/** How many pages a listing reads before it stops. At 100 records a page, that is 10,000 records. */
export const MAX_PAGES = 100;

export interface Page<T> {
  items: readonly T[];
  cursor?: string | null;
}

/**
 * Yields every item across pages. Stops at an empty page, a missing cursor, a cursor the server already
 * sent, or after `maxPages`, so a server that repeats or cycles its cursor cannot keep the caller looping.
 * With `pageSize`, a page shorter than it is the last one: a PDS sends a cursor with every page that has
 * records, so without this every listing costs one more round trip for an empty page.
 */
export async function* paginate<T>(
  fetchPage: (cursor: string | undefined) => Promise<Page<T>>,
  { maxPages = MAX_PAGES, pageSize }: { maxPages?: number; pageSize?: number } = {},
): AsyncGenerator<T> {
  const seen = new Set<string>();
  let cursor: string | undefined;
  for (let page = 0; page < maxPages; page++) {
    const { items, cursor: next } = await fetchPage(cursor);
    yield* items;
    if (items.length === 0 || (pageSize !== undefined && items.length < pageSize) || !next || seen.has(next)) return;
    seen.add(next);
    cursor = next;
  }
}