/**
 * Computes the checksum of a SDS011 message.
 *
 * This checksum is the low 8 bits of the sum of the data bytes.
 * The sensor will not react to commands with invalid checksum.
 */
byte sds011_checksum(byte* msg, size_t msg_size) {
  int sum = 0;
  for (int i = 2; i < msg_size - 2; i++) {
    sum += msg[i];
  }
  return (byte) 0xff & sum;
}