I’m trying to implement a template for libcanard (in plain C), and I’m running into troubles implementing UnionTypes. There seem to be two parts to this: the UnionType itself does not seem to set the tag bit length correctly, and then that incorrect value is being given to whatever calculates the offset.
From pydsdl._serializable._composite.UnionType, line 335.
tag_bit_length = 2 ** math.ceil(math.log2(max(8, unaligned_tag_bit_length)))
Which seems to just be calculating the bit length and then dumping it back into number of fields. To me this doesn’t seem to be correct behaviour, but I don’t know the code well enough to say that definitively (which is why I’m not posting this as an issue). I also can’t find any other reference to the tag bit length elsewhere in pydsdl.
If this code is wrong, I believe max(8 and the 2 ** should be removed from that line. Otherwise, I’m not sure how I’m supposed to implement the tag bit width.
Here’s a direct snippet from my UnionType.j2 template:
{%- for field, offset in T.iterate_fields_with_offsets() %}
case ({{T.full_name | c.macrofy}}_{{field.name | c.macrofy}}_TAG):
{{ encode.set_size(field, offset) | indent(width=2) }}
break;
{%- endfor %}
And here is it’s output:
case (SPEAR_DRIVE_DRIVECOMMAND_THING_TAG):
canardDSDLSetUxx(buf, offset + {8}, data->thing, 8);
size += 8;
break;
case (SPEAR_DRIVE_DRIVECOMMAND_ID_TAG):
size += uavcan_node_ID_encode(buf, offset + {8}, data->id);
break;
The offset, according to the spec, should be 1 (instead of 8, as it is right now), as that is the bit length required to represent the two data types.
From this DSDL:
@union
uint8 thing
uavcan.node.ID.1.0 id