import { existsSync } from 'node:fs';
import path from 'node:path';
import { Logger } from '@nestjs/common';
import fastifyStatic from '@fastify/static';
import type { NestFastifyApplication } from '@nestjs/platform-fastify';

/** Request paths that belong to the backend, never to the SPA fallback. */
const BACKEND_PREFIXES = ['/api', '/mcp', '/socket.io'] as const;

function isBackendPath(url: string): boolean {
  return BACKEND_PREFIXES.some((prefix) => url === prefix || url.startsWith(`${prefix}/`));
}

/**
 * Serve the built web SPA bundle (Phase P5 cutover, #1444).
 *
 * WEB_DIST_DIR unset: SPA serving is disabled — dev runs the Vite dev server,
 * which proxies /api and /socket.io here. WEB_DIST_DIR set but not holding a
 * built bundle: fail at boot, because a gateway configured to serve the UI
 * silently serving 404s is an outage, not a degraded mode.
 *
 * Static files get exact routes (wildcard: false, so nothing shadows the API
 * routes); every other GET/HEAD outside the backend prefixes falls back to
 * index.html so client-side routes deep-link correctly.
 */
export async function mountSpaStatic(app: NestFastifyApplication): Promise<void> {
  const logger = new Logger('SpaStatic');
  const distDir = process.env['WEB_DIST_DIR'];
  if (!distDir) {
    logger.log('WEB_DIST_DIR not set; SPA serving disabled (dev mode uses the Vite dev server)');
    return;
  }

  const root = path.resolve(distDir);
  const indexFile = path.join(root, 'index.html');
  if (!existsSync(indexFile)) {
    throw new Error(`WEB_DIST_DIR is '${distDir}' but '${indexFile}' does not exist`);
  }

  // Default cache semantics: public, max-age=0 with ETag/Last-Modified, so
  // every response revalidates (304 when unchanged). Always correct, including
  // for index.html after a deploy; immutable caching for hashed /assets/ files
  // is a P6 optimization.
  await app.register(
    fastifyStatic as never,
    {
      root,
      wildcard: false,
      index: false,
    } as never,
  );

  // A wildcard route, not setNotFoundHandler: Nest installs its own not-found
  // handler during init and Fastify allows only one. find-my-way matches
  // most-specific-first, so every declared route (API, static files) wins over
  // this catch-all; non-GET unmatched requests keep Fastify's stock 404.
  const fastify = app.getHttpAdapter().getInstance();
  fastify.get('/*', (req, reply) => {
    const url = req.raw.url ?? '';
    if (isBackendPath(url)) {
      // An unknown backend path is an API 404, never the SPA page.
      void reply.code(404).send({
        message: `Route ${req.raw.method ?? 'GET'}:${url} not found`,
        error: 'Not Found',
        statusCode: 404,
      });
      return;
    }
    // sendFile is decorated by @fastify/static; its type augmentation targets
    // a different fastify copy in the pnpm tree than the Nest adapter's.
    (reply as unknown as { sendFile: (file: string) => unknown }).sendFile('index.html');
  });

  logger.log(`Serving SPA bundle from ${root}`);
}