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

import assert from 'assert';
import * as fs from 'fs';
import * as http from 'http';
import * as path from 'path';
import { OAuth2Client } from 'google-auth-library';
import vscode from 'vscode';
import { CONFIG } from '../../colab-config';
import { log } from '../../common/logging';
import { LoopbackHandler, LoopbackServer } from '../../common/loopback-server';
import { CodeManager } from '../code-manager';
import {
  DEFAULT_AUTH_URL_OPTS,
  OAuth2Flow,
  OAuth2TriggerOptions,
  FlowResult,
} from './flows';

/**
 * An OAuth2 flow that uses a local server to handle the redirect URI.
 *
 * Since the flow when triggered spins up a local server to handle the redirect,
 * the {@link FlowResult} returns a `dispose` method. This is needed since the
 * endpoint should continue to serve assets like the favicon after the initial
 * trigger. Only when the login flow is complete are we "done" with the server.
 *
 * Since it's possible we'd want to dispose this class while there are in-flight
 * triggered flows, the {@link LocalServerFlow} is disposable and will clean-up
 * any owned servers from outstanding flows.
 */
export class LocalServerFlow implements OAuth2Flow, vscode.Disposable {
  private readonly codeManager = new CodeManager();
  private readonly handler: Handler;
  private readonly activeServers = new Set<vscode.Disposable>();

  constructor(
    private readonly vs: typeof vscode,
    private readonly serveRoot: string,
    private readonly oAuth2Client: OAuth2Client,
    extensionUri: string,
  ) {
    this.handler = new Handler(
      vs,
      this.serveRoot,
      this.codeManager,
      extensionUri,
    );
  }

  dispose() {
    this.codeManager.dispose();
    for (const disposable of this.activeServers) {
      disposable.dispose();
    }
    this.activeServers.clear();
  }

  /**
   * Trigger the OAuth2 flow by opening a local server to listen for the
   * redirect URI. Callers are expected to dispose the returned disposable when
   * the full flow is complete. It is their responsibility since this flow is
   * expected to serve assets until fully completed (for e.g., the favicon).
   */
  async trigger(options: OAuth2TriggerOptions): Promise<FlowResult> {
    const server = new LoopbackServer(this.handler);
    this.activeServers.add(server);
    try {
      const code = this.codeManager.waitForCode(options.nonce, options.cancel);
      options.cancel.onCancellationRequested(server.dispose.bind(server));
      const port = await server.start();
      const address = `http://127.0.0.1:${port.toString()}`;
      const authUrl = this.oAuth2Client.generateAuthUrl({
        ...DEFAULT_AUTH_URL_OPTS,
        redirect_uri: address,
        state: `nonce=${options.nonce}`,
        scope: options.scopes,
        code_challenge: options.pkceChallenge,
      });

      await this.vs.env.openExternal(this.vs.Uri.parse(authUrl));
      // TODO: We can't dispose of the server immediately since the favicon is
      // loaded asynchronously. Following a successful flow result (here), we
      // should TTL disposing of the server. It'll get cleaned up when the flow
      // is disposed, so not crucial to add now.
      return {
        code: await code,
        redirectUri: address,
      };
    } catch (err: unknown) {
      server.dispose();
      this.activeServers.delete(server);
      throw err;
    }
  }
}

class Handler implements LoopbackHandler {
  constructor(
    private readonly vs: typeof vscode,
    private readonly serveRoot: string,
    private readonly codeProvider: CodeManager,
    private readonly extensionUri: string,
  ) {}

  handleRequest(req: http.IncomingMessage, res: http.ServerResponse): void {
    // URL and Host are only missing on malformed requests.
    assert(req.url);
    assert(req.headers.host);
    const url = new URL(req.url, `http://${req.headers.host}`);
    if (req.method !== 'GET') {
      res.writeHead(405, { Allow: 'GET' });
      res.end('Method Not Allowed');
      return;
    }
    switch (url.pathname) {
      case '/': {
        const state = url.searchParams.get('state');
        if (!state) {
          throw new Error('Missing state in redirect URL');
        }
        const parsedState = new URLSearchParams(state);
        const nonce = parsedState.get('nonce');
        const code = url.searchParams.get('code');
        if (!nonce || !code) {
          throw new Error('Missing nonce or code in redirect URI');
        }
        this.codeProvider.resolveCode(nonce, code);

        void this.redirectSuccessfulAuth(res).catch((err: unknown) => {
          log.error('Unable to redirect the successful auth request', err);
        });
        break;
      }
      case '/favicon.ico': {
        const assetPath = url.pathname.substring(1);
        sendFile(res, path.join(this.serveRoot, assetPath));
        break;
      }
      default: {
        log.warn('Received unhandled request', req);
        res.writeHead(404);
        res.end('Not Found');
        break;
      }
    }
  }

  async redirectSuccessfulAuth(res: http.ServerResponse): Promise<void> {
    const authSuccessUri = await this.vs.env.asExternalUri(
      this.vs.Uri.parse(`${this.extensionUri}/auth-success`),
    );
    const successState = encodeURIComponent(authSuccessUri.toString());
    const redirectUri = `${CONFIG.ColabApiDomain}/vscode/auth-success?state=${successState}`;
    // Since we need to handle the request asynchronously, it's technically
    // possible that the response has already been closed by time we get here.
    // This is not foreseen to ever happen, under normal network conditions. In
    // that case, just no-op.
    if (res.headersSent) {
      return;
    }
    res.writeHead(302, { Location: redirectUri });
    res.end();
  }
}

function sendFile(res: http.ServerResponse, filepath: string): void {
  fs.readFile(filepath, (err, body) => {
    if (err) {
      log.error(`Unable to read file: ${filepath}`, err);
      res.writeHead(500);
      res.end('Internal Server Error');
    } else {
      res.setHeader('content-length', body.length);
      res.writeHead(200);
      res.end(body);
    }
  });
}