// AK-P3 (AD-28/S5) — shared SSE subscription for in-app notifications.
// Hosts (routes/NavBar) own this hook and inject the resulting items into the
// presentational screens as props; screens themselves never subscribe (DoD A.5).
// Stream records are mapped through the same pure VM mapper as loader data so
// live updates and initial payloads share one shape (deeplink, bounce keys).

import { useEffect, useRef, useState } from "react";
import {
  mergeNotificationCenterItems,
  toNotificationCenterItemVM,
  NOTIFICATION_CENTER_MAX_ITEMS,
  type NotificationCenterItemVM,
} from "@hivegrid/view-models";

const RECONNECT_DELAY_MS = 1_000;
const MAX_RECONNECT_DELAY_MS = 30_000;
/**
 * A connection only counts as healthy once it survived this long. Resetting the
 * backoff on every `open`/message meant a stream that dropped right after the
 * snapshot reconnected once per second forever instead of backing off.
 */
const STABLE_CONNECTION_MS = 15_000;

type StreamPayload = {
  cursor?: unknown;
  id?: unknown;
  items?: unknown;
  deliveries?: unknown;
  item?: unknown;
  delivery?: unknown;
  notification?: unknown;
};

function serializeDateField(value: unknown): string | null {
  if (value instanceof Date) return value.toISOString();
  return typeof value === "string" && value.trim().length > 0 ? value : null;
}

function normalizeStreamItem(value: unknown): NotificationCenterItemVM | null {
  if (!value || typeof value !== "object") return null;
  const record = value as Record<string, unknown>;
  if (typeof record.id !== "string" || record.id.trim().length === 0) return null;
  return toNotificationCenterItemVM({
    id: record.id,
    messageKey: typeof record.messageKey === "string" ? record.messageKey : "notification.delivery",
    category: typeof record.category === "string" ? record.category : "notification",
    channel: typeof record.channel === "string" ? record.channel : "IN_APP",
    status: typeof record.status === "string" ? record.status : "PENDING",
    attemptsMade: typeof record.attemptsMade === "number" && Number.isFinite(record.attemptsMade) ? record.attemptsMade : 0,
    createdAtIso: serializeDateField(record.createdAt),
    updatedAtIso: serializeDateField(record.updatedAt),
    sentAtIso: serializeDateField(record.sentAt),
    readAtIso: serializeDateField(record.readAt),
    lastError: typeof record.lastError === "string" ? record.lastError : null,
    payloadMetadata: typeof record.payloadMetadata === "object" ? record.payloadMetadata : undefined,
    deeplink: typeof record.deeplink === "string" ? record.deeplink : null,
  });
}

function getPayloadItems(payload: StreamPayload): NotificationCenterItemVM[] {
  const snapshotItems = Array.isArray(payload.items) ? payload.items : Array.isArray(payload.deliveries) ? payload.deliveries : null;
  if (snapshotItems) return snapshotItems.map(normalizeStreamItem).filter((item): item is NotificationCenterItemVM => Boolean(item));
  const singleItem = normalizeStreamItem(payload.item ?? payload.delivery ?? payload.notification ?? payload);
  return singleItem ? [singleItem] : [];
}

function getEventCursor(event: MessageEvent, payload: StreamPayload): string | null {
  if (typeof payload.cursor === "string" && payload.cursor.trim().length > 0) return payload.cursor;
  if (typeof payload.id === "string" && payload.id.trim().length > 0) return payload.id;
  return event.lastEventId?.trim() ? event.lastEventId : null;
}

function parseStreamEvent(event: MessageEvent): { cursor: string | null; items: NotificationCenterItemVM[] } | null {
  try {
    const payload = JSON.parse(event.data || "{}");
    if (!payload || typeof payload !== "object") return null;
    return { cursor: getEventCursor(event, payload as StreamPayload), items: getPayloadItems(payload as StreamPayload) };
  } catch {
    return null;
  }
}

function buildReconnectUrl(streamUrl: string, cursor: string | null) {
  if (!cursor) return streamUrl;
  const url = new URL(streamUrl, window.location.origin);
  url.searchParams.set("afterCursor", cursor);
  return url.toString();
}

export type NotificationsStreamState = {
  items: NotificationCenterItemVM[];
  unreadCount: number;
  connectionStatusKey: string;
};

export function useNotificationsStream(
  streamUrl: string | null | undefined,
  initialItems: readonly NotificationCenterItemVM[]
): NotificationsStreamState {
  const [items, setItems] = useState<NotificationCenterItemVM[]>(() => initialItems.slice(0, NOTIFICATION_CENTER_MAX_ITEMS));
  const [connectionStatusKey, setConnectionStatusKey] = useState(
    streamUrl ? "notifications.center.realtime.connecting" : "notifications.center.realtime.unavailable"
  );
  const cursorRef = useRef<string | null>(null);
  const reconnectTimerRef = useRef<number | null>(null);
  const reconnectDelayRef = useRef(RECONNECT_DELAY_MS);
  const connectedAtRef = useRef<number | null>(null);

  useEffect(() => {
    // Revalidated loader data merges over the live state (never truncates it).
    // merge(current, incoming): incoming wins per id — the fresh loader data
    // must be the incoming side, otherwise stale live state shadows it.
    setItems((currentItems) => mergeNotificationCenterItems(currentItems, initialItems.slice(0, NOTIFICATION_CENTER_MAX_ITEMS)));
  }, [initialItems]);

  useEffect(() => {
    if (!streamUrl || typeof window === "undefined" || typeof window.EventSource === "undefined") return;
    const activeStreamUrl = streamUrl;
    let closed = false;
    let source: EventSource | null = null;
    function clearReconnectTimer() {
      if (reconnectTimerRef.current) {
        window.clearTimeout(reconnectTimerRef.current);
        reconnectTimerRef.current = null;
      }
    }
    function connect() {
      clearReconnectTimer();
      connectedAtRef.current = null;
      source = new EventSource(buildReconnectUrl(activeStreamUrl, cursorRef.current));
      setConnectionStatusKey("notifications.center.realtime.connecting");
      function handleMessage(event: MessageEvent) {
        const parsed = parseStreamEvent(event);
        if (!parsed) return;
        if (parsed.cursor && parsed.items.length > 0) cursorRef.current = parsed.cursor;
        setItems((currentItems) => mergeNotificationCenterItems(currentItems, parsed.items));
        setConnectionStatusKey("notifications.center.realtime.connected");
      }
      function handleOpen() {
        connectedAtRef.current = Date.now();
        setConnectionStatusKey("notifications.center.realtime.connected");
      }
      function handleError() {
        if (closed) return;
        source?.close();
        setConnectionStatusKey("notifications.center.realtime.reconnecting");
        // Only a connection that actually held counts as healthy. Otherwise a
        // stream that dies right after opening keeps resetting the backoff and
        // reconnects once per second forever.
        const openedAt = connectedAtRef.current;
        if (openedAt !== null && Date.now() - openedAt >= STABLE_CONNECTION_MS) {
          reconnectDelayRef.current = RECONNECT_DELAY_MS;
        }
        connectedAtRef.current = null;
        const reconnectDelay = reconnectDelayRef.current;
        reconnectDelayRef.current = Math.min(reconnectDelay * 2, MAX_RECONNECT_DELAY_MS);
        reconnectTimerRef.current = window.setTimeout(connect, reconnectDelay);
      }
      source.addEventListener("open", handleOpen);
      source.addEventListener("notification.snapshot", handleMessage);
      source.addEventListener("notification.update", handleMessage);
      source.addEventListener("error", handleError);
    }
    connect();
    return () => {
      closed = true;
      clearReconnectTimer();
      source?.close();
    };
  }, [streamUrl]);

  return {
    items,
    unreadCount: items.filter((item) => !item.isRead).length,
    connectionStatusKey,
  };
}