import { useCallback, useEffect, useRef, useState } from 'react';
import { GripVertical, Plus, Trash2 } from 'lucide-react';
import { Button } from '@/components/ui/button';
import { Input } from '@/components/ui/input';
import { useClient } from 'hooks/useClient';
import { MediaPicker } from '../MediaPicker';
import { readableError } from '../errors';
import { mediaThumbUrl } from '../thumbnail';

/**
 * One carousel's contents: what is in it, in what order, and what each picture is called.
 *
 * ── Why it is here and not a link to Carousels ──
 *
 * A carousel block on a page IS a carousel, and being sent to another section to reorder it means
 * losing the page you were arranging it for. The set is a thing of its own — that is why it has its
 * own section — but editing it is something you do while looking at where it appears.
 *
 * ── It reads the carousel itself ──
 *
 * `GET /carousels/:id`, rather than being handed one. The block knows an id and nothing else, and a
 * component that fetches what it needs can be dropped anywhere something knows an id.
 *
 * ── Duplication, stated plainly ──
 *
 * `CarouselsPanel` still draws its own copy of this list. That is a real duplicate and it should adopt
 * this component — its version carries the same operations against the same routes, plus creating and
 * deleting whole carousels, which this deliberately does not.
 */

type Item = { id: number; url: string; alt: string | null };
type Carousel = { id: number; name: string; slug: string; items: Item[] };

export const CarouselItems = ({ carouselId, onChanged }: { carouselId: number; onChanged?: () => void }) => {
  const client = useClient();
  const clientRef = useRef(client);
  clientRef.current = client;

  const [carousel, setCarousel] = useState<Carousel | null>(null);
  const [busy, setBusy] = useState(false);
  const [error, setError] = useState<string | null>(null);
  const [picking, setPicking] = useState(false);

  const load = useCallback(async () => {
    try {
      setCarousel((await clientRef.current.get<{ data: Carousel }>(`/fofos-admin/carousels/${carouselId}`)).data);
    } catch (failure) {
      setError(readableError(failure));
    }
  }, [carouselId]);

  useEffect(() => {
    void load();
  }, [load]);

  /** Every write goes through here: one busy flag, one reload, one place errors surface. */
  const send = async (call: () => Promise<unknown>) => {
    setBusy(true);
    setError(null);
    try {
      await call();
      await load();
      // The page draws this carousel, so it is now showing the old one.
      onChanged?.();
    } catch (failure) {
      setError(readableError(failure));
    } finally {
      setBusy(false);
    }
  };

  /*
    Reordering by dragging, the same way the page rows do it.

    The list rearranges as the pointer crosses a row rather than on drop, so what you see while
    dragging is what you will get. The write happens once, at the end, and it sends EVERY id in order —
    which is what makes two items claiming a position impossible.

    It replaced a pair of arrows per row. Nine items meant eighteen buttons and four presses to move
    something from the bottom to the top.
  */
  const [dragged, setDragged] = useState<number | null>(null);
  const orderedRef = useRef<Item[] | null>(null);

  const moveOver = (event: React.DragEvent, overId: number) => {
    if (dragged === null || !carousel) return;
    // Without this the drop is refused and the browser animates the row back to where it started.
    event.preventDefault();
    if (dragged === overId) return;

    const next = [...carousel.items];
    const from = next.findIndex((one) => one.id === dragged);
    const to = next.findIndex((one) => one.id === overId);
    if (from < 0 || to < 0) return;

    next.splice(to, 0, next.splice(from, 1)[0]!);
    orderedRef.current = next;
    setCarousel({ ...carousel, items: next });
  };

  const commitOrder = () => {
    const ordered = orderedRef.current;
    setDragged(null);
    orderedRef.current = null;
    // Dropped where it started, or outside the list. Nothing moved, so nothing is written.
    if (!ordered) return;

    void send(() =>
      clientRef.current.put(`/fofos-admin/carousels/${carouselId}/items/order`, { ids: ordered.map((one) => one.id) }),
    );
  };

  if (!carousel) return <p className="text-xs text-muted-foreground">{error ?? 'Loading…'}</p>;

  return (
    <div className="flex flex-col gap-2">
      <div className="flex items-baseline gap-2">
        <span className="min-w-0 flex-1 truncate text-xs font-medium">{carousel.name}</span>
        <span className="shrink-0 text-[10px] text-muted-foreground">
          {carousel.items.length} item{carousel.items.length === 1 ? '' : 's'}
        </span>
      </div>

      <ul className="flex flex-col gap-1">
        {carousel.items.map((item) => (
          <li
            key={item.id}
            /* The row is the drop target; the grip is what drags. See the note on the handler. */
            onDragOver={(event) => moveOver(event, item.id)}
            onDrop={commitOrder}
            className={`flex items-center gap-2 rounded border border-border/60 p-1.5 ${
              dragged === item.id ? 'opacity-40' : ''
            }`}
          >
            <span
              draggable
              onDragStart={(event) => {
                setDragged(item.id);
                // Firefox starts no drag at all without data on the transfer. The value is unused.
                event.dataTransfer.effectAllowed = 'move';
                event.dataTransfer.setData('text/plain', String(item.id));
              }}
              onDragEnd={commitOrder}
              className="shrink-0 cursor-grab text-muted-foreground/60 active:cursor-grabbing"
              aria-label="Reorder"
            >
              <GripVertical className="size-4" />
            </span>
            {/*
              From the DERIVED ladder, which is the whole point of the pipeline: most of these are HEIC
              and a browser cannot show the original at all.
            */}
            <img
              src={mediaThumbUrl(item.url)}
              alt=""
              loading="lazy"
              className="size-24 shrink-0 rounded bg-muted object-cover"
              // A file whose conversion has not run has no rung. Hidden rather than left as a broken
              // glyph, which reads as a corrupt picture rather than a pending one.
              onError={(event) => {
                event.currentTarget.style.visibility = 'hidden';
              }}
            />

            <span className="min-w-0 flex-1">
              {/*
                Saved on blur, not on every keystroke: a caption is a sentence somebody is in the
                middle of, and a request per character would race with itself.
              */}
              <Input
                defaultValue={item.alt ?? ''}
                onBlur={(event) => {
                  const value = event.target.value.trim();
                  if (value === (item.alt ?? '')) return;
                  void send(() =>
                    clientRef.current.patch(`/fofos-admin/carousels/${carouselId}/items/${item.id}`, {
                      alt: value || null,
                    }),
                  );
                }}
                placeholder="Caption"
                className="h-7 text-xs"
              />
            </span>

            <span className="flex shrink-0">
              <Button
                variant="ghost"
                size="sm"
                className="h-7 px-1.5 text-destructive"
                disabled={busy}
                onClick={() =>
                  void send(() => clientRef.current.delete(`/fofos-admin/carousels/${carouselId}/items/${item.id}`))
                }
              >
                <Trash2 className="size-3.5" />
              </Button>
            </span>
          </li>
        ))}
      </ul>

      <Button variant="outline" size="sm" className="h-8 self-start text-xs" disabled={busy} onClick={() => setPicking(true)}>
        <Plus className="size-3.5" /> Add media…
      </Button>

      {error && <p className="text-xs text-destructive">{error}</p>}

      <MediaPicker
        open={picking}
        onClose={() => setPicking(false)}
        onPick={(url) => {
          setPicking(false);
          void send(() => clientRef.current.post(`/fofos-admin/carousels/${carouselId}/items`, { urls: [url] }));
        }}
        title="Add to this carousel"
      />
    </div>
  );
};