CellID is long long int in DD4hep, but uint64_t in EDM4hep

[wdconinc]

There is a (potentially relevant) inconsistency between CellID in DD4hep and EDM4hep.

• In DD4hep, CellID is defined as a signed long long int in DDCore/include/Parsers/Primitives.h. https://github.com/AIDASoft/DD4hep/blob/master/DDCore/include/Parsers/Primitives.h#L44
• In EDM4hep, CellID is defined as an unsigned uint64_t, e.g. in https://github.com/key4hep/EDM4hep/blob/master/edm4hep.yaml#L325

This leads to some ergonomics issues in downstream code where care and explicit casts are needed. I understand (of course) that changing the EDM4hep side of this is hard considering the volume of stored data files. I also understand that changing DD4hep is potentially very invasive as well. So, I guess I'm just complaining ;-)

[tmadlener]

Argh, that seems like an oversight in the original definition. Since it is used as a bitfield (in hopefully all cases), I wouldn't rule out a change of EDM4hep a priori. On the other hand since it is used as a bitfield potentially also DD4hep could profit from at least making this a fixed width integer instead of just long long int.

Let's discuss this in a bit more detail in the next Key4hep meeting. We can then also take this to a DD4hep meeting to see how this can be reconciled.

[jmcarcell]

I prefer the change from long long int to uint64_t for two reasons:

• If you only use positive CellIDs the change from long long int to uint64_t is trivial while the opposite is not true if the CellIDs are very large
• long long int is at least 64 bits but it may be larger on other platforms. Uncommon and unlikely, but it's still true.

[MarkusFrankATcernch]

The signed'ness is probably not the issue, but it may be more interesting to specify the exact bitfield width already in the type rather rely on a less strict definition of "long long".
Probably more an academic issue: x86 will be around for a long time.

[jmcarcell]

Closing since the discussion moved to AIDASoft/DD4hep#1090, which has now been fixed