"use client";

/**
 * usePostScrollFeed — anchor-first, keyset-paginated loader for the
 * Instagram-style profile scroll feeds (/u/me/posts and /u/[handle]/posts).
 *
 * Why: the grid thumbnails link to `…/posts#post-<id>`. Loading the whole
 * timeline and then scrolling to the hash fails for old posts (they aren't
 * loaded yet). Instead we:
 *   • read `#post-<id>` and fetch THAT post directly (posts.get) so it renders
 *     at the top instantly, then page the posts older than it;
 *   • with no hash, start from the newest page;
 *   • load older pages as a bottom sentinel nears the viewport (dedupe by id,
 *     stop when a page is short) — the same idiom as the home feed.
 *
 * `fetchPage(before, limit)` is supplied by the caller so the same hook serves
 * the owner feed (posts.mine) and the public feed (profile.feed). `setFeed` is
 * returned so the owner page can optimistically prune on delete/hide.
 */

import { useCallback, useEffect, useRef, useState } from "react";
import { posts as postsApi, type FeedCursor, type Post } from "@/lib/social";

const PAGE = 10;

type FetchPage = (before: FeedCursor | null, limit: number) => Promise<Post[]>;

const cursorOf = (p: Post): FeedCursor => ({
  created_at: p.created_at,
  id: p.id,
});

export function usePostScrollFeed(fetchPage: FetchPage) {
  const [feed, setFeed] = useState<Post[]>([]);
  const [loading, setLoading] = useState(true);
  const [hasMore, setHasMore] = useState(true);
  const [error, setError] = useState<string | null>(null);

  const sentinelRef = useRef<HTMLDivElement | null>(null);
  const fetchingRef = useRef(false);
  const startedRef = useRef(false);
  const cursorRef = useRef<FeedCursor | null>(null);
  const anchorIdRef = useRef<string | null>(null);

  // Append the next older page, keyed off the last loaded post.
  const loadOlder = useCallback(async () => {
    if (fetchingRef.current || !hasMore) return;
    fetchingRef.current = true;
    try {
      const rows = (await fetchPage(cursorRef.current, PAGE)) ?? [];
      setFeed((prev) => {
        const seen = new Set(prev.map((p) => p.id));
        const merged = [...prev, ...rows.filter((p) => !seen.has(p.id))];
        if (merged.length > 0) cursorRef.current = cursorOf(merged[merged.length - 1]);
        return merged;
      });
      if (rows.length < PAGE) setHasMore(false);
    } catch {
      setHasMore(false);
      setError((e) => e ?? "Could not load posts.");
    } finally {
      fetchingRef.current = false;
      setLoading(false);
    }
  }, [fetchPage, hasMore]);

  // First load: anchor-first when arriving with #post-<id>, else newest page.
  useEffect(() => {
    if (startedRef.current) return;
    startedRef.current = true;

    const hash =
      typeof window !== "undefined" ? window.location.hash : "";
    const anchorId = hash.startsWith("#post-") ? hash.slice("#post-".length) : "";

    (async () => {
      if (anchorId) {
        try {
          const anchor = await postsApi.get(anchorId);
          if (anchor) {
            anchorIdRef.current = anchor.id;
            cursorRef.current = cursorOf(anchor);
            setFeed([anchor]);
            setLoading(false);
            await loadOlder(); // posts older than the anchor
            return;
          }
        } catch {
          // Anchor missing/deleted — fall through to the newest page.
        }
      }
      await loadOlder();
    })();
  }, [loadOlder]);

  // Scroll the anchored post into view once it's in the DOM.
  useEffect(() => {
    const id = anchorIdRef.current;
    if (!id || feed.length === 0) return;
    anchorIdRef.current = null;
    requestAnimationFrame(() => {
      const el = document.getElementById(`post-${id}`);
      if (el) el.scrollIntoView({ block: "start", behavior: "auto" });
    });
  }, [feed.length]);

  // Infinite scroll: load older as the sentinel nears the viewport.
  useEffect(() => {
    const el = sentinelRef.current;
    if (!el || !hasMore) return;
    const io = new IntersectionObserver(
      (entries) => {
        if (entries[0]?.isIntersecting) void loadOlder();
      },
      { rootMargin: "600px" },
    );
    io.observe(el);
    return () => io.disconnect();
  }, [loadOlder, hasMore]);

  return { feed, setFeed, loading, hasMore, error, setError, sentinelRef };
}