import { useEffect, useState, type CSSProperties } from "react";

// Scroll-affordance for a scroll container (typically one whose native scrollbar is hidden with
// `.no-scrollbar`, but the page scroller keeps its bar and takes the fade anyway):
// fades the top/bottom edge of the content to hint there's more to scroll, but only on the edge
// that's actually clipped (no fade at a boundary, none when it all fits). Returns a callback ref
// to attach to the scroller and the mask `style` to spread onto it. Uses a node-state callback
// ref (not useRef) so the effect (re)runs whenever the element attaches — e.g. a SidePanel that
// mounts collapsed and only renders its scroll body once expanded still wires up correctly.
// `fadeTop` gates the top edge only. Pass false when the scroller's top is a sticky element (a
// table's sticky header): fading there would eat into the header instead of hinting at content, and
// the header would visibly change as the fade turns on with the first scroll.
export function useScrollFade(
  fadePx = 20,
  fadeTop = true,
): {
  ref: (node: HTMLElement | null) => void;
  style: CSSProperties;
} {
  const [el, setEl] = useState<HTMLElement | null>(null);
  const [fade, setFade] = useState({ up: false, down: false });

  useEffect(() => {
    if (!el) return;
    // Bail out when neither edge changed: `scroll` fires per frame, and the page scroller's
    // hook sits above the whole page tree, so a new object here would re-render on every frame
    // of every scroll. The two booleans only flip at the ends of the range.
    const update = () => {
      const up = el.scrollTop > 1;
      const down = el.scrollTop + el.clientHeight < el.scrollHeight - 1;
      setFade((prev) => (prev.up === up && prev.down === down ? prev : { up, down }));
    };
    update();
    el.addEventListener("scroll", update, { passive: true });
    // Watch the scroller AND its content. Content that grows or shrinks — a lazy page loading in,
    // a thread's messages decrypting, a list being filtered — changes neither the scroller's own
    // box nor its scrollTop, so without the children nothing would re-evaluate the edges.
    const ro = new ResizeObserver(update);
    ro.observe(el);
    for (const child of Array.from(el.children)) ro.observe(child);
    // Children get SWAPPED, and a ResizeObserver holding a detached node simply never fires
    // again. Measured: the Plex page opened with no bottom fade over a 3119px list, because the
    // node observed at mount was the Suspense fallback the real page then replaced. (Same
    // lazy-page race that broke BackToTop.) So follow the child list: the records name exactly
    // which nodes arrived and left, which keeps this O(1) per change rather than re-observing
    // every child of a long list on each append.
    const mo = new MutationObserver((records) => {
      for (const rec of records) {
        for (const node of Array.from(rec.addedNodes))
          if (node instanceof Element) ro.observe(node);
        for (const node of Array.from(rec.removedNodes))
          if (node instanceof Element) ro.unobserve(node);
      }
      update();
    });
    mo.observe(el, { childList: true });
    return () => {
      el.removeEventListener("scroll", update);
      ro.disconnect();
      mo.disconnect();
    };
  }, [el]);

  const mask = `linear-gradient(to bottom, transparent, #000 ${fade.up && fadeTop ? fadePx : 0}px, #000 calc(100% - ${
    fade.down ? fadePx : 0
  }px), transparent)`;
  return { ref: setEl, style: { maskImage: mask, WebkitMaskImage: mask } };
}
