/**
 * C-015 + P-006/008/009: Gateway Connection Lifecycle
 * 
 * - Register device with Gateway on startup
 * - Heartbeat keepalive every N seconds
 * - Exponential backoff reconnection on disconnect
 * - Connectivity check before registration
 */

import { type DeviceConfig } from "@yuzu-gca/shared";

export interface ConnectionState {
  status: "disconnected" | "connecting" | "connected" | "reconnecting";
  lastHeartbeat: number;
  reconnectAttempt: number;
  registeredAt: number | null;
}

/**
 * Check if the Gateway is reachable.
 * Returns true if a HEAD request to gateway.url succeeds.
 */
export async function checkConnectivity(gatewayUrl: string): Promise<boolean> {
  try {
    const resp = await fetch(`${gatewayUrl}/health`, {
      method: "HEAD",
      signal: AbortSignal.timeout(5000),
    });
    return resp.ok;
  } catch {
    return false;
  }
}

/**
 * Register this device with the Gateway.
 * POST /devices/register with device identity and capabilities.
 * Returns the assigned device token on success.
 */
export async function registerWithGateway(
  gatewayUrl: string,
  config: DeviceConfig,
): Promise<{ token: string } | null> {
  try {
    const resp = await fetch(`${gatewayUrl}/devices/register`, {
      method: "POST",
      headers: { "Content-Type": "application/json" },
      body: JSON.stringify({
        id: config.identity.id,
        name: config.identity.name,
        platform: config.identity.platform,
        arch: config.identity.arch,
        hostname: config.identity.hostname,
        capabilities: config.capabilities,
        transport: config.gateway.transport,
      }),
      signal: AbortSignal.timeout(10000),
    });
    if (!resp.ok) {
      console.error(`[Gateway] Registration failed: HTTP ${resp.status}`);
      return null;
    }
    const data = await resp.json() as { token: string };
    console.log(`[Gateway] Registered as ${config.identity.name} (${config.identity.id})`);
    return { token: data.token };
  } catch (err) {
    console.error(`[Gateway] Registration error:`, (err as Error).message);
    return null;
  }
}

/**
 * Send a heartbeat to the Gateway.
 * POST /devices/:id/heartbeat
 * Returns true if Gateway acknowledged.
 */
export async function sendHeartbeat(
  gatewayUrl: string,
  deviceId: string,
  token: string,
): Promise<boolean> {
  try {
    const resp = await fetch(`${gatewayUrl}/devices/${deviceId}/heartbeat`, {
      method: "POST",
      headers: {
        "Content-Type": "application/json",
        "Authorization": `Bearer ${token}`,
      },
      body: JSON.stringify({ timestamp: Date.now() }),
      signal: AbortSignal.timeout(5000),
    });
    return resp.ok;
  } catch {
    return false;
  }
}

/**
 * Unregister from Gateway.
 * DELETE /devices/:id
 */
export async function unregisterFromGateway(
  gatewayUrl: string,
  deviceId: string,
  token: string,
): Promise<void> {
  try {
    await fetch(`${gatewayUrl}/devices/${deviceId}`, {
      method: "DELETE",
      headers: { "Authorization": `Bearer ${token}` },
      signal: AbortSignal.timeout(5000),
    });
  } catch {
    // Best effort
  }
}

/**
 * Compute exponential backoff delay with jitter.
 */
export function backoffDelay(attempt: number, maxMs: number = 30000): number {
  const base = Math.min(1000 * Math.pow(2, attempt), maxMs);
  const jitter = Math.random() * base * 0.3; // 0-30% jitter
  return base + jitter;
}

/**
 * Connection manager — handles the full lifecycle:
 *   check → register → heartbeat loop → disconnect → reconnect with backoff
 */
export class ConnectionManager {
  private state: ConnectionState = {
    status: "disconnected",
    lastHeartbeat: 0,
    reconnectAttempt: 0,
    registeredAt: null,
  };
  private heartbeatTimer: ReturnType<typeof setInterval> | null = null;
  private token: string = "";
  private stopped: boolean = false;

  constructor(
    private config: DeviceConfig,
    private onStateChange?: (state: ConnectionState) => void,
  ) {}

  get currentState(): ConnectionState { return { ...this.state }; }

  /** Start the connection lifecycle (check → register → heartbeat) */
  async start(): Promise<void> {
    this.stopped = false;
    await this.connect();
  }

  /** Stop everything */
  stop(): void {
    this.stopped = true;
    this.clearHeartbeat();
    if (this.token) {
      unregisterFromGateway(this.config.gateway.url, this.config.identity.id, this.token)
        .catch(() => {});
    }
    this.setState({ status: "disconnected", registeredAt: null });
  }

  private async connect(): Promise<void> {
    if (this.stopped) return;
    this.setState({ status: "connecting" });

    const reachable = await checkConnectivity(this.config.gateway.url);
    if (!reachable) {
      console.warn("[Gateway] Unreachable, retrying...");
      this.setState({ status: "reconnecting" });
      this.scheduleReconnect();
      return;
    }

    const result = await registerWithGateway(this.config.gateway.url, this.config);
    if (!result) {
      this.setState({ status: "reconnecting" });
      this.scheduleReconnect();
      return;
    }

    this.token = result.token;
    this.state.reconnectAttempt = 0;
    this.setState({ status: "connected", registeredAt: Date.now() });
    this.startHeartbeat();
  }

  private startHeartbeat(): void {
    this.clearHeartbeat();
    this.heartbeatTimer = setInterval(async () => {
      if (this.stopped) return;
      const ok = await sendHeartbeat(
        this.config.gateway.url,
        this.config.identity.id,
        this.token,
      );
      this.state.lastHeartbeat = Date.now();
      if (!ok && !this.stopped) {
        console.warn("[Gateway] Heartbeat failed, reconnecting...");
        this.clearHeartbeat();
        this.setState({ status: "reconnecting" });
        this.scheduleReconnect();
      }
    }, this.config.gateway.heartbeatInterval);
  }

  private clearHeartbeat(): void {
    if (this.heartbeatTimer) {
      clearInterval(this.heartbeatTimer);
      this.heartbeatTimer = null;
    }
  }

  private scheduleReconnect(): void {
    if (this.stopped) return;
    const delay = backoffDelay(
      this.state.reconnectAttempt++,
      this.config.gateway.maxReconnectDelay,
    );
    console.log(`[Gateway] Reconnecting in ${Math.round(delay)}ms (attempt ${this.state.reconnectAttempt})`);
    setTimeout(() => this.connect(), delay);
  }

  private setState(partial: Partial<ConnectionState>): void {
    this.state = { ...this.state, ...partial };
    this.onStateChange?.(this.currentState);
  }
}