import { useEffect, useRef } from 'react';

/** How long the quiet has to hold before the screen calls itself ready. */
const quietMs = 150;

/**
 * The readiness signal for the screenshot loop. `networkidle` will not do and cannot do: a live
 * run keeps the SSE open by construction. The screen says by itself when it has decided what to
 * draw — and "I am waiting" is a decision too, for a route that is shot for the sake of the
 * waiting.
 *
 * ⚠ The flag is ONE-WAY: once it has settled it never goes out again. It answers the screenshot
 * loop's question "has the screen come together already?", not "is the network quiet right now":
 * a background refetch on focus return and an update from a live run happen constantly and by
 * construction, and without the latch the frame would be shot in a race with them. A route shot for
 * the sake of the waiting does not touch the latch — it never gets as far as "ready".
 *
 * Two conditions of the latch, and each one was bought with a frame:
 *
 * 1. While NOT A SINGLE request has flown, there is no attribute at all. On the first effect
 *    nothing is in flight, and the previous edition announced `ready` at that moment — the gate
 *    shot the frame before the first read had started (found by acceptance S3.6). The absence of
 *    the attribute the loop waits out silently, and the library is read by every screen.
 * 2. The quiet has to HOLD. The reads of the second wave — the book, the chapters, the bank — turn
 *    on only after the library has answered, and between the waves the counter honestly drops to
 *    zero: a latch without the hold closed in that pit, and `/scale` was shot with the line
 *    "loading chapters…" (measured with a frame, not with reasoning).
 */
export function useScreenshotFlag(waiting: boolean): void {
  const started = useRef(false);
  const settled = useRef(false);

  useEffect(() => {
    if (waiting) started.current = true;
    if (!started.current) return;
    if (waiting || settled.current) {
      document.documentElement.dataset.screen = settled.current ? 'ready' : 'loading';
      return;
    }

    const timer = setTimeout(() => {
      settled.current = true;
      document.documentElement.dataset.screen = 'ready';
    }, quietMs);
    return () => {
      clearTimeout(timer);
    };
  }, [waiting]);
}