/**
 * Shared "which cities currently have events" derivation used by both the
 * `/city` hub page and `sitemap.ts`. Both previously re-implemented this
 * `Set` + filter independently after fetching discovery events; extracted
 * here so there's one place computing it.
 *
 * NOTE: there's no dedicated "cities with active events" aggregation
 * endpoint yet — this only reflects whatever discovery events the caller
 * passes in. `/city` intentionally samples just the first page of public
 * discovery events (backend max pageSize 100) to derive this cheaply; that
 * remains a stub, revisit with a real aggregation once saza1's foundation
 * lands if event volume grows past one page. `sitemap.ts` instead passes
 * every page (via `fetchAllSeoDiscoveryEvents`), so it isn't subject to
 * that same page-1 limit.
 */

interface ActiveCityEventCandidate {
  cityId: number
}

interface ActiveCityCandidate {
  id: number
}

/** Derives the set of city ids present among the given discovery events. */
export function getActiveCityIds(events: ActiveCityEventCandidate[]): Set<number> {
  return new Set(events.map((event) => event.cityId))
}

/** Filters `cities` down to those whose id is in `activeCityIds`. */
export function filterActiveCities<T extends ActiveCityCandidate>(cities: T[], activeCityIds: Set<number>): T[] {
  return cities.filter((city) => activeCityIds.has(city.id))
}