/**
 * The live stream.
 *
 * The console reads through TanStack Query on a 30-second poll. That is the
 * floor, not the ceiling: a bill rung at 10:00:01 waits until 10:00:30 to
 * appear. This connects to the backend's event stream and, when a till actually
 * sells something, invalidates the queries that just went stale — so the screen
 * updates in about a second instead of averaging fifteen.
 *
 * ── What it does NOT do ──────────────────────────────────────────────────────
 *
 * It never writes data into the cache. An event says "outlet 1185 sold
 * something, products 42 and 77 were involved" and nothing more; the numbers
 * still come from the API, because the API is the only thing that knows the
 * total after the backend's locks, dedup and rejections have had their say.
 * Painting figures straight from a broker message would put totals on screen
 * that the database never agreed to.
 *
 * It also does not replace the poll. Every hook keeps its `refetchInterval`. If
 * this connection drops, the broker is down, or MQTT_URL is unset on the
 * server, the console behaves exactly as it did before — slower, never wrong.
 * That is the whole reason this is safe to add.
 */

import { useEffect } from 'react';
import { useQueryClient } from '@tanstack/react-query';
import { API_BASE } from '@/api/client';
import { queryKeys } from './keys';

/** Mirrors `messaging.LiveEvent` on the backend. */
interface LiveEvent {
  type: 'sale' | 'customer' | 'terminal';
  locationid: number;
  productids?: number[];
  terminalid?: string;
  at: string;
}

/**
 * Watch one outlet.
 *
 * Pass `undefined` for either id and nothing connects — which is what the Store
 * Admin's "All branches" view does, since a stream is per-outlet and opening
 * one per branch would mean six connections for a screen that is already
 * polling. Under All branches the poll remains the only refresh, deliberately.
 */
export function useLiveEvents(tenantid: number | undefined, locationid: number | undefined) {
  const queryClient = useQueryClient();

  useEffect(() => {
    if (!tenantid || !locationid) return;

    const url = `${API_BASE}/live/api/v1/web/live/events?tenantid=${tenantid}&locationid=${locationid}`;
    const source = new EventSource(url);

    /**
     * A sale moved stock and added a bill.
     *
     * Invalidated by prefix rather than by exact key: the product list is keyed
     * by page and the statement by its filters, and this has no idea which page
     * or filter the operator is looking at. `invalidateQueries` with a prefix
     * only refetches what is actually mounted, so the cost is the queries on
     * screen — not every page ever visited.
     */
    const onSale = () => {
      void queryClient.invalidateQueries({ queryKey: queryKeys.stock.all });
      void queryClient.invalidateQueries({ queryKey: queryKeys.products.all });
      void queryClient.invalidateQueries({ queryKey: queryKeys.insights.posSales(locationid) });
      void queryClient.invalidateQueries({
        queryKey: [...queryKeys.insights.all, 'pos-bills', locationid],
      });
      void queryClient.invalidateQueries({
        queryKey: [...queryKeys.insights.all, 'pos-summary', locationid],
      });
    };

    /** A heartbeat — the presence board, and nothing else. */
    const onTerminal = () => {
      void queryClient.invalidateQueries({ queryKey: queryKeys.insights.posHealth(locationid) });
    };

    const onCustomer = () => {
      void queryClient.invalidateQueries({ queryKey: queryKeys.customers.all });
    };

    source.addEventListener('sale', onSale);
    source.addEventListener('terminal', onTerminal);
    source.addEventListener('customer', onCustomer);

    /**
     * Errors are left alone on purpose.
     *
     * `EventSource` reconnects by itself, honouring the `retry:` the server
     * sends, and it fires `error` on every one of those attempts. Logging here
     * fills the console with noise during an ordinary backend deploy, and
     * surfacing it to the operator would be reporting a failure that costs them
     * nothing — the poll is still running underneath.
     */

    return () => {
      source.removeEventListener('sale', onSale);
      source.removeEventListener('terminal', onTerminal);
      source.removeEventListener('customer', onCustomer);
      source.close();
    };
  }, [tenantid, locationid, queryClient]);
}

export type { LiveEvent };