otel/-top)))
      (let ((trace  (if parent-ctx (get parent-ctx :trace) (otel/gen-trace-id)))
            (span   (otel/gen-span-id))
            (parent (if parent-ctx (get parent-ctx :span) nil)))
        (let ((t0 (begin (otel/-push! {:span span :trace trace}) (otel/now-ns))))
          (let ((result (thunk)))
            (let ((t1 (otel/now-ns)))
              (begin
                (otel/-pop!)
                (otel/record!
                  {:trace trace :span span :parent parent :name name
                   :t0 t0 :t1 t1
                   :attrs (merge attrs (finalize result))
                   :events (list)})
                result))))))))

;; with-span — a plain timed span; no result-derived attrs.
(define otel/with-span
  (fn (name attrs thunk)
    (otel/-timed name attrs (fn (r) {}) thunk)))

;; ── auto-instrument HTTP handlers ─────────────────────────────────────
;; A trace is a composition; a request handler is a timed effect. Wrap one Dream
;; route's handler so dispatching it records a root span "METHOD /route" with
;; {:http.method :http.route :http.status}. The status is only known after the
;; handler runs, so it's a finalize attr; the result is coerced the same way the
;; router coerces it, so a bare-string handler still reports 200. The raw handler
;; result is returned untouched so dr/run-route's own coercion is unchanged.
(define otel/instrument-route
  (fn (r)
    (let ((method  (dream-route-method r))
          (path    (dream-route-path r))
          (handler (dream-route-handler r)))
      (dream-route method path
        (fn (req)
          (otel/-timed
            (str method " " path)
            {:http.method method :http.route path}
            (fn (resp) {:http.status (dream-status (dream-coerce-response resp))})
            (fn () (handler req))))))))

;; Flatten nested route groups and instrument each route — the host/make-app seam.
(define otel/instrument-routes
  (fn (routes) (map otel/instrument-route (dr/flatten-routes routes))))