;; 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))))
          ;; guard the body: on a raise, pop + record an ERROR span (status +
          ;; exception event) as a SIDE EFFECT in a clause test that returns false,
          ;; so the condition auto-propagates to the outer handler. (An explicit
          ;; `(raise e)` inside a guard handler re-enters this guard and hangs.)
          (guard (e ((begin
                       (let ((t1 (otel/now-ns)))
                         (begin
                           (otel/-pop!)
                           (otel/record!
                             {:trace trace :span span :parent parent :name name
                              :t0 t0 :t1 t1 :status "error" :attrs attrs
                              :events (list {:name "exception" :time t1 :message (str e)})})))
                       false)
                     nil))
            (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 :status "ok"
                     :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))))

;; ── P4: render-fold → SVG waterfall ───────────────────────────────────
;; A trace is a tree of spans; a waterfall is a fold over that tree into SVG (the
;; same discipline as the compose render-fold: walk the structure, emit per node).
;; Layout: x ∝ (t0 − trace.t0), width ∝ duration, y ∝ depth (ancestor count via
;; parent links). Geometry is computed as plain rect data first (otel/waterfall-
;; rects, testable), then folded into <svg> markup (otel/waterfall).
(define otel/-row-h 18)
(define otel/-svg-w 640)
(define otel/-pad 4)

;; all spans belonging to one trace, from the ring.
(define otel/trace-spans
  (fn (trace-id) (filter (fn (s) (= (get s :trace) trace-id)) (otel/recent))))

(define otel/-span-by-id
  (fn (spans id) (first (filter (fn (s) (= (get s :span) id)) spans))))

;; depth = number of ancestors reachable via :parent within this trace's spans.
(define otel/-depth
  (fn (s spans)
    (if (nil? (get s :parent))
      0
      (let ((p (otel/-span-by-id spans (get s :parent))))
        (if (nil? p) 0 (+ 1 (otel/-depth p spans)))))))

;; trace start = earliest t0; end = latest t1 (folds over the spans).
(define otel/-min-t0
  (fn (spans) (reduce (fn (m s) (min m (get s :t0))) (get (first spans) :t0) spans)))
(define otel/-max-t1
  (fn (spans) (reduce (fn (m s) (max m (get s :t1))) (get (first spans) :t1) spans)))

;; a rect per span: {:span :name :depth :x :y :w}. x/width are scaled to the
;; drawable width; a zero-duration span still gets a 1px sliver so it's visible.
(define otel/waterfall-rects
  (fn (trace-id)
    (let ((spans (otel/trace-spans trace-id)))
      (if (empty? spans)
        (list)
        (let ((t0 (otel/-min-t0 spans))
              (t1 (otel/-max-t1 spans))
              (iw (- otel/-svg-w (* 2 otel/-pad))))
          (let ((total (if (> (- t1 t0) 0) (- t1 t0) 1)))
            (map
              (fn (s)
                (let ((d (otel/-depth s spans))
                      (dur (- (get s :t1) (get s :t0))))
                  (let ((raw-w (* (/ dur total) iw)))
                    {:span (get s :span)
                     :name (get s :name)
                     :depth d
                     :dur dur
                     :status (get s :status)
                     :x (+ otel/-pad (* (/ (- (get s :t0) t0) total) iw))
                     :y (+ otel/-pad (* d otel/-row-h))
                     :w (if (> raw-w 1) raw-w 1)})))
              spans)))))))

;; ns → a compact "Nms" / "Nus" label.
(define otel/-ms
  (fn (ns)
    (if (>= ns 1000000)
      (str (quotient ns 1000000) "ms")
      (str (quotient ns 1000) "us"))))

;; error spans render red, everything else teal.
(define otel/-bar-fill
  (fn (status) (if (= status "error") "#e45756" "#4c9a8f")))

;; one <g> (bar + name + duration) per rect. The bar is the only <rect>, so
;; count(rect) == count(span); axis chrome uses <line>/<text>.
(define otel/-rect->g
  (fn (r)
    (quasiquote
      (g
        (rect :x (unquote (get r :x)) :y (unquote (get r :y))
              :width (unquote (get r :w)) :height (unquote (- otel/-row-h 2))
              :fill (unquote (otel/-bar-fill (get r :status))) :rx 2)
        (text :x (unquote (+ (get r :x) 3)) :y (unquote (+ (get r :y) 12))
              :font-size 10 :fill "#ffffff"
              (unquote (str (get r :name) "  " (otel/-ms (get r :dur)))))))))

;; max depth across rects (0 when empty) → svg height.
(define otel/-max-depth
  (fn (rects) (reduce (fn (m r) (max m (get r :depth))) 0 rects)))

;; the trace as an inline <svg> waterfall.
(define otel/waterfall
  (fn (trace-id)
    (let ((rects (otel/waterfall-rects trace-id)))
      (let ((h (+ (* 2 otel/-pad) (* (+ (otel/-max-depth rects) 1) otel/-row-h))))
        (quasiquote
          (svg :width (unquote otel/-svg-w) :height (unquote h)
               :xmlns "http://www.w3.org/2000/svg"
               (splice-unquote (map otel/-rect->g rects))))))))

;; count nodes whose head symbol prints as `head` — a small SVG-tree assertion aid.
(define otel/-tree-count
  (fn (tree head)
    (if (list? tree)
      (let ((self (if (and (not (empty? tree)) (= (str (first tree)) head)) 1 0)))
        (reduce (fn (acc n) (+ acc (otel/-tree-count n head))) self tree))
      0)))

;; ── P5: metrics (aggregate-fold) ──────────────────────────────────────
;; Fold the recent spans into per-route request counts + a latency histogram
;; (p50/p95/p99 from durations). No `sort` primitive here, so percentiles ride a
;; tiny insertion sort; nearest-rank keeps the maths exact for the tests.
(define otel/-insert
  (fn (x sorted)
    (if (empty? sorted)
      (list x)
      (if (<= x (first sorted))
        (cons x sorted)
        (cons (first sorted) (otel/-insert x (rest sorted)))))))
(define otel/-sort-nums
  (fn (lst) (reduce (fn (acc x) (otel/-insert x acc)) (list) lst)))

;; nearest-rank percentile of an ASCENDING list: rank = ceil(p/100 · N), 1-based.
(define otel/-percentile
  (fn (sorted p)
    (if (empty? sorted)
      0
      (let ((n (len sorted)))
        (let ((idx (- (ceil (* (/ p 100) n)) 1)))
          (let ((i (if (< idx 0) 0 (if (>= idx n) (- n 1) idx))))
            (nth sorted i)))))))

;; a span's route label: the http.route attr, else the span name.
(define otel/-span-route
  (fn (s) (or (get (get s :attrs) :http.route) (get s :name))))

(define otel/-span-dur (fn (s) (- (get s :t1) (get s :t0))))

;; distinct values, order-preserving.
(define otel/-distinct
  (fn (lst)
    (reduce
      (fn (acc x) (if (some (fn (y) (= y x)) acc) acc (append acc (list x))))
      (list)
      lst)))

;; the aggregate for one route: count + latency percentiles over its durations.
(define otel/route-metrics
  (fn (spans route)
    (let ((rs (filter (fn (s) (= (otel/-span-route s) route)) spans)))
      (let ((durs (otel/-sort-nums (map otel/-span-dur rs))))
        {:route route
         :count (len rs)
         :p50 (otel/-percentile durs 50)
         :p95 (otel/-percentile durs 95)
         :p99 (otel/-percentile durs 99)}))))

;; fold spans → {:total-requests N :routes (per-route metrics …)}.
(define otel/metrics
  (fn (spans)
    (let ((routes (otel/-distinct (map otel/-span-route spans))))
      {:total-requests (len spans)
       :routes (map (fn (r) (otel/route-metrics spans r)) routes)})))

;; convenience: metrics over the current ring.
(define otel/metrics-recent (fn () (otel/metrics (otel/recent))))

;; ── P6: live dashboard (SSR + SSE) ────────────────────────────────────
;; GET /otel renders a dashboard — metrics strip + the latest trace's waterfall +
;; a recent-traces list — as server-rendered HTML carrying Datastar-style reactive
;; attributes that subscribe to GET /otel/stream, the SSE feed of new span events
;; (SXTP events, the host's Datastar-borrowed wire format). SSR + declarative
;; reactive attrs + SSE patches IS the reactive-island model here. (Live client
;; hydration is a deploy concern; SSR, the event feed, and the data are tested.)

;; recent traces, newest-first: {:trace :name :spans}. root name = the parentless
;; span (fallback: first recorded).
(define otel/-trace-ids
  (fn () (otel/-distinct (map (fn (s) (get s :trace)) (otel/recent)))))
(define otel/-trace-root-name
  (fn (trace-id)
    (let ((spans (otel/trace-spans trace-id)))
      (let ((roots (filter (fn (s) (nil? (get s :parent))) spans)))
        (cond
          ((not (empty? roots)) (get (first roots) :name))
          ((not (empty? spans)) (get (first spans) :name))
          (else ""))))))
(define otel/trace-summary
  (fn (trace-id)
    {:trace trace-id
     :name (otel/-trace-root-name trace-id)
     :spans (len (otel/trace-spans trace-id))}))
(define otel/recent-traces
  (fn () (reverse (map otel/trace-summary (otel/-trace-ids)))))
(define otel/latest-trace
  (fn ()
    (let ((r (otel/recent)))
      (if (empty? r) nil (get (last r) :trace)))))

;; ── SSE span events (SXTP) ────────────────────────────────────────────
(define otel/span-event
  (fn (s)
    (sxtp/event "otel.span"
      {:id (get s :span)
       :body {:trace (get s :trace) :span (get s :span) :parent (get s :parent)
              :name (get s :name) :t0 (get s :t0) :t1 (get s :t1)
              :attrs (get s :attrs)}
       :time (get s :t1)})))
(define otel/latest-span-event
  (fn ()
    (let ((r (otel/recent)))
      (if (empty? r) nil (otel/span-event (last r))))))
;; one SSE frame: `event: otel.span\n data: <sxtp event>\n\n`; "" when no spans.
(define otel/-stream-body
  (fn ()
    (let ((e (otel/latest-span-event)))
      (if (nil? e)
        ""
        (str "event: otel.span\ndata: " (sxtp/serialize e) "\n\n")))))

;; ── dashboard markup (plain HTML tags → render-to-html SSRs cleanly) ──
(define otel/-metrics-strip
  (fn (m)
    (quasiquote
      (table :class "otel-metrics"
        (tr (th "route") (th "count") (th "p50") (th "p95") (th "p99"))
        (splice-unquote
          (map
            (fn (r)
              (quasiquote
                (tr (td (unquote (str (get r :route))))
                    (td (unquote (str (get r :count))))
                    (td (unquote (str (get r :p50))))
                    (td (unquote (str (get r :p95))))
                    (td (unquote (str (get r :p99)))))))
            (get m :routes)))))))

(define otel/-traces-list
  (fn (traces)
    (quasiquote
      (ul :class "otel-traces"
        (splice-unquote
          (map
            (fn (t)
              (quasiquote
                (li :data-trace (unquote (str (get t :trace)))
                  (unquote (str (get t :name) " — " (get t :spans) " spans")))))
            traces))))))

;; a per-route latency bar chart: each row is a nested bar — teal p50 inside amber
;; p95 inside light-red p99 — so the tail (red beyond amber) is visible at a glance.
(define otel/-latency-chart
  (fn (m)
    (let ((routes (get m :routes)))
      (if (empty? routes)
        (quasiquote (p :class "otel-empty" "no metrics yet"))
        (let ((maxp99 (reduce (fn (mx r) (max mx (get r :p99))) 1 routes))
              (label-w 150)
              (bar-w 340)
              (row-h 24))
          (quasiquote
            (svg :width 560 :height (unquote (+ (* (len routes) row-h) 6))
                 :xmlns "http://www.w3.org/2000/svg" :font-family "monospace"
              (splice-unquote
                (map-indexed
                  (fn (i r)
                    (let ((y (* i row-h)))
                      (let ((p99w (max 1 (* (/ (get r :p99) maxp99) bar-w)))
                            (p95w (max 1 (* (/ (get r :p95) maxp99) bar-w)))
                            (p50w (max 1 (* (/ (get r :p50) maxp99) bar-w))))
                        (quasiquote
                          (g
                            (text :x 0 :y (unquote (+ y 15)) :font-size 11 :fill "#333"
                                  (unquote (get r :route)))
                            (rect :x (unquote label-w) :y (unquote (+ y 5)) :width (unquote p99w)
                                  :height 14 :fill "#f2c2c2" :rx 3)
                            (rect :x (unquote label-w) :y (unquote (+ y 5)) :width (unquote p95w)
                                  :height 14 :fill "#f2a63b" :rx 3)
                            (rect :x (unquote label-w) :y (unquote (+ y 5)) :width (unquote p50w)
                                  :height 14 :fill "#4c9a8f" :rx 3)
                            (text :x (unquote (+ label-w p99w 5)) :y (unquote (+ y 15))
                                  :font-size 9 :fill "#999"
                                  (unquote (str "p99 " (otel/-ms (get r :p99))))))))))
                  routes)))))))))

(define otel/dashboard
  (fn ()
    (let ((m (otel/metrics (otel/recent)))
          (lt (otel/latest-trace))
          (traces (otel/recent-traces)))
      (quasiquote
        (div :id "otel-dashboard"
          ;; The host serves single-body responses (no server-push SSE), so the
          ;; dashboard stays live by reloading itself — every 3s it re-renders the
          ;; latest metrics + trace. /otel/stream remains a snapshot of the newest
          ;; span for any client that wants to poll it.
          (meta :http-equiv "refresh" :content "3")
          (h1 "OpenTelemetry")
          (p :style "font-size:0.8em;opacity:0.7" "live · auto-refreshes every 3s")
          (h2 "latency by route")
          (p :style "font-size:0.75em;opacity:0.7"
            (span :style "color:#4c9a8f" "▉ p50") "  "
            (span :style "color:#f2a63b" "▉ p95") "  "
            (span :style "color:#f2c2c2" "▉ p99 (tail)"))
          (unquote (otel/-latency-chart m))
          (h2 "metrics")
          (unquote (otel/-metrics-strip m))
          (h2 "latest trace")
          (unquote
            (if (nil? lt)
              (quasiquote (p :class "otel-empty" "no traces yet"))
              (otel/waterfall lt)))
          (h2 "recent traces")
          (unquote (otel/-traces-list traces)))))))

;; ── routes ────────────────────────────────────────────────────────────
(define otel/dashboard-route
  (dream-get "/otel"
    (fn (req) (dream-html (render-to-html (otel/dashboard) {})))))
(define otel/stream-route
  (dream-get "/otel/stream"
    (fn (req)
      (dream-response 200 {:content-type "text/event-stream"} (otel/-stream-body)))))
(define otel/routes (list otel/dashboard-route otel/stream-route))

;; ── P7: OTLP-JSON export ──────────────────────────────────────────────
;; Serialize spans to the OTLP/JSON schema (resourceSpans → scopeSpans → spans)
;; for interop with Jaeger/Grafana. traceId/spanId/parentSpanId are hex strings;
;; timestamps are uint64-as-string; attributes are typed {key,value}. Export is a
;; fold (span → OTLP span); POST goes through an INJECTED transport so it's
;; testable without a live collector.

;; our "trace-N"/"span-N" ids → a fixed-width lowercase-hex id (real OTLP widths:
;; traceId 16 bytes = 32 hex, spanId 8 bytes = 16 hex). Deterministic from the id.
(define otel/-id-num
  (fn (id)
    (let ((n (string->number (last (split id "-")))))
      (if (nil? n) 0 n))))
(define otel/-zeros
  (fn (k) (if (<= k 0) "" (str "0" (otel/-zeros (- k 1))))))
(define otel/-pad-hex
  (fn (n width)
    (let ((h (number->string n 16)))
      (str (otel/-zeros (- width (len h))) h))))
(define otel/-trace-hex (fn (id) (otel/-pad-hex (otel/-id-num id) 32)))
(define otel/-span-hex  (fn (id) (otel/-pad-hex (otel/-id-num id) 16)))

;; a value → OTLP AnyValue: numbers are intValue (uint64-as-string), else stringValue.
(define otel/-otlp-value
  (fn (v)
    (if (= (type-of v) "number")
      {:intValue (str v)}
      {:stringValue (str v)})))
(define otel/-otlp-attrs
  (fn (attrs)
    (map (fn (k) {:key (str k) :value (otel/-otlp-value (get attrs k))}) (keys attrs))))

;; span kind: 2 = SERVER for an instrumented request (has http.method), else
;; 1 = INTERNAL.
(define otel/-otlp-kind
  (fn (attrs) (if (get attrs :http.method) 2 1)))

(define otel/-otlp-span
  (fn (s)
    (let ((attrs (or (get s :attrs) {})))
      (let ((base
              {:traceId (otel/-trace-hex (get s :trace))
               :spanId (otel/-span-hex (get s :span))
               :name (get s :name)
               :kind (otel/-otlp-kind attrs)
               :startTimeUnixNano (str (get s :t0))
               :endTimeUnixNano (str (get s :t1))
               :attributes (otel/-otlp-attrs attrs)}))
        (if (nil? (get s :parent))
          base
          (assoc base :parentSpanId (otel/-span-hex (get s :parent))))))))

;; spans → the OTLP export envelope (a JSON-shaped SX value).
(define otel/export-otlp
  (fn (spans)
    {:resourceSpans
      (list
        {:resource
          {:attributes (list {:key "service.name"
                              :value {:stringValue "rose-ash-host"}})}
         :scopeSpans
          (list
            {:scope {:name "otel-on-sx" :version "0.1.0"}
             :spans (map otel/-otlp-span spans)})})}))

(define otel/export-otlp-json
  (fn (spans) (dream-json-encode (otel/export-otlp spans))))

;; POST the OTLP payload through an injected transport. `transport` is a fn taking
;; a request dict {:method :url :headers :body} — real deploys pass an http POST;
;; tests pass a recorder. Returns whatever the transport returns.
(define otel/post-otlp
  (fn (endpoint spans transport)
    (transport
      {:method "POST"
       :url endpoint
       :headers {:content-type "application/json"}
       :body (otel/export-otlp-json spans)})))

;; ── P8: W3C traceparent context propagation ───────────────────────────
;; traceparent = "<version>-<trace-id>-<parent-id>-<flags>" (RFC: version 00,
;; trace-id 32 hex, parent-id 16 hex, flags 2 hex; low bit = sampled). Emitting it
;; on outbound inter-service calls and parsing it on inbound requests lets ONE
;; trace span multiple services. Our internal ids hex-encode via the OTLP helpers.
(define otel/format-traceparent
  (fn (trace-id span-id)
    (str "00-" (otel/-trace-hex trace-id) "-" (otel/-span-hex span-id) "-01")))

;; the traceparent for the CURRENT span (nil outside any span) — for outbound calls.
(define otel/current-traceparent
  (fn ()
    (let ((t (otel/-top)))
      (if (nil? t) nil (otel/format-traceparent (get t :trace) (get t :span))))))

;; parse an inbound traceparent → {:version :trace-id :parent-id :flags :sampled};
;; nil if absent or malformed (wrong field count / hex widths).
(define otel/parse-traceparent
  (fn (header)
    (if (or (nil? header) (= header ""))
      nil
      (let ((parts (split header "-")))
        (if (= (len parts) 4)
          (let ((ver (nth parts 0)) (tid (nth parts 1))
                (pid (nth parts 2)) (flags (nth parts 3)))
            (if (and (= (len tid) 32) (= (len pid) 16))
              {:version ver :trace-id tid :parent-id pid :flags flags
               :sampled (not (= flags "00"))}
              nil))
          nil)))))