/**
 * @license
 * Copyright 2025 Google LLC
 * SPDX-License-Identifier: Apache-2.0
 */

import type {
  AgentCard,
  Message,
  MessageSendParams,
  Task,
  TaskStatusUpdateEvent,
  TaskArtifactUpdateEvent,
} from '@a2a-js/sdk';
import {
  type Client,
  ClientFactory,
  ClientFactoryOptions,
  DefaultAgentCardResolver,
  RestTransportFactory,
  JsonRpcTransportFactory,
  type AuthenticationHandler,
  createAuthenticatingFetchWithRetry,
} from '@a2a-js/sdk/client';
import { v4 as uuidv4 } from 'uuid';
import { Agent as UndiciAgent, ProxyAgent } from 'undici';
import type { Config } from '../config/config.js';
import { debugLogger } from '../utils/debugLogger.js';
import { safeLookup } from '../utils/fetch.js';
import { classifyAgentError } from './a2a-errors.js';

// Remote agents can take 10+ minutes (e.g. Deep Research).
// Use a dedicated dispatcher so the global 5-min timeout isn't affected.
const A2A_TIMEOUT = 1800000; // 30 minutes

export type SendMessageResult =
  | Message
  | Task
  | TaskStatusUpdateEvent
  | TaskArtifactUpdateEvent;

/**
 * Manages A2A clients and caches loaded agent information.
 * Follows a singleton pattern to ensure a single client instance.
 */
export class A2AClientManager {
  private static instance: A2AClientManager;

  // Each agent should manage their own context/taskIds/card/etc
  private clients = new Map<string, Client>();
  private agentCards = new Map<string, AgentCard>();

  private a2aDispatcher: UndiciAgent | ProxyAgent;
  private a2aFetch: typeof fetch;

  private constructor(config?: Config) {
    const proxyUrl = config?.getProxy();
    const agentOptions = {
      headersTimeout: A2A_TIMEOUT,
      bodyTimeout: A2A_TIMEOUT,
      connect: {
        lookup: safeLookup, // SSRF protection at connection level
      },
    };

    if (proxyUrl) {
      this.a2aDispatcher = new ProxyAgent({
        uri: proxyUrl,
        ...agentOptions,
      });
    } else {
      this.a2aDispatcher = new UndiciAgent(agentOptions);
    }

    this.a2aFetch = (input, init) =>
      // eslint-disable-next-line no-restricted-syntax -- TODO: Migrate to safeFetch for SSRF protection
      fetch(input, { ...init, dispatcher: this.a2aDispatcher } as RequestInit);
  }

  /**
   * Gets the singleton instance of the A2AClientManager.
   */
  static getInstance(config?: Config): A2AClientManager {
    if (!A2AClientManager.instance) {
      A2AClientManager.instance = new A2AClientManager(config);
    }
    return A2AClientManager.instance;
  }

  /**
   * Resets the singleton instance. Only for testing purposes.
   * @internal
   */
  static resetInstanceForTesting() {
    // @ts-expect-error - Resetting singleton for testing
    A2AClientManager.instance = undefined;
  }

  /**
   * Loads an agent by fetching its AgentCard and caches the client.
   * @param name The name to assign to the agent.
   * @param agentCardUrl The full URL to the agent's card.
   * @param authHandler Optional authentication handler to use for this agent.
   * @returns The loaded AgentCard.
   */
  async loadAgent(
    name: string,
    agentCardUrl: string,
    authHandler?: AuthenticationHandler,
  ): Promise<AgentCard> {
    if (this.clients.has(name) && this.agentCards.has(name)) {
      throw new Error(`Agent with name '${name}' is already loaded.`);
    }

    // Authenticated fetch for API calls (transports).
    let authFetch: typeof fetch = this.a2aFetch;
    if (authHandler) {
      authFetch = createAuthenticatingFetchWithRetry(
        this.a2aFetch,
        authHandler,
      );
    }

    // Use unauthenticated fetch for the agent card unless explicitly required.
    // Some servers reject unexpected auth headers on the card endpoint (e.g. 400).
    const cardFetch = async (
      input: RequestInfo | URL,
      init?: RequestInit,
    ): Promise<Response> => {
      // Try without auth first
      const response = await this.a2aFetch(input, init);

      // Retry with auth if we hit a 401/403
      if ((response.status === 401 || response.status === 403) && authFetch) {
        return authFetch(input, init);
      }

      return response;
    };

    const resolver = new DefaultAgentCardResolver({ fetchImpl: cardFetch });

    const options = ClientFactoryOptions.createFrom(
      ClientFactoryOptions.default,
      {
        transports: [
          new RestTransportFactory({ fetchImpl: authFetch }),
          new JsonRpcTransportFactory({ fetchImpl: authFetch }),
        ],
        cardResolver: resolver,
      },
    );

    try {
      const factory = new ClientFactory(options);
      const client = await factory.createFromUrl(agentCardUrl, '');
      const agentCard = await client.getAgentCard();

      this.clients.set(name, client);
      this.agentCards.set(name, agentCard);

      debugLogger.debug(
        `[A2AClientManager] Loaded agent '${name}' from ${agentCardUrl}`,
      );

      return agentCard;
    } catch (error: unknown) {
      throw classifyAgentError(name, agentCardUrl, error);
    }
  }

  /**
   * Invalidates all cached clients and agent cards.
   */
  clearCache(): void {
    this.clients.clear();
    this.agentCards.clear();
    debugLogger.debug('[A2AClientManager] Cache cleared.');
  }

  /**
   * Sends a message to a loaded agent and returns a stream of responses.
   * @param agentName The name of the agent to send the message to.
   * @param message The message content.
   * @param options Optional context and task IDs to maintain conversation state.
   * @returns An async iterable of responses from the agent (Message or Task).
   * @throws Error if the agent returns an error response.
   */
  async *sendMessageStream(
    agentName: string,
    message: string,
    options?: { contextId?: string; taskId?: string; signal?: AbortSignal },
  ): AsyncIterable<SendMessageResult> {
    const client = this.clients.get(agentName);
    if (!client) {
      throw new Error(`Agent '${agentName}' not found.`);
    }

    const messageParams: MessageSendParams = {
      message: {
        kind: 'message',
        role: 'user',
        messageId: uuidv4(),
        parts: [{ kind: 'text', text: message }],
        contextId: options?.contextId,
        taskId: options?.taskId,
      },
    };

    yield* client.sendMessageStream(messageParams, {
      signal: options?.signal,
    });
  }

  /**
   * Retrieves a loaded agent card.
   * @param name The name of the agent.
   * @returns The agent card, or undefined if not found.
   */
  getAgentCard(name: string): AgentCard | undefined {
    return this.agentCards.get(name);
  }

  /**
   * Retrieves a loaded client.
   * @param name The name of the agent.
   * @returns The client, or undefined if not found.
   */
  getClient(name: string): Client | undefined {
    return this.clients.get(name);
  }

  /**
   * Retrieves a task from an agent.
   * @param agentName The name of the agent.
   * @param taskId The ID of the task to retrieve.
   * @returns The task details.
   */
  async getTask(agentName: string, taskId: string): Promise<Task> {
    const client = this.clients.get(agentName);
    if (!client) {
      throw new Error(`Agent '${agentName}' not found.`);
    }
    try {
      return await client.getTask({ id: taskId });
    } catch (error: unknown) {
      const prefix = `A2AClient getTask Error [${agentName}]`;
      if (error instanceof Error) {
        throw new Error(`${prefix}: ${error.message}`, { cause: error });
      }
      throw new Error(`${prefix}: Unexpected error: ${String(error)}`);
    }
  }

  /**
   * Cancels a task on an agent.
   * @param agentName The name of the agent.
   * @param taskId The ID of the task to cancel.
   * @returns The cancellation response.
   */
  async cancelTask(agentName: string, taskId: string): Promise<Task> {
    const client = this.clients.get(agentName);
    if (!client) {
      throw new Error(`Agent '${agentName}' not found.`);
    }
    try {
      return await client.cancelTask({ id: taskId });
    } catch (error: unknown) {
      const prefix = `A2AClient cancelTask Error [${agentName}]`;
      if (error instanceof Error) {
        throw new Error(`${prefix}: ${error.message}`, { cause: error });
      }
      throw new Error(`${prefix}: Unexpected error: ${String(error)}`);
    }
  }
}