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+# 2026-05-01 — Availability Algorithm
+
+> Same-day successor to `2026-05-01-schema-and-seed.md`.
+
+## Milestone
+
+The pure availability algorithm from `Initial.md` §5 is implemented and exhaustively tested. Closes Step 3 of `Initial.md` §9.
+
+## What landed
+
+- `src/lib/availability.ts` — pure module exporting `findSlots`, `therapistAvailable`, `roomAvailable`, `inWorkingHours` plus the `*State` types the algorithm consumes.
+- `test/availability.test.ts` — **28 tests** across `inWorkingHours`, `therapistAvailable`, and `findSlots` integration.
+
+## What's verified
+
+- `pnpm test` — **38/38 green** (10 schema + 28 availability)
+- `pnpm lint` — clean
+- `pnpm exec tsc --noEmit` — clean
+
+## Coverage of the algorithm
+
+`inWorkingHours` (6): inside hours, before-open, exactly-at-close, non-working-day, DST spring-forward day, `effectiveFrom` bracket, cross-midnight rejected.
+
+`therapistAvailable` (4): free in hours, booking-overlap blocks, BLOCK override blocks, EXTRA_HOURS opens out-of-hours.
+
+`findSlots` integration (18): baseline grid math, empty therapist/room lists, therapist tag mismatch, room tag mismatch, ServiceTherapist allowlist, inactive therapist/room, BLOCK PTO, room buffer enforcement, room block, preferred-therapist filter, off-grid `from` rounds up, out-of-hours window, DST day, slot ordering, partial-availability slot still emitted with surviving candidates.
+
+## Decisions ratified
+
+| Decision | Resolution |
+|---|---|
+| Pure / impure boundary | Algorithm is pure; DB loader is separate (not yet written) |
+| Slot grid orientation | UTC, not local. DST handled implicitly |
+| Slot granularity | Default 15 min; configurable via `slotGranularityMin` |
+| Cross-midnight slots | Rejected outright |
+| EXTRA_HOURS vs working hours | Union — slot is OK if either covers it |
+| BLOCK vs everything | Always wins — any overlap kills the slot |
+| Booking overlap semantics | Half-open `[)`, matching the exclusion constraints |
+| TZ extraction library | `Intl.DateTimeFormat` (not `date-fns-tz` `toZonedTime`) — see Gotcha 1 |
+| Output shape | Slot returns *all* candidate therapists/rooms; assignment happens at commit time |
+
+## Gotchas hit
+
+### 1. `date-fns-tz` v3 `toZonedTime` is system-TZ-dependent
+
+`toZonedTime(date, tz)` returns a `Date` whose value is shifted such that `getHours()` (system-local) reads as the target zone's wall clock — but **only if you call the system-local accessors**, and only if you understand the shift. Calling `.getUTCHours()` on the result gives UTC, not the target. On an EDT developer machine targeting `America/New_York`, the call is a no-op and the bug is silent.
+
+**Fix**: use `Intl.DateTimeFormat` with `formatToParts` directly. Cached per zone; ~50 LOC helper. Portable across system timezones, no library quirks.
+
+`date-fns-tz` is still installed; we'll use `formatInTimeZone` for display formatting later (which uses Intl internally and is fine).
+
+### 2. `Intl.DateTimeFormat` returns `"24"` for midnight in some locales with `hour12: false`
+
+Quirky and well-documented. Normalized to `0` in the helper.
+
+## Repo deltas this session
+
+```
+src/lib/availability.ts    NEW   ~250 LOC
+test/availability.test.ts  NEW   ~330 LOC
+package.json               + date-fns + date-fns-tz, + tsx
+prisma/schema.prisma       (full schema from earlier today)
+prisma/migrations/         (full_schema migration from earlier today)
+prisma/seed.ts, src/lib/seed.ts   (from earlier today)
+test/booking-exclusion.test.ts    (refactored to fixtures, +3 FK tests)
+docs/progress/             2 new snapshots (this and schema-and-seed)
+```
+
+## Open questions still unresolved
+
+1. Practice timezone (`APP_TZ` placeholder)
+2. Customer-visible brand name (TouchBase or other)
+3. Currency
+4. Stripe account ownership
+
+## Roadmap status (per `Initial.md` §9)
+
+1. ~~Spike: docker-compose + exclusion-constraint migration + tests~~ **done 2026-04-30**
+2. ~~Schema + migrations + seed~~ **done 2026-05-01**
+3. ~~Pure availability algorithm + tests~~ **done 2026-05-01**
+4. **next**: DB loader for the algorithm + first end-to-end story (admin creates booking on behalf of customer, confirmation email sent)
+5. Public self-booking → Stripe deposits → reminders
+
+## Recommended next step
+
+**(a)** DB loader — `loadAvailabilityState(db, { from, to, serviceId })` that turns DB state into the `TherapistState[]` / `RoomState[]` the algorithm consumes. Roughly half a day. Unblocks any UI work that needs real availability data.
+
+Then the first end-to-end story (Step 4) becomes very close — admin picks slot via `findSlots`, calls a `createHold` server action that inserts a Booking with HOLD status, transactional with payment-intent issuance later. Auth.js + UI can stay deferred since admin-only flow doesn't need public auth yet.