static String modifyQRIS(String qrString, int amount) {
  List<Map<String, dynamic>> fields = _parseEMV(qrString);

  // Remove previous CRC
  fields.removeWhere((f) => f['id'] == '63');

  String amountStr = amount.toString();
  int idx54 = fields.indexWhere((f) => f['id'] == '54');

  if (idx54 != -1) {
    fields[idx54]['value'] = amountStr;
  } else {
    int idx53 = fields.indexWhere((f) => f['id'] == '53');
    int insertAt = idx53 != -1 ? idx53 + 1 : fields.length;

    fields.insert(insertAt, {
      'id': '54',
      'value': amountStr,
    });
  }

  String payload = _buildEMV(fields) + '6304';

  String crc = _crc16ccitt(payload);

  return payload + crc;
}

static String _crc16ccitt(String str) {
  int crc = 0xFFFF;

  for (int i = 0; i < str.length; i++) {
    crc ^= (str.codeUnitAt(i) << 8);

    for (int j = 0; j < 8; j++) {
      if ((crc & 0x8000) != 0) {
        crc = (crc << 1) ^ 0x1021;
      } else {
        crc <<= 1;
      }

      crc &= 0xFFFF;
    }
  }

  return crc
      .toRadixString(16)
      .toUpperCase()
      .padLeft(4, '0');
}