def sum16(data: bytes) -> int:
    return sum(data) & 0xFFFF


def build_frame(cmd: int, device_id: int, payload: bytes = b"") -> bytes:
    frame = bytearray()
    frame += b'\x23\xA9'                  # 帧头
    frame += b'\x00\x00'                  # 长度占位
    frame += device_id.to_bytes(4, 'little')
    frame += bytes([cmd])
    frame += payload

    length = len(frame) + 2               # 加上校验和
    frame[2:4] = length.to_bytes(2, 'big')

    s = sum16(frame)
    frame += s.to_bytes(2, 'big')
    return bytes(frame)


def parse_device_id(frame: bytes) -> int:
    return int.from_bytes(frame[4:8], 'little')