import { describe, expect, it } from "vitest";
import {
  MAX_LOOKAHEAD_ROWS,
  PREFETCH_ROWS,
  PREFETCH_VIEWPORTS,
  readAheadLookahead,
  shouldPrefetchNext,
} from "./readAhead";

describe("readAheadLookahead", () => {
  it("floors at PREFETCH_ROWS when nothing (or little) is rendered", () => {
    expect(readAheadLookahead(0)).toBe(PREFETCH_ROWS);
    expect(readAheadLookahead(1)).toBe(PREFETCH_ROWS); // 1*2=2 < floor 4
  });

  it("scales with the rendered row count for a normal viewport", () => {
    expect(readAheadLookahead(10)).toBe(10 * PREFETCH_VIEWPORTS); // 20
    expect(readAheadLookahead(15)).toBe(30);
  });

  it("caps at MAX_LOOKAHEAD_ROWS so a tall window doesn't prefetch unboundedly", () => {
    // 30 rendered rows would be 60 uncapped — the cap is what bounds per-mount feed prefetch.
    expect(readAheadLookahead(30)).toBe(MAX_LOOKAHEAD_ROWS);
    expect(readAheadLookahead(1000)).toBe(MAX_LOOKAHEAD_ROWS);
  });
});

describe("shouldPrefetchNext", () => {
  it("fetches once the last rendered row enters the read-ahead window", () => {
    // 100 rows loaded, viewport renders 10 → lookahead 20. Fetch when lastIndex >= 100-1-20 = 79.
    expect(shouldPrefetchNext(78, 100, 10)).toBe(false);
    expect(shouldPrefetchNext(79, 100, 10)).toBe(true);
    expect(shouldPrefetchNext(99, 100, 10)).toBe(true);
  });

  it("fetches eagerly while fewer rows are loaded than the lookahead (early pages)", () => {
    // Only 15 rows loaded, lookahead 20 → the whole list is inside the window.
    expect(shouldPrefetchNext(8, 15, 10)).toBe(true);
  });

  it("does not fetch near the top of a long loaded list", () => {
    expect(shouldPrefetchNext(12, 500, 10)).toBe(false);
  });
});