events: waitlist + auto-promotion + 21 tests

When full, ev/waitlist! queues actors FIFO (:waitlist/:unwaitlist on the
booking stream; waiting fold independent of the seat fold). ev/waitlist,
ev/waitlist-position, ev/leave-waitlist!. ev/cancel-promote! frees a seat and
auto-promotes the head of the queue to a confirmed booking. Idempotent.
240/240 green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/events/booking.sx

@@ -40,6 +40,9 @@
       ((= (first xs) x) i)
       (else (ev-bk-index (rest xs) x (+ i 1))))))
 
+(define ev-bk-append (fn (xs a) (append xs (list a))))
+(define ev-bk-remove (fn (xs a) (filter (fn (x) (not (= x a))) xs)))
+
 ;; ---- per-actor state association list: ((actor state) ...) in join order ----
 
 (define
@@ -269,3 +272,101 @@
   (fn
     (b occ-key capacity)
     (max 0 (- capacity (ev-booking-count b occ-key)))))
+
+;; ---- waitlist ----
+;; When an occurrence is full, actors join a FIFO waitlist (:waitlist /
+;; :unwaitlist events on the same stream). Taking a seat (:booking / :hold)
+;; removes an actor from the queue, so the waitlist fold is independent of the
+;; seat fold. Cancelling/releasing a seat can auto-promote the head of the
+;; queue (a :booking appended for them).
+
+(define
+  ev-fold-waiting
+  (fn
+    (events)
+    (reduce
+      (fn
+        (acc e)
+        (let
+          ((typ (persist/event-type e))
+            (actor (get (persist/event-data e) :actor)))
+          (cond
+            ((= typ :waitlist) (if (ev-bk-member? actor acc) acc (ev-bk-append acc actor)))
+            ((= typ :unwaitlist) (ev-bk-remove acc actor))
+            ((= typ :booking) (ev-bk-remove acc actor))
+            ((= typ :hold) (ev-bk-remove acc actor))
+            (else acc))))
+      (list)
+      events)))
+
+;; The current waitlist queue (FIFO, oldest first).
+(define
+  ev/waitlist
+  (fn (b occ-key) (ev-fold-waiting (persist/read b (ev-booking-stream occ-key)))))
+
+;; 1-based queue position for an actor (0 if not waiting).
+(define
+  ev/waitlist-position
+  (fn (b occ-key actor) (ev-seat-of (ev/waitlist b occ-key) actor)))
+
+;; Book if a seat is free, else join the waitlist. Idempotent: already seated →
+;; :already; already queued → :already-waiting.
+(define
+  ev/waitlist!
+  (fn
+    (b occ-key capacity actor)
+    (let
+      ((seats (ev-booked-actors b occ-key))
+        (waiting (ev/waitlist b occ-key)))
+      (cond
+        ((ev-bk-member? actor seats)
+          {:status :already :seat (ev-seat-of seats actor) :actor actor})
+        ((ev-bk-member? actor waiting)
+          {:status :already-waiting :position (ev-seat-of waiting actor) :actor actor})
+        (else
+          (let
+            ((r (ev/book! b occ-key capacity actor)))
+            (if
+              (= (get r :status) :booked)
+              r
+              (begin
+                (persist/append b (ev-booking-stream occ-key) :waitlist 0 {:actor actor})
+                {:status :waitlisted
+                  :position (+ (len waiting) 1)
+                  :actor actor}))))))))
+
+;; Leave the waitlist. :left or :not-waiting.
+(define
+  ev/leave-waitlist!
+  (fn
+    (b occ-key actor)
+    (if
+      (ev-bk-member? actor (ev/waitlist b occ-key))
+      (begin
+        (persist/append b (ev-booking-stream occ-key) :unwaitlist 0 {:actor actor})
+        {:status :left :actor actor})
+      {:status :not-waiting :actor actor})))
+
+;; Cancel a seat and, if that frees capacity, auto-promote the head of the
+;; waitlist (a confirmed booking). Returns the cancel result plus :promoted
+;; (the actor promoted, or nil).
+(define
+  ev/cancel-promote!
+  (fn
+    (b occ-key capacity actor)
+    (let
+      ((c (ev/cancel! b occ-key actor)))
+      (if
+        (= (get c :status) :cancelled)
+        (let
+          ((waiting (ev/waitlist b occ-key))
+            (seats (ev-booked-actors b occ-key)))
+          (if
+            (and (not (empty? waiting)) (< (len seats) capacity))
+            (let
+              ((promoted (first waiting)))
+              (begin
+                (persist/append b (ev-booking-stream occ-key) :booking 0 {:actor promoted})
+                {:status :cancelled :actor actor :promoted promoted}))
+            {:status :cancelled :actor actor :promoted nil}))
+        c))))

lib/events/scoreboard.json

@@ -1,17 +1,17 @@
 {
   "lang": "events",
-  "total_passed": 219,
+  "total_passed": 240,
   "total_failed": 0,
-  "total": 219,
+  "total": 240,
   "suites": [
     {"name":"calendar","passed":37,"failed":0,"total":37},
     {"name":"availability","passed":22,"failed":0,"total":22},
     {"name":"api","passed":24,"failed":0,"total":24},
-    {"name":"booking","passed":61,"failed":0,"total":61},
+    {"name":"booking","passed":82,"failed":0,"total":82},
     {"name":"ticket","passed":31,"failed":0,"total":31},
     {"name":"notify","passed":7,"failed":0,"total":7},
     {"name":"reminders","passed":14,"failed":0,"total":14},
     {"name":"federation","passed":23,"failed":0,"total":23}
   ],
-  "generated": "2026-06-07T05:31:56+00:00"
+  "generated": "2026-06-07T05:59:03+00:00"
 }

lib/events/scoreboard.md

@@ -1,13 +1,13 @@
 # events scoreboard
 
-**219 / 219 passing** (0 failure(s)).
+**240 / 240 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
 | calendar | 37 | 37 | ok |
 | availability | 22 | 22 | ok |
 | api | 24 | 24 | ok |
-| booking | 61 | 61 | ok |
+| booking | 82 | 82 | ok |
 | ticket | 31 | 31 | ok |
 | notify | 7 | 7 | ok |
 | reminders | 14 | 14 | ok |

lib/events/tests/booking.sx

@@ -359,6 +359,65 @@
             (ev-booking-count b "hi")
             1))))))
 
+;; ---- waitlist ----
+(define
+  ev-bk-wl-run-all!
+  (fn
+    ()
+    (do
+      ;; join the waitlist when full; book directly when a seat is free
+      (let
+        ((b (persist/open)))
+        (do
+          (ev-bk-check! "waitlist! books when a seat is free" (get (ev/waitlist! b "w" 2 (quote a)) :status) :booked)
+          (ev-bk-check! "second booking still fits" (get (ev/waitlist! b "w" 2 (quote c)) :status) :booked)
+          (ev-bk-check! "third joins the waitlist when full" (get (ev/waitlist! b "w" 2 (quote x)) :status) :waitlisted)
+          (ev-bk-check! "fourth is next in line" (get (ev/waitlist! b "w" 2 (quote y)) :position) 2)
+          (ev-bk-check! "waitlist is FIFO" (ev/waitlist b "w") (list (quote x) (quote y)))
+          (ev-bk-check! "seats unaffected by waitlisting" (ev/roster b "w") (list (quote a) (quote c)))
+          (ev-bk-check! "waitlist-position reports a queued actor" (ev/waitlist-position b "w" (quote y)) 2)
+          (ev-bk-check! "waitlist-position 0 for a seated actor" (ev/waitlist-position b "w" (quote a)) 0)))
+      ;; idempotency
+      (let
+        ((b (persist/open)))
+        (do
+          (ev/waitlist! b "wi" 1 (quote a))
+          (ev/waitlist! b "wi" 1 (quote x))
+          (ev-bk-check! "re-joining as a seated actor is :already" (get (ev/waitlist! b "wi" 1 (quote a)) :status) :already)
+          (ev-bk-check! "re-joining the queue is :already-waiting" (get (ev/waitlist! b "wi" 1 (quote x)) :status) :already-waiting)
+          (ev-bk-check! "queue did not grow on re-join" (ev/waitlist b "wi") (list (quote x)))))
+      ;; leaving the waitlist
+      (let
+        ((b (persist/open)))
+        (do
+          (ev/waitlist! b "wl" 1 (quote a))
+          (ev/waitlist! b "wl" 1 (quote x))
+          (ev/waitlist! b "wl" 1 (quote y))
+          (ev-bk-check! "leave-waitlist reports left" (get (ev/leave-waitlist! b "wl" (quote x)) :status) :left)
+          (ev-bk-check! "leaving removes from the queue" (ev/waitlist b "wl") (list (quote y)))
+          (ev-bk-check! "leaving when not queued is not-waiting" (get (ev/leave-waitlist! b "wl" (quote z)) :status) :not-waiting)))
+      ;; auto-promotion on cancel
+      (let
+        ((b (persist/open)))
+        (do
+          (ev/waitlist! b "wp" 1 (quote a))
+          (ev/waitlist! b "wp" 1 (quote x))
+          (ev/waitlist! b "wp" 1 (quote y))
+          (let
+            ((r (ev/cancel-promote! b "wp" 1 (quote a))))
+            (do
+              (ev-bk-check! "cancel-promote cancels the seat holder" (get r :status) :cancelled)
+              (ev-bk-check! "cancel-promote promotes the head of the queue" (get r :promoted) (quote x))))
+          (ev-bk-check! "promoted actor now holds the seat" (ev/roster b "wp") (list (quote x)))
+          (ev-bk-check! "promoted actor left the queue" (ev/waitlist b "wp") (list (quote y)))
+          (ev-bk-check! "promoted seat is confirmed" (ev/seat-state b "wp" (quote x)) :confirmed)
+          ;; cancelling with an empty waitlist promotes nobody
+          (ev/leave-waitlist! b "wp" (quote y))
+          (let
+            ((r2 (ev/cancel-promote! b "wp" 1 (quote x))))
+            (ev-bk-check! "cancel with empty waitlist promotes nobody" (get r2 :promoted) nil))
+          (ev-bk-check! "seat is free after the last cancel" (ev/seats-left b "wp" 1) 1))))))
+
 (define
   ev-booking-tests-run!
   (fn
@@ -368,4 +427,5 @@
       (set! ev-bk-fail 0)
       (set! ev-bk-failures (list))
       (ev-bk-run-all!)
+      (ev-bk-wl-run-all!)
       {:failures ev-bk-failures :total (+ ev-bk-pass ev-bk-fail) :passed ev-bk-pass :failed ev-bk-fail})))

plans/events-on-sx.md

@@ -18,7 +18,7 @@ capacity rules, transactional booking, and a flow-driven notification dispatcher
 
 ## Status (rolling)
 
-`bash lib/events/conformance.sh` → **219/219** (Phases 1-4 + ext: federated free/busy)
+`bash lib/events/conformance.sh` → **240/240** (Phases 1-4 + ext: federated free/busy, waitlist)
 
 ## Ground rules
 
@@ -86,6 +86,13 @@ lib/events/api.sx ── (events/schedule) (events/book) (events/agenda) ──
 
 ## Progress log
 
+- 2026-06-07 — Waitlist + auto-promotion (extension). When an occurrence is
+  full, `ev/waitlist!` queues actors FIFO (:waitlist/:unwaitlist events on the
+  same stream; waiting fold is independent of the seat fold since taking a seat
+  removes from the queue). `ev/waitlist` (queue), `ev/waitlist-position`,
+  `ev/leave-waitlist!`. `ev/cancel-promote!` cancels a seat and auto-promotes
+  the head of the queue to a confirmed booking when capacity opens. Idempotent
+  (:already / :already-waiting). +21 tests, 240/240 green.
 - 2026-06-07 — Federated free/busy (extension). Peers publish BUSY intervals
   per actor (iCal free/busy model — privacy-preserving, not event details).
   `ev/peer-with-busy`, `ev/peer-busy`; `ev/federated-busy` unions local