marriage/src/app/questions-list/section-prefetch.test.ts

import { describe, expect, it, vi } from "vitest";
import type { QuestionListItem } from "@/lib/schema-adapter";
import { prefetchSectionsWithBoundedConcurrency } from "./section-prefetch";

function section(slug: string): QuestionListItem {
  return { slug } as QuestionListItem;
}

describe("prefetchSectionsWithBoundedConcurrency", () => {
  it("starts promptly and runs at most two section requests at once", async () => {
    const sections = [section("one"), section("two"), section("three")];
    const releases: Array<() => void> = [];
    let activeRequests = 0;
    let maximumActiveRequests = 0;
    const prefetch = vi.fn(async () => {
      activeRequests += 1;
      maximumActiveRequests = Math.max(maximumActiveRequests, activeRequests);
      await new Promise<void>((resolve) => releases.push(resolve));
      activeRequests -= 1;
    });

    const queue = prefetchSectionsWithBoundedConcurrency(
      sections,
      prefetch,
      () => false,
    );

    expect(prefetch).toHaveBeenCalledTimes(2);
    expect(maximumActiveRequests).toBe(2);

    releases.shift()?.();
    await vi.waitFor(() => expect(prefetch).toHaveBeenCalledTimes(3));
    expect(maximumActiveRequests).toBe(2);

    releases.splice(0).forEach((release) => release());
    await queue;
  });

  it("continues after a background request fails", async () => {
    const prefetch = vi
      .fn<(item: QuestionListItem) => Promise<void>>()
      .mockRejectedValueOnce(new Error("offline"))
      .mockResolvedValue(undefined);

    await prefetchSectionsWithBoundedConcurrency(
      [section("one"), section("two"), section("three")],
      prefetch,
      () => false,
    );

    expect(prefetch).toHaveBeenCalledTimes(3);
  });
});