/**
 * Native Messaging protocol: Chrome/Firefox spec.
 * - 4-byte little-endian unsigned 32-bit length prefix
 * - UTF-8 JSON payload
 * - Max 1 MB message from native host to browser
 */

const MAX_MESSAGE_LENGTH = 1024 * 1024; // 1 MB

/**
 * Create a messenger that reads length-prefixed JSON from input and writes to output.
 * @param {object} options
 * @param {import('stream').Readable} options.input - e.g. process.stdin
 * @param {import('stream').Writable} options.output - e.g. process.stdout
 * @param {function(object): void} options.onMessage - called with parsed JSON message
 * @param {function(Error): void} [options.onError] - called on parse/read errors
 */
function createMessenger({ input, output, onMessage, onError }) {
  let buffer = Buffer.alloc(0);
  let needed = 4; // first read: 4-byte length
  let readingLength = true;

  function fail(err) {
    if (onError) onError(err);
    else console.error(err);
  }

  function processChunk(chunk) {
    if (!chunk || chunk.length === 0) return;
    buffer = Buffer.concat([buffer, chunk]);

    while (buffer.length >= needed) {
      const slice = buffer.subarray(0, needed);
      buffer = buffer.subarray(needed);

      if (readingLength) {
        readingLength = false;
        needed = slice.readUInt32LE(0);
        if (needed > MAX_MESSAGE_LENGTH) {
          fail(new Error(`Message length ${needed} exceeds max ${MAX_MESSAGE_LENGTH}`));
          return;
        }
        if (needed === 0) {
          readingLength = true;
          needed = 4;
        }
        continue;
      }

      try {
        const msg = JSON.parse(slice.toString('utf8'));
        onMessage(msg);
      } catch (e) {
        fail(e);
      }

      readingLength = true;
      needed = 4;
    }
  }

  input.on('data', processChunk);
  input.on('error', fail);
  input.resume?.();

  return {
    /**
     * Send a message to the browser (length prefix + JSON).
     * @param {object} msg - JSON-serializable object
     */
    send(msg) {
      try {
        const json = JSON.stringify(msg);
        const buf = Buffer.from(json, 'utf8');
        const len = Buffer.allocUnsafe(4);
        len.writeUInt32LE(buf.length, 0);
        output.write(len);
        output.write(buf);
      } catch (e) {
        fail(e);
      }
    },

    destroy() {
      input.removeAllListeners('data');
      input.removeAllListeners('error');
    },
  };
}

module.exports = { createMessenger, MAX_MESSAGE_LENGTH };