- As fast as possible (only slightly slower than packed structs, ideally)
- Support for variable length arrays and optional types
- Allow deserialization into user-defined, possibly nested, types. See [ResultList](src/targeting/resultlist.h) for an example of this.
## Design
The code for a single type is split across 3 files. Let's look at PnpResult:
- [The struct definition](src/struct/pnpresult_struct.h): This is the data the object holds. Auto-generated. The data this object holds can be primitives or other, fully-deserialized types (like Vec2)
- [The user class](src/targeting/pnpresult_struct.h): This is the fully-deserialized PnpResult type. This contains extra functions users might need to expose like `Ambiguity`, or other computed helper things.
## Deviations from WPI's Struct Schema Typestrings
- Enum types are disallowed
- Bitfields and bit packing are disallowed
- Only variable length arrays are supported (no fixed-length arrays)
- Arrays must be no more than 127 elements long
- Members can be either VLAs or optional, but not both
- A top-level NT topic type shall be a single type (eg TargetCorner), and cannot an array of types (eg TargetCorner[] or TargetCorner[?])
-`float` and `double` types will be replaced with float32/float64 when generating message schema strings. This means that `float32 x;` and `float x;` will result in the same message hash.
For example, this is a valid PhotonStruct schema. Note the WPILib `Transform3d`, the Photon-defined `TargetCorner`, optional prefix, and VLA suffix.
Dynamic decoding is facilitated by publishing schemas to the `.schema` table in NT, and by encoding the `message_uuid` as a property on a `photonstruct` publisher. Schema names in the .schema table shall be formatted as `photonstruct:{Type Name}:{Message UUID}`. For example, here I've published Photon results to `/photonvision/WPI2024/rawBytes`. This topic has the typestring `photonstruct:PhotonPipelineResult:ed36092eb95e9fc254ebac897e2a74df`, with properties `{message_uuid': 'ed36092eb95e9fc254ebac897e2a74df'}`. It shall be legal to have published multiple versions of the same message, as long as their UUIDs are unique (which they'd better be).
The struct definition for PhotonPipelineResult we retrieved from the struct schema database shown above (via the command `python.exe scripts/catnt.py --echo /.schema/photonstruct:PhotonPipelineResult:ed36092eb95e9fc254ebac897e2a74df`) is:
If we were decoding this, we'd go retrieve the struct definitions for all our nested types. For example, `PhotonTrackedTarget:4387ab389a8a78b7beb4492f145831b4` is defined by it's .schema table entry be the following. This type also demonstrates a mix of WPILib struct types (such as Transform3d), intrinsic types (such as float64), and Photon struct types (such as TargetCorner).
