import { readFileSync } from "node:fs";
import { expect, it } from "vitest";

// One invariant, two languages, no shared wire format — so a test is the only thing that can hold
// them together.
//
// The backend finishes an item when a progress checkpoint lands within `_FINISH_MARGIN_S` of the
// duration: watched, position cleared, scrobbled to Plex. The player's refused-finish rollback must
// therefore never post a position inside that window, or the branch named "take the finish back"
// performs, through the backend, the very write that was just refused (rounds 6 and 7 both landed
// on this). The frontend mirrors the number so it knows where the window starts.
//
// Both were 30 and exactly complementary, but nothing related them, so a backend-only change would
// have re-opened the defect silently — a rule stated in a comment and not enforced in code, the one
// thing this repo's house rules single out. The direction matters: the frontend's margin must be at
// LEAST the backend's. Larger is merely conservative (a rollback skipped where it could have run);
// smaller is the bug.
//
// Both values are read from source rather than imported: `plexPlayerHooks` pulls in the i18n module
// (and with it `localStorage`), and booting half the app to read one integer would make this test
// about the module graph instead of about the two numbers.
function constantIn(path: string, pattern: RegExp): number {
  const source = readFileSync(new URL(path, import.meta.url), "utf8");
  const m = pattern.exec(source);
  // If this fails the constant was renamed or moved — find it and re-point this test, because the
  // invariant it guards did not go away.
  expect(m, `${pattern} not found in ${path}`).not.toBeNull();
  return Number(m![1]);
}

it("never lets the player's finish margin fall below the backend's", () => {
  const backend = constantIn(
    "../../../backend/app/routes/plex/playback.py",
    /^_FINISH_MARGIN_S\s*=\s*(\d+)/m,
  );
  const frontend = constantIn("./plexPlayerHooks.ts", /^const FINISH_MARGIN_S\s*=\s*(\d+)/m);
  expect(backend).toBeGreaterThan(0);
  expect(frontend).toBeGreaterThanOrEqual(backend);
});