---
phase: 05-web-push-notifications
plan: 03
type: tdd
wave: 2
depends_on: [05-01]
files_modified:
  - apps/api/src/lib/pushCoalescer.ts
  - apps/api/tests/lib/pushCoalescer.test.ts
autonomous: true
requirements: [NOTIF-02]

must_haves:
  truths:
    - 'A burst of N coalesceListPush calls for the same (listId, actorId) within the window fires exactly ONE dispatch with count=N (D-01)'
    - "The coalesced dispatch passes the actor's userId as excludeUserId so the actor is never notified of their own change (D-03)"
    - "The coalesced notification copy is generic: title 'ActorName updated ListName', body 'N change(s)' — no item text (D-02)"
    - 'A new burst after the window fired starts a fresh count (timer/map entry cleared)'
  artifacts:
    - path: 'apps/api/src/lib/pushCoalescer.ts'
      provides: 'coalesceListPush(listId, actorId, actorName, listName, dispatch, windowMs) — per-(list,actor) debounce'
      exports: ['coalesceListPush']
      min_lines: 25
  key_links:
    - from: 'apps/api/src/lib/pushCoalescer.ts'
      to: 'dispatch callback'
      via: 'setTimeout fires once per window with excludeUserId=actorId'
      pattern: 'setTimeout'
---

<objective>
TDD the list-change coalescing debounce (D-01): collapse a rapid burst of edits to one list by one member into a single push, naming the actor + list + change count (D-02/D-03 generic copy). Reorder changes are excluded upstream (Plan 05-05 does not call this for position changes).

Purpose: Lists are the chattier, lower-stakes source. Without coalescing a grocery burst would fire one push per keystroke-save. The debounce is pure in-memory logic (single process, D-12) and is the load-bearing anti-spam primitive for NOTIF-02.

Output: `apps/api/src/lib/pushCoalescer.ts` with `coalesceListPush`, turning the Plan 05-01 RED scaffold GREEN.
</objective>

<context>
@.planning/PROJECT.md
@.planning/ROADMAP.md
@apps/api/src/lib/listEmitter.ts
@.planning/phases/05-web-push-notifications/05-RESEARCH.md
@.planning/phases/05-web-push-notifications/05-UI-SPEC.md
</context>

<feature>
  <name>pushCoalescer — per-(list,actor) debounce</name>
  <files>apps/api/src/lib/pushCoalescer.ts, apps/api/tests/lib/pushCoalescer.test.ts</files>
  <read_first>
    - apps/api/src/lib/listEmitter.ts (module-level Map singleton idiom)
    - .planning/phases/05-web-push-notifications/05-RESEARCH.md (Pattern 6 pushCoalescer; D-01 window 30-60s)
    - .planning/phases/05-web-push-notifications/05-UI-SPEC.md (## Notification Content Contract → List change: title "{ActorName} updated {ListName}", body "{N} change{s}", tag "list-change-{listId}", data.url "/lists/{listId}")
  </read_first>
  <behavior>
    - coalesceListPush(listId, actorId, actorName, listName, dispatch, windowMs=45000): keyed by `${listId}:${actorId}`.
      - First call: count=1, sets a setTimeout(windowMs).
      - Subsequent calls within window: count++, clearTimeout + reset timer (sliding window).
      - On timer fire: delete the map entry, call dispatch(payload, actorId) where payload = { title:`${actorName} updated ${listName}`, body:`${count} change${count===1?'':'s'}`, tag:`list-change-${listId}`, navigate:`/lists/${listId}` }.
    - Cases (vi.useFakeTimers):
      - 3 calls within window then advance time → dispatch called once, body "3 changes", excludeUserId=actorId.
      - 1 call then advance → body "1 change".
      - burst, advance past window, second burst, advance → dispatch called twice, each fresh count.
      - two different actorIds on the same list → two independent entries → two dispatches.
  </behavior>
  <implementation>
    Module-level `const pending = new Map<string, {count:number; timer: ReturnType<typeof setTimeout>}>()`. dispatch is injected (Plan 05-05 passes a closure over dispatchPush+subscription-fan-out) so the coalescer stays pure and testable. The `excludeUserId=actorId` argument is how D-03 self-suppression is plumbed; the caller's fan-out filters `WHERE userId != excludeUserId`. Export coalesceListPush only.
  </implementation>
  <verify>
    <automated>cd apps/api && pnpm exec vitest run tests/lib/pushCoalescer.test.ts</automated>
  </verify>
  <acceptance_criteria>
    Test green: N-burst → 1 dispatch count=N; "1 change" singular/plural; window reset; per-actor isolation; excludeUserId=actorId asserted.
  </acceptance_criteria>
</feature>

<threat_model>

## Trust Boundaries

| Boundary   | Description                                                                     |
| ---------- | ------------------------------------------------------------------------------- |
| in-process | coalescer holds no external input; actorName/listName come from trusted DB rows |

## STRIDE Threat Register

| Threat ID | Category               | Component               | Disposition | Mitigation Plan                                                                        |
| --------- | ---------------------- | ----------------------- | ----------- | -------------------------------------------------------------------------------------- |
| T-05-06   | Information Disclosure | list-change copy        | mitigate    | D-02 generic copy — no item text in payload; only actor name + count + list name       |
| T-05-07   | Spoofing               | actor self-notification | mitigate    | excludeUserId=actorId threaded to the fan-out (D-03); caller filters userId != actorId |
| T-05-08   | Denial of Service      | unbounded pending map   | accept      | Two-person household, per-(list,actor) keys bounded; entries self-delete on fire       |

</threat_model>

<verification>
- RED precedes GREEN; pushCoalescer.test.ts green.
- `pnpm --filter @familysync/api typecheck` passes.
</verification>

<success_criteria>

- Failing test committed (RED).
- coalesceListPush implemented; test passes (GREEN).
- Burst→single, plural rules, window reset, per-actor isolation, self-suppression all verified.
  </success_criteria>

<output>
Create `.planning/phases/05-web-push-notifications/05-03-SUMMARY.md` with RED/GREEN commits.
</output>
