events: next-free slot search + 6 tests

ev-next-free finds the earliest free slot >= after for a duration within a
horizon, probing 'after' + busy-interval ends via the busy_in rule (ev-free?).
Finds gaps, skips too-short gaps, half-open at edges. 59/59 green.

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

lib/events/availability.sx

@@ -16,7 +16,7 @@
 ;; adjacent slots (E == next start) do NOT conflict. Conflict pairs are
 ;; canonical (O1 < O2 by key) so each overlap is reported once. The same `busy`
 ;; rule answers "is A free in [QS,QE)?" (busy_in is empty) and feeds "when is A
-;; next free?" — same rules, different bindings.
+;; next free?" (ev-next-free probes candidate slots with the same rule).
 
 ;; A stable key for an occurrence dict {:id :start :end}.
 (define ev-occ-key (fn (occ) (str (get occ :id) "@" (get occ :start))))
@@ -98,6 +98,16 @@
   ev-sort-lists
   (fn (xs) (reduce (fn (acc x) (ev-list-insert x acc)) (list) xs)))
 
+(define
+  ev-dedup-sorted
+  (fn
+    (xs)
+    (cond
+      ((empty? xs) xs)
+      ((empty? (rest xs)) xs)
+      ((= (first xs) (first (rest xs))) (ev-dedup-sorted (rest xs)))
+      (else (cons (first xs) (ev-dedup-sorted (rest xs)))))))
+
 ;; All busy intervals (list S E) for an actor, ascending by start.
 (define
   ev-busy
@@ -129,3 +139,39 @@
       (let
         ((rows (dl-query db (list (quote busy_in) actor (quote QS) (quote QE)))))
         (begin (dl-retract! db (ev-qwindow-fact qs qe)) (empty? rows))))))
+
+;; ---- next-free slot search ----
+;; The earliest start s >= `after` such that [s, s+duration) is entirely free
+;; for `actor` and ends at or before `horizon`, or nil if none. The earliest
+;; such slot must begin either at `after` or immediately after some busy
+;; interval ends (classic interval packing), so those are the only candidates
+;; we probe — each probe reuses the busy_in rule via ev-free?.
+
+(define
+  ev-first-free
+  (fn
+    (db actor cands duration horizon)
+    (cond
+      ((empty? cands) nil)
+      (else
+        (let
+          ((s (first cands)))
+          (if
+            (and
+              (<= (+ s duration) horizon)
+              (ev-free? db actor s (+ s duration)))
+            s
+            (ev-first-free db actor (rest cands) duration horizon)))))))
+
+(define
+  ev-next-free
+  (fn
+    (db actor after duration horizon)
+    (let
+      ((ends (filter (fn (e) (>= e after)) (map (fn (iv) (first (rest iv))) (ev-busy db actor)))))
+      (ev-first-free
+        db
+        actor
+        (ev-dedup-sorted (sort (cons after ends)))
+        duration
+        horizon))))

lib/events/scoreboard.json

@@ -1,11 +1,11 @@
 {
   "lang": "events",
-  "total_passed": 53,
+  "total_passed": 59,
   "total_failed": 0,
-  "total": 53,
+  "total": 59,
   "suites": [
     {"name":"calendar","passed":37,"failed":0,"total":37},
-    {"name":"availability","passed":16,"failed":0,"total":16}
+    {"name":"availability","passed":22,"failed":0,"total":22}
   ],
-  "generated": "2026-06-07T00:21:06+00:00"
+  "generated": "2026-06-07T00:49:23+00:00"
 }

lib/events/scoreboard.md

@@ -1,8 +1,8 @@
 # events scoreboard
 
-**53 / 53 passing** (0 failure(s)).
+**59 / 59 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
 | calendar | 37 | 37 | ok |
-| availability | 16 | 16 | ok |
+| availability | 22 | 22 | ok |

lib/events/tests/availability.sx

@@ -64,6 +64,33 @@
             (quote lunch)
             (ev-dt 2026 6 1 12 0)))))))
 
+;; Disjoint fixture for slot search: 09:00–10:00 then 10:30–11:30 (a 30m gap).
+(define
+  ev-av-gap-db
+  (fn
+    ()
+    (ev-avail-db
+      (list
+        (ev-occ
+          (quote a)
+          (ev-dt 2026 6 1 9 0)
+          60)
+        (ev-occ
+          (quote b)
+          (ev-dt 2026 6 1 10 30)
+          60))
+      (list
+        (list
+          (quote sam)
+          (ev-av-key
+            (quote a)
+            (ev-dt 2026 6 1 9 0)))
+        (list
+          (quote sam)
+          (ev-av-key
+            (quote b)
+            (ev-dt 2026 6 1 10 30)))))))
+
 (define
   ev-av-run-all!
   (fn
@@ -203,6 +230,63 @@
             (list
               (ev-dt 2026 6 1 12 0)
               (ev-dt 2026 6 1 13 0))))
+        (let
+          ((gdb (ev-av-gap-db)))
+          (do
+            (ev-av-check!
+              "next-free finds the gap between bookings"
+              (ev-next-free
+                gdb
+                (quote sam)
+                (ev-dt 2026 6 1 9 0)
+                30
+                (ev-dt 2026 6 1 18 0))
+              (ev-dt 2026 6 1 10 0))
+            (ev-av-check!
+              "next-free skips a gap too short for the duration"
+              (ev-next-free
+                gdb
+                (quote sam)
+                (ev-dt 2026 6 1 9 0)
+                60
+                (ev-dt 2026 6 1 18 0))
+              (ev-dt 2026 6 1 11 30))
+            (ev-av-check!
+              "next-free returns `after` when already free"
+              (ev-next-free
+                gdb
+                (quote sam)
+                (ev-dt 2026 6 1 14 0)
+                60
+                (ev-dt 2026 6 1 18 0))
+              (ev-dt 2026 6 1 14 0))
+            (ev-av-check!
+              "next-free returns nil when nothing fits before horizon"
+              (ev-next-free
+                gdb
+                (quote sam)
+                (ev-dt 2026 6 1 9 0)
+                120
+                (ev-dt 2026 6 1 11 0))
+              nil)
+            (ev-av-check!
+              "next-free for actor with no bookings is `after`"
+              (ev-next-free
+                gdb
+                (quote nobody)
+                (ev-dt 2026 6 1 9 0)
+                60
+                (ev-dt 2026 6 1 18 0))
+              (ev-dt 2026 6 1 9 0))
+            (ev-av-check!
+              "next-free at exact edge of a booking (half-open)"
+              (ev-next-free
+                gdb
+                (quote sam)
+                (ev-dt 2026 6 1 10 0)
+                30
+                (ev-dt 2026 6 1 18 0))
+              (ev-dt 2026 6 1 10 0))))
         (let
           ((daily (ev-expand (ev-event (quote class) (ev-dt 2026 6 1 9 0) 60 {:freq :daily :count 3} 1) (ev-date 2026 6 1) (ev-date 2026 7 1))))
           (let

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` → **53/53** (Phase 1: calendar + availability)
+`bash lib/events/conformance.sh` → **59/59** (Phase 1: calendar + availability)
 
 ## Ground rules
 
@@ -57,8 +57,9 @@ lib/events/api.sx ── (events/schedule) (events/book) (events/agenda) ──
 - [x] `calendar.sx` — event facts, RRULE expansion in a window (DAILY/WEEKLY)
 - [x] `calendar.sx` — MONTHLY (bymonthday + nth-weekday byday)
 - [x] `availability.sx` — free/busy rules (busy/conflict/busy_in on Datalog)
-- [ ] `availability.sx` — next-free slot search (same rules, different bindings)
-- [ ] `api.sx` + tests + scoreboard + conformance.sh
+- [x] `availability.sx` — next-free slot search (same rules, different bindings)
+- [ ] `api.sx` — public entry points (schedule/agenda/free-check)
+- [ ] tests + scoreboard + conformance.sh [done incrementally; api.sx pending]
 
 ## Phase 2 — Ticketing + booking
 - [ ] capacity rules; transactional booking → `persist` (no overbooking)
@@ -78,6 +79,11 @@ lib/events/api.sx ── (events/schedule) (events/book) (events/agenda) ──
 
 ## Progress log
 
+- 2026-06-07 — `next-free` slot search: earliest start ≥ after where
+  [s,s+duration) is free and ends ≤ horizon, else nil. Candidates are `after`
+  plus each busy-interval end (interval-packing); each probe reuses the
+  `busy_in` Datalog rule via `ev-free?`. Finds gaps between bookings, skips
+  too-short gaps, half-open at edges. +6 tests, 59/59.
 - 2026-06-07 — `availability.sx`: free/busy + conflict detection as forward-
   chained Datalog over `occurrence`/`booking` EDB. Rules `busy(A,S,E)`,
   `conflict(A,O1,O2)` (canonical `O1<O2`, half-open overlap `S1<E2 ∧ S2<E1`),