// @vitest-environment jsdom
//
// A next-page call while that page is still loading joins it instead of sending it again:
// on prod every feed page went out 2–3 times with the same cursor.
import { InfiniteQueryObserver } from "@tanstack/react-query";
import { describe, expect, it } from "vitest";

import { mayReadAhead, NEXT_PAGE, nextPageOnClick } from "./nextPage";
import { createQueryClient } from "./queryClient";

function probe(sent: (string | null)[], delay = 5) {
  return new InfiniteQueryObserver(createQueryClient(), {
    queryKey: ["probe"],
    initialPageParam: null as string | null,
    queryFn: async ({ pageParam }) => {
      sent.push(pageParam);
      await new Promise((r) => setTimeout(r, delay));
      return { next: `c${sent.length}` };
    },
    getNextPageParam: (last: { next: string }) => last.next,
  });
}

describe("NEXT_PAGE", () => {
  it("sends each page once when the read-ahead calls twice in a row", async () => {
    const sent: (string | null)[] = [];
    const observer = new InfiniteQueryObserver(createQueryClient(), {
      queryKey: ["probe"],
      initialPageParam: null as string | null,
      queryFn: async ({ pageParam }) => {
        sent.push(pageParam);
        await new Promise((r) => setTimeout(r, 5));
        return { next: `c${sent.length}` };
      },
      getNextPageParam: (last: { next: string }) => last.next,
    });
    const unsubscribe = observer.subscribe(() => {});
    await observer.refetch();

    // Two scroll frames before `isFetchingNextPage` reaches the component.
    void observer.fetchNextPage(NEXT_PAGE);
    await observer.fetchNextPage(NEXT_PAGE);

    expect(sent).toEqual([null, "c1"]);
    expect(observer.getCurrentResult().data?.pages).toHaveLength(2);
    unsubscribe();
  });

  // The join takes ANY fetch in flight. During a background refetch (an invalidation, a
  // poll) it reloads the pages already there and appends none — why a read-ahead waits it out.
  it("joins a background refetch and appends no page", async () => {
    const sent: (string | null)[] = [];
    const observer = probe(sent);
    const unsubscribe = observer.subscribe(() => {});
    await observer.refetch();

    void observer.refetch();
    await observer.fetchNextPage(NEXT_PAGE);

    expect(observer.getCurrentResult().data?.pages).toHaveLength(1);
    unsubscribe();
  });
});

describe("a read-ahead", () => {
  it("waits while the list fetches anything", () => {
    expect(mayReadAhead(true, false)).toBe(true);
    expect(mayReadAhead(true, true)).toBe(false);
    expect(mayReadAhead(false, false)).toBe(false);
  });
});

describe("a next page asked for by a click", () => {
  it("loads the page even while a background refetch runs", async () => {
    const sent: (string | null)[] = [];
    const observer = probe(sent);
    const unsubscribe = observer.subscribe(() => {});
    await observer.refetch();

    void observer.refetch();
    await observer.fetchNextPage(nextPageOnClick(false));

    expect(observer.getCurrentResult().data?.pages).toHaveLength(2);
    unsubscribe();
  });

  it("joins a next page already loading", () => {
    expect(nextPageOnClick(true)).toBe(NEXT_PAGE);
  });
});