import type { CatchUpWindow } from "./chat-utils";

type Page<T> = { messages: T[]; next: string; through: string; hasMore: boolean };

export function createMutationRecovery<T>(
  fetchPage: (cursor: CatchUpWindow) => Promise<Page<T>>,
  apply: (messages: T[]) => void,
  initialCursor: CatchUpWindow = { after: "0" },
  onCursor?: (cursor: CatchUpWindow) => void,
) {
  let cursor: CatchUpWindow = { ...initialCursor };
  let stopped = false;
  let active: Promise<void> | undefined;
  let requested = false;
  async function drain() {
    do {
      requested = false;
      let more: boolean;
      do {
        const page = await fetchPage({ ...cursor });
        if (stopped) return;
        if (
          BigInt(page.through) < BigInt(cursor.after) ||
          BigInt(page.next) < BigInt(cursor.after) ||
          BigInt(page.next) > BigInt(page.through) ||
          (cursor.through !== undefined && cursor.through !== page.through) ||
          (page.hasMore && BigInt(page.next) <= BigInt(cursor.after))
        ) throw new Error("Invalid mutation recovery window");
        apply(page.messages);
        cursor = page.hasMore ? { after: page.next, through: page.through } : { after: page.through };
        onCursor?.({ ...cursor });
        more = page.hasMore;
      } while (more && !stopped);
    } while (requested && !stopped);
  }
  return {
    run(): Promise<void> {
      if (stopped) return Promise.resolve();
      if (active) { requested = true; return active; }
      active = drain().finally(() => { active = undefined; });
      return active;
    },
    stop() { stopped = true; },
  };
}