"use client";

/**
 * HomeFeed — the global, personalised feed of community posts.
 *
 * Architecture:
 *   • Window-virtualised list (dynamic row heights via measureElement) so a
 *     long feed stays cheap to render.
 *   • Infinite scroll: 10 posts per page, fetched as a bottom sentinel nears
 *     the viewport. The server orders not-yet-liked posts from people you
 *     follow first, then everyone else — seamless, no visible boundary.
 *   • When the feed is exhausted, a "new on the platform" strip (≤5 accounts
 *     that joined in the last 24h) caps it off — hidden if there are none.
 *
 * The card's Follow / Follow back affordance is enabled here (enableFollow);
 * the server hydrates each post's viewer↔author relationship.
 */

import { useCallback, useEffect, useRef, useState } from "react";
import { useWindowVirtualizer } from "@tanstack/react-virtual";
import { Loader2 } from "lucide-react";
import { feed, type Post, type UserSummary } from "@/lib/social";
import { PostScrollCard } from "@/components/social/post/post-scroll-card";
import { NewUsersStrip } from "./new-users-strip";

const PAGE = 10;

export function HomeFeed() {
  const [items, setItems] = useState<Post[]>([]);
  const [loading, setLoading] = useState(true);
  const [hasMore, setHasMore] = useState(true);
  const [newUsers, setNewUsers] = useState<UserSummary[]>([]);

  const fetchingRef = useRef(false);
  const offsetRef = useRef(0);
  const parentRef = useRef<HTMLDivElement>(null);
  const sentinelRef = useRef<HTMLDivElement>(null);
  const [offsetTop, setOffsetTop] = useState(0);

  const loadPage = useCallback(async () => {
    if (fetchingRef.current) return;
    fetchingRef.current = true;
    try {
      const rows = (await feed.home(PAGE, offsetRef.current)) ?? [];
      offsetRef.current += rows.length;
      setItems((prev) => {
        const seen = new Set(prev.map((p) => p.id));
        return [...prev, ...rows.filter((p) => !seen.has(p.id))];
      });
      if (rows.length < PAGE) {
        setHasMore(false);
        // Feed exhausted → load the discovery strip once.
        feed
          .newUsers()
          .then((u) => setNewUsers(u ?? []))
          .catch(() => setNewUsers([]));
      }
    } catch {
      setHasMore(false);
    } finally {
      fetchingRef.current = false;
      setLoading(false);
    }
  }, []);

  // Initial page + measure where the list sits in the document (so the window
  // virtualiser offsets correctly below the streak component).
  useEffect(() => {
    void loadPage();
  }, [loadPage]);
  useEffect(() => {
    if (parentRef.current) setOffsetTop(parentRef.current.offsetTop);
  }, [loading]);

  const virtualizer = useWindowVirtualizer({
    count: items.length,
    estimateSize: () => 680,
    overscan: 4,
    scrollMargin: offsetTop,
  });

  // Infinite scroll — fetch the next page as the sentinel approaches.
  useEffect(() => {
    const node = sentinelRef.current;
    if (!node || !hasMore) return;
    const io = new IntersectionObserver(
      (entries) => {
        if (entries[0].isIntersecting && hasMore && !fetchingRef.current) {
          void loadPage();
        }
      },
      { rootMargin: "600px 0px" },
    );
    io.observe(node);
    return () => io.disconnect();
  }, [hasMore, loadPage, items.length]);

  if (loading && items.length === 0) {
    return (
      <div className="flex justify-center py-16">
        <Loader2 className="size-6 animate-spin text-muted-foreground" />
      </div>
    );
  }

  if (!loading && items.length === 0) {
    return (
      <div className="space-y-4">
        <p className="rounded-xl bg-card ring-1 ring-foreground/10 px-5 py-10 text-center text-sm text-muted-foreground">
          Your feed is quiet for now. Follow a few devotees, or share a verse to
          get things started.
        </p>
        <NewUsersStrip users={newUsers} />
      </div>
    );
  }

  const virtualItems = virtualizer.getVirtualItems();

  return (
    <div>
      <div
        ref={parentRef}
        className="relative w-full"
        style={{ height: virtualizer.getTotalSize() }}
      >
        {virtualItems.map((vi) => {
          const post = items[vi.index];
          return (
            <div
              key={post.id}
              data-index={vi.index}
              ref={virtualizer.measureElement}
              className="absolute left-0 top-0 w-full"
              style={{ transform: `translateY(${vi.start - offsetTop}px)` }}
            >
              <div className="pb-4">
                <PostScrollCard post={post} enableFollow />
              </div>
            </div>
          );
        })}
      </div>

      {hasMore ? (
        <div ref={sentinelRef} className="flex justify-center py-6">
          <Loader2 className="size-5 animate-spin text-muted-foreground" />
        </div>
      ) : (
        <NewUsersStrip users={newUsers} />
      )}
    </div>
  );
}