swarm-ui: test which alert instances the alerts page lists

The firing-only filter and the helpers beside it move out of the page
component into a plain module, with tests for the state match, the
never-active time and the row key.

frontend/packages/swarm-ui/src/pages/alerts.ts

@@ -0,0 +1,34 @@
+// What the alerts page lists, apart from how it draws it: the shape of one
+// instance from swarm-controller's `GET /api/alerts` and which of them count
+// as firing.
+
+export interface Alert {
+  name: string;
+  // Grafana's state for the instance: `Alerting`, `Pending`, `NoData` or
+  // `Error`.
+  state: string;
+  active_at: string | null;
+  summary: string | null;
+  labels: Record<string, string>;
+}
+
+const FIRING = "Alerting";
+
+// The instances the page shows: the ones Grafana says are firing now. Pending,
+// no-data and errored ones are left to Grafana's own list.
+export function firingAlerts(alerts: Alert[]): Alert[] {
+  return alerts.filter((a) => a.state === FIRING);
+}
+
+// Grafana reports an instance that never became active with the zero time,
+// which `Date.parse` reads as year 1.
+export function activeSinceMs(a: Alert): number | null {
+  if (a.active_at === null) return null;
+  const ms = Date.parse(a.active_at);
+  return Number.isNaN(ms) || ms <= 0 ? null : ms;
+}
+
+// A row needs a key that is unique among instances of the same rule.
+export function alertKey(a: Alert): string {
+  return JSON.stringify(Object.entries(a.labels).sort());
+}