import { useContext, useEffect, useRef, useState, useSyncExternalStore } from "react";
import { streamReconnected } from "../lib/buildWatch";
import { AuthCtx } from "./context";
import { isScreen, SCREEN_LABELS } from "./roles";
import type { PresenceUser } from "./types";

interface EventHandlers {
  /** Called when the server revokes this browser's own session. */
  onRevoked: () => void;
  /** Called for every `changed` event. Later phases refresh their data from it. */
  onChanged?: (payload: unknown) => void;
  /** Phase 8: this browser's own session was locked or unlocked, here or in another tab. */
  onLockChange?: (action: "locked" | "unlocked") => void;
}

export interface EventsOptions {
  /** The screen in view (the first path segment when it is a screen). The server shows it as the role. */
  screen?: string | null;
  /** While locked the stream stays open but takes no new screen or packet. */
  locked?: boolean;
}

/** Phase 5: presence carries the packet each user has open. */
export interface PacketPresenceUser extends PresenceUser {
  packet_id?: number | string | null;
}

function parse(data: unknown): unknown {
  if (typeof data !== "string") return null;
  try {
    return JSON.parse(data) as unknown;
  } catch {
    return null;
  }
}

/*
 * The packet this browser is looking at. The packet page sets it (usePacketPresence) and the one EventSource
 * reopens with ?packet_id= so the server's presence knows it. A module store keeps AuthProvider unchanged.
 */
let focusPacket: number | null = null;
const focusListeners = new Set<() => void>();

export function setPresencePacket(id: number | null): void {
  if (focusPacket === id) return;
  focusPacket = id;
  for (const fn of focusListeners) fn();
}

function subscribeFocus(fn: () => void): () => void {
  focusListeners.add(fn);
  return () => {
    focusListeners.delete(fn);
  };
}

const getFocus = () => focusPacket;

/** The stream URL for the screen and the packet in view. */
export function eventsUrl(packetId: number | null, screen: string | null = null): string {
  const sp = new URLSearchParams();
  if (screen) sp.set("screen", screen);
  if (packetId) sp.set("packet_id", String(packetId));
  const q = sp.toString();
  return q ? `/api/events?${q}` : "/api/events";
}

/** The screen of a path: its first segment when that is a screen. */
export function screenOf(pathname: string): string | null {
  const seg = pathname.split("/")[1];
  return isScreen(seg) ? seg : null;
}

/**
 * Opens /api/events with EventSource while `sessionId` is set.
 * A `session` event with this session id and action "revoked" calls onRevoked.
 * `presence` is kept in state. The stream reopens when the packet in view changes.
 */
export function useEvents(sessionId: string | null, handlers: EventHandlers, opts: EventsOptions = {}): PresenceUser[] {
  const [presence, setPresence] = useState<PresenceUser[]>([]);
  const focus = useSyncExternalStore(subscribeFocus, getFocus, getFocus);
  const ref = useRef(handlers);
  useEffect(() => {
    ref.current = handlers;
  });
  // While locked the stream keeps what it had: nothing new is sent about where this browser is.
  const [held, setHeld] = useState<{ packetId: number | null; screen: string | null }>({ packetId: focus, screen: opts.screen ?? null });
  const wantScreen = opts.screen ?? null;
  if (!opts.locked && (held.packetId !== focus || held.screen !== wantScreen)) setHeld({ packetId: focus, screen: wantScreen });
  const { packetId, screen } = held;

  useEffect(() => {
    if (!sessionId || typeof EventSource === "undefined") return;
    const es = new EventSource(eventsUrl(packetId, screen), { withCredentials: true });

    // A second open on the same EventSource is a reconnect: the server may have restarted on a new build.
    let opened = false;
    es.addEventListener("open", () => {
      if (opened) streamReconnected();
      opened = true;
    });

    es.addEventListener("session", (ev) => {
      const p = parse((ev as MessageEvent).data);
      if (p && typeof p === "object") {
        const o = p as { session_id?: unknown; action?: unknown };
        if (o.session_id !== sessionId) return;
        if (o.action === "revoked") ref.current.onRevoked();
        else if (o.action === "locked" || o.action === "unlocked") ref.current.onLockChange?.(o.action);
      }
    });
    es.addEventListener("presence", (ev) => {
      const p = parse((ev as MessageEvent).data);
      if (p && typeof p === "object" && Array.isArray((p as { users?: unknown }).users)) {
        setPresence((p as { users: PresenceUser[] }).users);
      }
    });
    es.addEventListener("changed", (ev) => {
      ref.current.onChanged?.(parse((ev as MessageEvent).data));
    });
    // Phase 3: the worker finished a feed run. Fanned out with the changed events, marked event "feed".
    es.addEventListener("feed", (ev) => {
      const p = parse((ev as MessageEvent).data);
      ref.current.onChanged?.({ ...(p && typeof p === "object" ? p : {}), event: "feed" });
    });

    return () => {
      es.close();
      setPresence([]);
    };
  }, [sessionId, packetId, screen]);

  return presence;
}

/** The other people on `packetId`, from the presence list. One entry per user (one with a role first), sorted by name. */
export function othersOn(users: readonly PacketPresenceUser[], packetId: number, myId: string | null | undefined): PacketPresenceUser[] {
  const byUser = new Map<string, PacketPresenceUser>();
  for (const u of users) {
    if (u.packet_id === null || u.packet_id === undefined || Number(u.packet_id) !== packetId) continue;
    if (myId && u.user_id === myId) continue;
    const had = byUser.get(u.user_id);
    if (!had || (!presenceRole(had) && presenceRole(u))) byUser.set(u.user_id, u);
  }
  return [...byUser.values()].sort((a, b) => a.display_name.localeCompare(b.display_name));
}

/** The role shown next to a name: the server's role label, else the label of the screen the person has open. */
export function presenceRole(u: Pick<PresenceUser, "role" | "screen">): string {
  if (u.role) return u.role;
  return u.screen && isScreen(u.screen) ? SCREEN_LABELS[u.screen] : "";
}

/** "Also in this packet now: Andy Welder (Weld Stall) · Wes Super (Supervisor)". Empty when alone. */
export function presenceText(others: readonly PresenceUser[]): string {
  if (!others.length) return "";
  const names = others.map((u) => {
    const r = presenceRole(u);
    return r ? `${u.display_name} (${r})` : u.display_name;
  });
  return `Also in this packet now: ${names.join(" · ")}`;
}

/**
 * Marks `packetId` as the packet in view while mounted, and returns the other people looking at it.
 * Works under AuthProvider; outside it (a bare test render) it returns an empty list.
 */
export function usePacketPresence(packetId: number | null): PacketPresenceUser[] {
  const ctx = useContext(AuthCtx);
  useEffect(() => {
    if (!packetId) return;
    setPresencePacket(packetId);
    return () => setPresencePacket(null);
  }, [packetId]);
  if (!ctx || !packetId) return [];
  return othersOn(ctx.presence as PacketPresenceUser[], packetId, ctx.me?.user.id);
}