import { useEffect } from "react";

type OnReachEnd = () => void;

/**
 * Watches a ShadCN <ScrollArea> (via its root ref) and calls `onReachEnd`
 * once you scroll past `threshold` (default 90%) of its inner viewport,
 * debounced by `debounceMs` (default 200ms).
 */
export function useScrollAreaEnd(
  rootRef: React.RefObject<HTMLElement | null>,
  onReachEnd: OnReachEnd,
  threshold = 0.9,
  debounceMs = 200,
) {
  useEffect(() => {
    const root = rootRef.current;
    if (!root) return;

    // find the Radix viewport inside the ShadCN ScrollArea
    const viewport = root.querySelector<HTMLElement>(
      "[data-radix-scroll-area-viewport]",
    );
    if (!viewport) return;

    let ticking = false;
    let timer: number | null = null;

    const handleScroll = () => {
      const { scrollTop, scrollHeight, clientHeight } = viewport;
      if (scrollTop + clientHeight >= scrollHeight * threshold) {
        // clear any pending call
        if (timer !== null) {
          clearTimeout(timer);
        }
        // schedule the reach-end callback
        timer = window.setTimeout(() => {
          onReachEnd();
          timer = null;
        }, debounceMs);
      }
      ticking = false;
    };

    const onScroll = () => {
      if (!ticking) {
        window.requestAnimationFrame(handleScroll);
        ticking = true;
      }
    };

    viewport.addEventListener("scroll", onScroll);
    return () => {
      viewport.removeEventListener("scroll", onScroll);
      if (timer !== null) {
        clearTimeout(timer);
      }
    };
  }, [rootRef, onReachEnd, threshold, debounceMs]);
}