import type { DragEvent } from 'react';
import { useState } from 'react';
import { Link, useSearchParams } from 'react-router';
import ReactMarkdown from 'react-markdown';
import remarkGfm from 'remark-gfm';
import { AlertTriangle, CalendarClock, X } from 'lucide-react';
import { cn } from '@/lib/utils';
import { errorText } from 'helpers/error-text';
import type { TodoItem } from './useKanban';
import { useSelectedProject, useTodoItem, useTodoPatch, useTodoTree } from './useKanban';

/**
 * The board over one project's todo folder — one file per item.
 *
 * ── Carried across from the platform almost unchanged ──
 *
 * This was `TodoBoard/TodoBoardPanel.tsx`. The columns, the drag handling, the ordering and the detail
 * pane are as they were; what changed is where the data comes from — every read and write now names a
 * project, because there is more than one.
 *
 * Selection and the theme filter live in the URL (`?selected=`, `?theme=`, and now `?project=`), per the
 * navigation audit: a card is a real link, a filtered board is an addressable page, and back means what
 * it should.
 *
 * Writes go through `PATCH /kanban/item`, which touches frontmatter only and regenerates the index —
 * this board never holds state the files do not; a drag is a file edit and a refetch.
 */

const COLUMNS = [
  { status: 'open', label: 'Open' },
  { status: 'doing', label: 'Doing' },
  { status: 'blocked', label: 'Blocked' },
  { status: 'backburner', label: 'Backburner' },
  { status: 'done', label: 'Done' },
] as const;

export const BoardPanel = () => {
  // Read from the URL rather than passed in: this panel and the projects panel are siblings that never
  // speak to each other, per the navigation audit. The selector writes `?project=`; this reads it.
  const { selected: project } = useSelectedProject();
  const { data, isLoading, error } = useTodoTree(project);
  const patch = useTodoPatch(project);
  const [params] = useSearchParams();
  const selected = params.get('selected');
  const themeFilter = params.get('theme');
  const [dragOver, setDragOver] = useState<string | null>(null);

  const href = (changes: Record<string, string | null>): string => {
    const next = new URLSearchParams(params);
    for (const [key, value] of Object.entries(changes)) {
      if (value === null) next.delete(key);
      else next.set(key, value);
    }
    const qs = next.toString();
    return qs ? `?${qs}` : '.';
  };

  if (project === null) {
    return (
      <div className="flex h-full items-center justify-center p-8 text-center text-sm text-muted-foreground">
        Choose a project on the left, or add one.
      </div>
    );
  }

  /*
    THE ERROR, not a guess at it.

    This said "the todo board is owner-only, and this account was refused" — true of the version in the
    platform, where `/todos` was `kind: 'admin'`, and false here, where the permission is grantable. It
    survived the extraction as a sentence describing a rule that no longer exists, and the first real
    failure it met was a 500 from a mis-pointed project, which it reported as a permission problem.

    A wrong explanation is worse than none: it sends somebody to check their account when the answer is
    on screen in the panel beside them.
  */
  if (error) {
    return (
      <div className="flex h-full flex-col items-center justify-center gap-2 p-8 text-center text-sm text-muted-foreground">
        <span>This board could not be read.</span>
        <span className="max-w-md text-xs text-destructive">{errorText(error, 'unknown error')}</span>
      </div>
    );
  }
  if (isLoading || !data) {
    return <div className="flex h-full items-center justify-center text-sm text-muted-foreground">Reading…</div>;
  }

  /*
    No themes at all is almost always the same mistake — the project points at a repository rather than
    at the folder inside it that holds the themes. Said here, where somebody is looking, rather than left
    as an empty board that reads like "no work outstanding".
  */
  if (data.themes.length === 0) {
    return (
      <div className="flex h-full flex-col items-center justify-center gap-2 p-8 text-center text-sm text-muted-foreground">
        <span>No themes in this folder.</span>
        <span className="max-w-md text-xs">
          A theme is a directory with an <code>_index.md</code> in it. If you pointed this at a repository, the folder
          you want is usually <code>todo/</code> inside it.
        </span>
      </div>
    );
  }

  const themes = data.themes.filter((t) => t.items.length > 0);
  const items = themes.flatMap((t) => t.items);
  const visible = themeFilter ? items.filter((i) => i.theme === themeFilter) : items;

  const drop = (status: string) => (ev: DragEvent) => {
    ev.preventDefault();
    setDragOver(null);
    const id = ev.dataTransfer.getData('text/todo-id');
    if (id) patch.mutate({ id, status });
  };

  return (
    <div className="flex h-full min-h-0">
      <div className="flex min-w-0 flex-1 flex-col">
        <div className="flex flex-wrap items-center gap-1.5 border-b border-border px-4 py-2">
          <Link
            to={href({ theme: null })}
            className={cn(
              'rounded-full px-2.5 py-0.5 text-xs',
              !themeFilter
                ? 'bg-primary text-primary-foreground'
                : 'bg-muted text-muted-foreground hover:text-foreground',
            )}
          >
            All · {items.filter((i) => i.status !== 'done').length}
          </Link>
          {themes.map((t) => {
            const open = t.items.filter((i) => i.status !== 'done').length;
            return (
              <Link
                key={t.theme}
                to={href({ theme: t.theme === themeFilter ? null : t.theme })}
                className={cn(
                  'rounded-full px-2.5 py-0.5 text-xs',
                  themeFilter === t.theme
                    ? 'bg-primary text-primary-foreground'
                    : 'bg-muted text-muted-foreground hover:text-foreground',
                )}
              >
                {t.heading.split('—')[0]?.trim()} · {open}
              </Link>
            );
          })}
        </div>

        <div className="flex min-h-0 flex-1 gap-3 overflow-x-auto p-4">
          {COLUMNS.map((col) => {
            const cards = visible
              .filter((i) => i.status === col.status)
              .sort((a, b) => (b.priority === 'urgent' ? 1 : 0) - (a.priority === 'urgent' ? 1 : 0));
            return (
              <div
                key={col.status}
                onDragOver={(ev) => {
                  ev.preventDefault();
                  setDragOver(col.status);
                }}
                onDragLeave={() => setDragOver(null)}
                onDrop={drop(col.status)}
                className={cn(
                  'flex w-64 shrink-0 flex-col rounded-lg border border-border bg-muted/30',
                  dragOver === col.status && 'border-primary bg-primary/5',
                )}
              >
                <div className="flex items-baseline justify-between px-3 py-2 text-xs font-semibold text-muted-foreground">
                  <span>{col.label}</span>
                  <span>{cards.length}</span>
                </div>
                <div className="flex min-h-0 flex-1 flex-col gap-2 overflow-y-auto px-2 pb-2">
                  {cards.map((item) => (
                    <TodoCard
                      key={item.id}
                      item={item}
                      href={href({ selected: item.id })}
                      selected={selected === item.id}
                    />
                  ))}
                </div>
              </div>
            );
          })}
        </div>
      </div>

      {selected ? <TodoDetail id={selected} closeHref={href({ selected: null })} /> : null}
    </div>
  );
};

type TodoCardProps = { item: TodoItem; href: string; selected: boolean };

const TodoCard = ({ item, href, selected }: TodoCardProps) => (
  <Link
    to={href}
    draggable
    onDragStart={(ev) => ev.dataTransfer.setData('text/todo-id', item.id)}
    className={cn(
      'block rounded-md border border-border bg-card p-2.5 text-left shadow-sm hover:border-primary/50',
      selected && 'border-primary',
    )}
  >
    <div className="text-xs leading-snug text-card-foreground">{item.title}</div>
    <div className="mt-1.5 flex flex-wrap items-center gap-1.5 text-[10px] text-muted-foreground">
      <span className="rounded bg-muted px-1 py-px">{item.theme}</span>
      {item.priority === 'urgent' && (
        <span className="flex items-center gap-0.5 rounded bg-destructive/15 px-1 py-px text-destructive">
          <AlertTriangle className="h-2.5 w-2.5" /> urgent
        </span>
      )}
      {item.due && (
        <span className="flex items-center gap-0.5">
          <CalendarClock className="h-2.5 w-2.5" /> {item.due}
        </span>
      )}
      <span>{item.created}</span>
    </div>
  </Link>
);

type TodoDetailProps = { id: string; closeHref: string };

const TodoDetail = ({ id, closeHref }: TodoDetailProps) => {
  // Same source as the board above it. Threading it down as a prop would work equally well; reading the
  // URL keeps the detail pane openable from a link without the board having rendered first.
  const { selected: project } = useSelectedProject();
  const { data, isLoading } = useTodoItem(project, id);
  const patch = useTodoPatch(project);
  const status = data?.meta.status ?? 'open';

  return (
    <div className="flex w-[28rem] shrink-0 flex-col border-l border-border">
      <div className="flex items-center justify-between gap-2 border-b border-border px-4 py-2">
        <span className="truncate font-mono text-xs text-muted-foreground">{id}</span>
        <div className="flex items-center gap-2">
          <select
            value={status}
            onChange={(ev) => patch.mutate({ id, status: ev.target.value })}
            className="rounded border border-border bg-background px-1.5 py-0.5 text-xs"
          >
            {COLUMNS.map((c) => (
              <option key={c.status} value={c.status}>
                {c.label}
              </option>
            ))}
          </select>
          <Link to={closeHref} className="text-muted-foreground hover:text-foreground">
            <X className="h-4 w-4" />
          </Link>
        </div>
      </div>
      <div className="file-viewer-md min-h-0 flex-1 overflow-y-auto px-4 py-3 text-sm">
        {isLoading || !data ? (
          <span className="text-sm text-muted-foreground">Reading…</span>
        ) : (
          <ReactMarkdown remarkPlugins={[remarkGfm]}>{data.body}</ReactMarkdown>
        )}
      </div>
    </div>
  );
};