Support dynamic start/end offsets for CRCField

[mikewadsten]

Suppose there exists a protocol whose messages are laid out as follows:

Offset	Size (octets)	Description
0	1	Bitfield (request ID, header type, etc)
1	varies	Header (length depends on type bits)
1 + len(header)	2	CRC16 from header through end of payload (CRC value = 0)
1 + len(header) + 2	varies	Payload (0+ bytes)

We want to be able to place the CRC field into the message structure, and have it:

• Dynamically compute the beginning offset of the data to be CRC'ed
• Dynamically compute the end offset of the data to be CRC'ed
• (Ideally) be able to know the offset and size of the CRC field itself, to replace it with 0 during CRC computation

Basically I would envision the result to look something like this:

class Message(Structure):
    bits = BitField(8, ..., type=BitNum(4))
    header = DispatchTarget(None, bits.type, { ... })
    crc = CRCField(UBInt16(), algo=(ALGORITHM),
                   start=header, end=None,
                   replacement=0x0000)
    payload = Payload()