Choosing 50 from 1,400

The tile grid renders on the order of 1400 cameras. On every render and every ~10-second poll, three things have to happen without the list stuttering or the operator losing their scroll position: decide which cameras are visible, decide what order they go in, and decide which small subset is worth asking the backend for fresh data about.

Only about 50 cameras can be polled at a time, so the third decision is a budget, not a preference. Offline / AI / Active are all decided by the backend — the client holds Set<string> results and falls back to real-time store data only until the API answers.

The pieces, in order: queries/useCameraFilterIds.ts fetches the sets through pages/api/camera-filters/[filter].ts; hooks/useCameraFilters.ts scopes them and computes the polling budget; data/cameraControlOptions.ts holds the predicate; and components/organisms/TileView/CameraList/CameraListComponent.tsx runs the filter/sort memo.

The predicate

shouldCameraBeFiltered returns true when a camera should be shown. Its structure is not the obvious one:

1. Area scope is an AND gate, evaluated first. If areaFilteredCameraIds is present and does not contain this camera, it is hidden regardless of every other filter. An empty Set is a real answer ("this area has no cameras") and hides everything; only undefined means no area is selected.
2. Display All Cameras, or no conditions at all, short-circuits to true.
3. Everything after that is an OR. Selecting Offline and PTZ shows cameras that are offline or PTZ, not both.

Each server-backed condition has the same two-branch shape — use the API Set if it has arrived, otherwise fall back to whatever real-time data is already in the store:

Condition	With API data	Fallback
Offline / Online	offlineCameraIds.has(cId), OR'd with !isOnline	isCameraOffline() offline tiers
AiDetected	aiDetectionIds.has(cId)	hasAiDetectionHits
Active	activeCameraIds.has(cId)	isRecentlyMoved
PTZ / Fixed	—	ptz (static, no API involved)
Non-CA	—	st !== 'CA'

Edit the conditions and the sets and watch the predicate resolve:

shouldCameraBeFiltered — show or hide?

// Mirrors shouldCameraBeFiltered from data/cameraControlOptions.ts.
// The offline-tier fallback is simplified to a boolean; the rest is the real control flow.

function shouldCameraBeFiltered(realTime, conditions, cam, sets) {
const id = cam.cId;

// 1. Area scope — AND gate. undefined = no area. An EMPTY Set hides everything.
if (sets.areaFilteredCameraIds && !sets.areaFilteredCameraIds.has(id)) return false;

// 2. Escape hatch.
if (conditions.indexOf('Display All Cameras') !== -1 || conditions.length === 0) return true;

// 3. Everything remaining is OR'd.
let match = false;
const has = function (c) { return conditions.indexOf(c) !== -1; };

const isOffline = sets.offlineCameraIds
  ? (sets.offlineCameraIds.has(id) || !cam.isOnline)
  : !!realTime.isOfflineFallback;

if (has('Offline') && isOffline) match = true;
if (has('Online') && !isOffline) match = true;
if (has('PTZ') && cam.ptz) match = true;
if (has('Fixed') && !cam.ptz) match = true;
if (has('Non-CA') && cam.st !== 'CA') match = true;

if (has('Active') && (sets.activeCameraIds ? sets.activeCameraIds.has(id) : realTime.isRecentlyMoved))
  match = true;
if (has('AiDetected') && (sets.aiDetectionIds ? sets.aiDetectionIds.has(id) : realTime.hasAiDetectionHits))
  match = true;

return match;
}

// --- Inputs: edit these -------------------------------------------------
const conditions = ['Offline', 'PTZ'];
const sets = {
offlineCameraIds: new Set(['CAM-002']),
aiDetectionIds: new Set(['CAM-003']),
activeCameraIds: undefined,
areaFilteredCameraIds: undefined, // try: new Set(['CAM-001', 'CAM-002'])
};
const cameras = [
{ cId: 'CAM-001', ptz: true,  st: 'CA', isOnline: true },
{ cId: 'CAM-002', ptz: false, st: 'CA', isOnline: true },
{ cId: 'CAM-003', ptz: false, st: 'CA', isOnline: true },
{ cId: 'CAM-004', ptz: true,  st: 'NV', isOnline: true },
];

const rows = cameras.map(function (cam) {
const rt = { cId: cam.cId, isRecentlyMoved: false, hasAiDetectionHits: false };
return cam.cId + '  ' + (shouldCameraBeFiltered(rt, conditions, cam, sets) ? 'SHOWN ' : 'hidden') +
       '   ptz=' + cam.ptz + ' state=' + cam.st;
});
render(<pre style={{margin: 0}}>{'conditions: [' + conditions.join(', ') + ']\n\n' + rows.join('\n')}</pre>);

Output

The AI duration filter can override a match

There is one place where the OR is not really an OR. When AiDetected is active and the camera has an AI hit and aiDurationFilter is anything other than 'Indefinite', a detection older than the threshold makes the function return false immediately — discarding a match an earlier condition already made. Offline + AiDetected with a 5-minute duration filter will therefore hide an offline camera whose AI detection happens to be old. Confirm that is intended before relying on it.

Sorting

Two comparators. When AiDetected is active, aiDetectionComparator sorts by latestAiDetectionStartDate descending — most recently started detection first, which is the same value the "Duration: Xm" badge on each tile is derived from. Sorting by latestAiDetectionDate would order by last-heartbeat recency, which drifts independently of the displayed duration, and the list would disagree with its own badges. Ties fall back to time descending, then cn.localeCompare, so the order is deterministic even with missing timestamps.

Otherwise memoizedSortFunction switches on sortCondition:

Condition	Order	Notes
alphabetical	name A→Z	plain cn comparison
activity	most recent first	prefers real-time time from getCameraById, falls back to the pin's static time
distance	nearest first	needs currLoc; when it is null the comparator calls onGetCurrLoc() and returns 0 for that pass

There is no sort snapshot. Earlier versions of this page described an aiSortSnapshotRef that froze the AI order until the operator scrolled back to the top; no such ref, no aiResortVersion, and no scroll-to-top re-sort exists in the current code. The AI list sorts live on every recompute. What keeps it from thrashing is that latestAiDetectionStartDate is the start of a detection and does not change while the detection is ongoing.

The 50-slot polling budget

useCameraFilters owns it. MAX_POLLING_CAMERA_IDS is 50. Both pin lists are scoped to areaFilteredCameraIds first, then one of three paths fills the budget and the result is sliced to 50:

Path	When	Order
API-filtered	any server-backed filter is on	pins that pass the API sets — viewport members first, then the rest
Zoomed in	50 or fewer scoped pins in the extent	the viewport list as-is
Wide extent	more than 50	tier 1 active and in viewport, tier 2 rest of viewport, tier 3 active and in the extent but not scrolled to

Two decisions inside that flow are easy to miss. Area scope is applied before everything else — a polling slot spent on a camera the operator has scoped out of view is a slot spent on nothing. And recently-moved is only fetched when it can pay for itself: useCameraFilterSets fetches it when the Active filter is explicitly on, or when the caller passes pollActiveRegardless, which useCameraFilters sets only at wide extent. Zoomed in, every camera on screen already fits inside the budget, so tiered prioritisation would buy nothing and the endpoint is never called.

There is one trap in how filterSets.activeCameraIds is exposed: it is populated only when Active is genuinely toggled on, even though the underlying query may still hold cached data. TanStack Query retains data when enabled flips to false; if that stale Set leaked into filterSets, hasApiFilter would stay true, every pin would fail the pre-filter, and pollingCameraIds would come back empty. The separate alwaysActiveCameraIds return value is what the tiering uses instead.

pollingCameraIds — who gets one of the 50 slots?

// Mirrors the pollingCameraIds memo in hooks/useCameraFilters.ts.
const MAX = 50;

function computePollingCameraIds(o) {
const area = o.areaFilteredCameraIds;
const scope = function (l) { return area ? l.filter(function (i) { return area.has(i); }) : l; };
const visible = scope(o.visiblePins), viewport = scope(o.viewportVisiblePins);
const inView = new Set(viewport), active = o.alwaysActiveCameraIds;

if (o.apiFilteredPinIds) {                       // path 1
  const a = [], b = [];
  o.apiFilteredPinIds.forEach(function (i) { (inView.has(i) ? a : b).push(i); });
  return { path: 'api-filtered', ids: a.concat(b).slice(0, MAX) };
}
if (visible.length <= MAX)                       // path 2
  return { path: 'zoomed-in', ids: viewport.slice(0, MAX) };

const inExtent = new Set(visible);               // path 3 — tiered
const t1 = [], t2 = [], t3 = [];
viewport.forEach(function (i) { (active && active.has(i) ? t1 : t2).push(i); });
if (active) active.forEach(function (i) { if (!inView.has(i) && inExtent.has(i)) t3.push(i); });
return { path: 'wide-extent', tiers: [t1.length, t2.length, t3.length],
         ids: t1.concat(t2, t3).slice(0, MAX) };
}

// --- Inputs: edit these -------------------------------------------------
const id = function (n) { return 'cam-' + n; };
const range = function (a, b) { const o = []; for (let i = a; i < b; i++) o.push(id(i)); return o; };

const r = computePollingCameraIds({
visiblePins: range(0, 400),          // everything in the map extent
viewportVisiblePins: range(0, 24),   // what is actually scrolled into view
alwaysActiveCameraIds: new Set([id(3), id(7), id(150), id(151), id(390)]),
areaFilteredCameraIds: undefined,    // try: new Set(range(0, 30))
apiFilteredPinIds: null,             // try: range(100, 180)
});

render(<pre style={{margin: 0}}>{'path: ' + r.path +
(r.tiers ? '\ntiers [active-in-view, rest-of-view, active-elsewhere]: ' + JSON.stringify(r.tiers) : '') +
'\nslots used: ' + r.ids.length + ' / ' + MAX +
'\nfirst 12: ' + r.ids.slice(0, 12).join(', ')}</pre>);

Output

"In the viewport" comes from an IntersectionObserver in CameraListComponent with rootMargin: '200px 0px 200px 0px', so a camera counts from 200 px before it scrolls into view. Those IDs land in pinsStore.viewportVisiblePins.

viewportVisiblePins is deliberately not a filter dependency

memoizedFilterFunc leaves it out of its useCallback deps. It changes on every scroll; including it would recreate the filter, recompute filteredAndSortedPins, re-observe every pin, and update viewportVisiblePins again — an infinite cascade. The value is still read inside the callback. It is a stale-closure trade made on purpose, and the source says so.

Adding a filter

Six edits, in this order: add /CameraFilters/<slug> on the backend; add the slug to ALLOWED_FILTERS (and PROTECTED_FILTERS if it needs a token) in the BFF route; add a QUERY_KEYS entry in queries/keys.ts rather than inlining an array; add the slug to CameraFilterType and QUERY_KEY_MAP in queries/useCameraFilterIds.ts, which gets you structural sharing for free; add the enable flag and filterSets field in hooks/useCameraFilters.ts, remembering it must be undefined when the filter is off; and add the condition branch to shouldCameraBeFiltered with a real-time fallback for the pre-API window.

Next: PinsStore, which owns visiblePins and viewportVisiblePins.