const http = require('http');
const fs = require('fs');
const path = require('path');

const hostname = process.env.IMAGE_HOST || '127.0.0.1';
const port = Number.parseInt(process.env.IMAGE_PORT || '3000', 10);
const directoryPath = path.resolve(__dirname, '../images');
const maxCacheBytes = Number.parseInt(process.env.IMAGE_CACHE_MAX_MB || '128', 10) * 1024 * 1024;
const maxCacheItemBytes =
  Number.parseInt(process.env.IMAGE_CACHE_ITEM_MAX_MB || '4', 10) * 1024 * 1024;
const browserCacheSeconds = Number.parseInt(
  process.env.IMAGE_BROWSER_CACHE_SECONDS || '3600',
  10,
);

const mimeTypes = {
  '.avif': 'image/avif',
  '.bmp': 'image/bmp',
  '.gif': 'image/gif',
  '.ico': 'image/x-icon',
  '.jpeg': 'image/jpeg',
  '.jpg': 'image/jpeg',
  '.png': 'image/png',
  '.svg': 'image/svg+xml',
  '.webp': 'image/webp',
};

/** @type {Map<string, { data: Buffer, contentType: string, etag: string, modified: string }>} */
const memoryCache = new Map();
/** @type {Map<string, Promise<{ data: Buffer, contentType: string, etag: string, modified: string }>>} */
const pendingReads = new Map();
let cachedBytes = 0;

function removeCacheEntry(filePath) {
  const entry = memoryCache.get(filePath);
  if (!entry) return;
  cachedBytes -= entry.data.byteLength;
  memoryCache.delete(filePath);
}

function cacheEntry(filePath, entry) {
  if (entry.data.byteLength > maxCacheItemBytes || maxCacheBytes <= 0) return;

  removeCacheEntry(filePath);
  while (cachedBytes + entry.data.byteLength > maxCacheBytes && memoryCache.size) {
    const oldestPath = memoryCache.keys().next().value;
    if (!oldestPath) break;
    removeCacheEntry(oldestPath);
  }

  memoryCache.set(filePath, entry);
  cachedBytes += entry.data.byteLength;
}

function getCachedEntry(filePath) {
  const entry = memoryCache.get(filePath);
  if (!entry) return undefined;
  memoryCache.delete(filePath);
  memoryCache.set(filePath, entry);
  return entry;
}

async function readImage(filePath) {
  const cached = getCachedEntry(filePath);
  if (cached) return cached;

  const pending = pendingReads.get(filePath);
  if (pending) return pending;

  const readPromise = (async () => {
    const handle = await fs.promises.open(filePath, 'r');
    try {
      const stat = await handle.stat();
      if (!stat.isFile()) {
        const error = new Error('Not a file');
        error.code = 'ENOENT';
        throw error;
      }

      const data = await handle.readFile();
      const entry = {
        data,
        contentType: mimeTypes[path.extname(filePath).toLowerCase()] || 'application/octet-stream',
        etag: `W/"${stat.size.toString(16)}-${Math.trunc(stat.mtimeMs).toString(16)}"`,
        modified: stat.mtime.toUTCString(),
      };
      cacheEntry(filePath, entry);
      return entry;
    } finally {
      await handle.close();
    }
  })();

  pendingReads.set(filePath, readPromise);
  try {
    return await readPromise;
  } finally {
    pendingReads.delete(filePath);
  }
}

function resolveImagePath(requestUrl) {
  let pathname;
  try {
    pathname = decodeURIComponent(new URL(requestUrl, 'http://localhost').pathname);
  } catch {
    return undefined;
  }

  const relativePath = pathname.replace(/^[/\\]+/, '');
  const filePath = path.resolve(directoryPath, relativePath);
  const relativeToRoot = path.relative(directoryPath, filePath);
  if (relativeToRoot.startsWith('..') || path.isAbsolute(relativeToRoot)) return undefined;
  return filePath;
}

function sendText(res, statusCode, message) {
  res.writeHead(statusCode, {
    'Content-Type': 'text/plain; charset=utf-8',
    'Content-Length': Buffer.byteLength(message),
  });
  res.end(message);
}

const ImageServer = http.createServer(async (req, res) => {
  if (req.method !== 'GET' && req.method !== 'HEAD') {
    res.setHeader('Allow', 'GET, HEAD');
    sendText(res, 405, 'Method not allowed');
    return;
  }

  const filePath = resolveImagePath(req.url || '/');
  if (!filePath) {
    sendText(res, 400, 'Invalid image path');
    return;
  }

  try {
    const entry = await readImage(filePath);
    const headers = {
      'Content-Type': entry.contentType,
      'Content-Length': entry.data.byteLength,
      'Cache-Control': `public, max-age=${browserCacheSeconds}`,
      ETag: entry.etag,
      'Last-Modified': entry.modified,
      'X-Content-Type-Options': 'nosniff',
    };

    if (req.headers['if-none-match'] === entry.etag) {
      res.writeHead(304, headers);
      res.end();
      return;
    }

    res.writeHead(200, headers);
    res.end(req.method === 'HEAD' ? undefined : entry.data);
  } catch (error) {
    if (error && (error.code === 'ENOENT' || error.code === 'EISDIR')) {
      sendText(res, 404, 'Image not found');
      return;
    }
    console.error(`Failed to serve ${filePath}:`, error);
    sendText(res, 500, 'Image server error');
  }
});

ImageServer.keepAliveTimeout = 65_000;
ImageServer.headersTimeout = 66_000;

ImageServer.listen(port, hostname, () => {
  console.log(`Image server running at http://${hostname}:${port}/`);
  console.log(
    `Memory cache: ${Math.round(maxCacheBytes / 1024 / 1024)} MB, browser cache: ${browserCacheSeconds}s`,
  );
  console.log('Press CTRL+C to stop the server');
});

exports.ImageServer = ImageServer;