Idea bit arrays

[NiJen]

I try to make a dataclass with not only byte size fields.
I have a 64bit word with the following structure:
8 Bits
4 Bits
8 Bits
8 Bits
1 Bit
7 Bits
16 bits
8 Bits
8 Bits

I try to create a dataclass but, I made something the wrong or did not have the right idea how to handle it, nothing works so far.

My idea is something like this:

class packet_header:
  header_length : dcs.U8 = field(default=0)
  data_width : dcs.UBits[4] = field(default=0)
  stream_type : dcs.U8 = field(default=0)
  sub_stream_type : dcs.U8 = field(default=0)
  crc : dcs.UBits[1] = field(default=False)
  pad : dcs.Ubits[3] = field(default=0, repr=False, init=False)
  point_packets : dcs.U16 = field(default=0)
  header_type : dcs.U8 = field(default=0)
  header_version : dcs.U8 = field(default=0)

So we have an array of bits, so we are not limited to the byte size?

Maybe this is possible?

[harrymander]

This is currently not possible with the library. You are essentially looking for something similar to C's bit-fields. I'm loath to add support for something like your example since it would require a fair bit of hacking to work with the stdlib struct module which is used by the library for packing/unpacking. A possible simpler API could look something like below, where you have to manually specify the type into which the subfields are packed:

@dcs.dataclass_struct()
class packet_header:
    header_length: Annotated[dcs.U32, dcs.BitField(8)] = 0
    data_width: Annotated[dcs.U32, dcs.BitField(4)] = 0
    stream_type: Annotated[dcs.U32, dcs.BitField(8)] = 0
    sub_stream_type: Annotated[dcs.U32, dcs.BitField(8)] = 0
    crc: Annotated[dcs.U32, dcs.BitField(1)] = 0
    pad: Annotated[dcs.U32, dcs.BitField(3)] = 0
    point_packets: dcs.U16 = 0
    header_type: dcs.U8 = 0
    header_version: dcs.U8 = 0

So the first 6 attributes (header_length, data_width, stream_type, sub_stream_type, crc, pad) are packed into a single 32-bit word such that the struct's byte representation is equivalent to

@dcs.dataclass_struct()
class packet_header:
    header_start: dcs.U32 = 0
    point_packets: dcs.U16 = 0
    header_type: dcs.U8 = 0
    header_version: dcs.U8 = 0

Unfortunately I don't really have the time to work on this, but would be open to reviewing a PR with full test coverage and proper validation.