import { Router, Request, Response } from 'express';
import { pool } from '../db';
import { asyncRoute } from '../asyncRoute';

const router = Router();

// Postgres unique-violation SQLSTATE — raised by the two partial indexes that
// stop siblings sharing a name.
const UNIQUE_VIOLATION = '23505';

// Walks down from a node, collecting it and every descendant. Used both for
// cycle detection on reparent and for reporting the blast radius of a delete.
const SUBTREE_CTE = `
  WITH RECURSIVE subtree AS (
    SELECT id FROM categories WHERE id = $1
    UNION ALL
    SELECT c.id FROM categories c JOIN subtree s ON c.parent_id = s.id
  )`;

function readName(value: unknown): string | null {
  if (typeof value !== 'string') return null;
  const trimmed = value.trim();
  return trimmed === '' ? null : trimmed;
}

// Distinguishes "not supplied" from "explicitly cleared to root".
function readParentId(value: unknown): number | null | undefined {
  if (value === undefined) return undefined;
  if (value === null || value === '') return null;
  const parsed = typeof value === 'number' ? value : Number(value);
  if (!Number.isSafeInteger(parsed) || parsed < 1) return undefined;
  return parsed;
}

async function parentExists(id: number): Promise<boolean> {
  const { rows } = await pool.query(`SELECT 1 FROM categories WHERE id = $1`, [id]);
  return rows.length > 0;
}

router.get('/', asyncRoute(async (_req: Request, res: Response) => {
  const { rows } = await pool.query(
    `SELECT c.id, c.name, c.parent_id, c.sort_order,
            (SELECT COUNT(*)::int FROM items i WHERE i.category_id = c.id) AS item_count
     FROM categories c
     ORDER BY c.sort_order, lower(c.name)`
  );
  res.json(rows);
}));

router.post('/', asyncRoute(async (req: Request, res: Response) => {
  const name = readName(req.body.name);
  if (!name) {
    return res.status(400).json({ error: 'name is required' });
  }

  const parentId = readParentId(req.body.parent_id);
  if (parentId === undefined && req.body.parent_id !== undefined) {
    return res.status(400).json({ error: 'invalid parent_id' });
  }
  const parent = parentId ?? null;
  if (parent !== null && !(await parentExists(parent))) {
    return res.status(400).json({ error: 'parent category does not exist' });
  }

  const sortOrder = Number.isSafeInteger(req.body.sort_order) ? req.body.sort_order : 0;

  try {
    const { rows } = await pool.query(
      `INSERT INTO categories (name, parent_id, sort_order) VALUES ($1, $2, $3)
       RETURNING id, name, parent_id, sort_order`,
      [name, parent, sortOrder]
    );
    res.status(201).json({ ...rows[0], item_count: 0 });
  } catch (err) {
    if ((err as { code?: string }).code === UNIQUE_VIOLATION) {
      return res.status(409).json({ error: 'a category with that name already exists here' });
    }
    throw err;
  }
}));

// Works out what parent_id an update should land on. Absent means "leave it
// alone", so the current value is echoed back rather than treated as a clear.
// Returns the refusal instead of sending it, keeping the response the
// handler's business and the two ways a parent can be invalid out of its body.
type ParentResolution = { error: string } | { parent: number | null };

async function resolveParentId(
  submitted: unknown,
  id: number,
  current: number | null
): Promise<ParentResolution> {
  if (submitted === undefined) {
    return { parent: current };
  }

  const parsed = readParentId(submitted);
  if (parsed === undefined) {
    return { error: 'invalid parent_id' };
  }
  if (parsed === null) {
    return { parent: null };
  }
  if (!(await parentExists(parsed))) {
    return { error: 'parent category does not exist' };
  }

  // Moving a node beneath itself or one of its own descendants would detach
  // that whole branch from the tree into an unreachable cycle.
  const { rows: cycle } = await pool.query(
    `${SUBTREE_CTE} SELECT 1 FROM subtree WHERE id = $2`,
    [id, parsed]
  );
  if (cycle.length) {
    return { error: 'a category cannot be moved beneath itself' };
  }

  return { parent: parsed };
}

router.put('/:id', asyncRoute(async (req: Request, res: Response) => {
  const id = Number(req.params.id);
  const existing = await pool.query(`SELECT id, name, parent_id, sort_order FROM categories WHERE id = $1`, [id]);
  if (!existing.rows.length) {
    return res.status(404).json({ error: 'not found' });
  }

  let name = existing.rows[0].name;
  if (req.body.name !== undefined) {
    const parsed = readName(req.body.name);
    if (!parsed) {
      return res.status(400).json({ error: 'name is required' });
    }
    name = parsed;
  }

  const resolved = await resolveParentId(req.body.parent_id, id, existing.rows[0].parent_id);
  if ('error' in resolved) {
    return res.status(400).json({ error: resolved.error });
  }
  const parent = resolved.parent;

  const sortOrder = Number.isSafeInteger(req.body.sort_order)
    ? req.body.sort_order
    : existing.rows[0].sort_order;

  try {
    const { rows } = await pool.query(
      `UPDATE categories SET name = $1, parent_id = $2, sort_order = $3 WHERE id = $4
       RETURNING id, name, parent_id, sort_order`,
      [name, parent, sortOrder, id]
    );
    res.json(rows[0]);
  } catch (err) {
    if ((err as { code?: string }).code === UNIQUE_VIOLATION) {
      return res.status(409).json({ error: 'a category with that name already exists here' });
    }
    throw err;
  }
}));

router.delete('/:id', asyncRoute(async (req: Request, res: Response) => {
  const id = Number(req.params.id);
  const { rows: subtree } = await pool.query(`${SUBTREE_CTE} SELECT id FROM subtree`, [id]);
  if (!subtree.length) {
    return res.status(404).json({ error: 'not found' });
  }

  const ids = subtree.map((row: { id: number }) => row.id);
  const { rows: affected } = await pool.query(
    `SELECT COUNT(*)::int AS n FROM items WHERE category_id = ANY($1::int[])`,
    [ids]
  );

  // The FK cascade takes the descendants; items fall back to NULL rather than
  // being deleted along with their category.
  await pool.query(`DELETE FROM categories WHERE id = $1`, [id]);

  res.json({ deleted_categories: ids.length, uncategorized_items: affected[0].n });
}));

export default router;