opack: implement dates

[thiccaxe]

I'd also like to add support for negative integers into opack, if possible. If anyone has a mac, I'd ask if you could test this piece of code https://github.com/fabianfreyer/opack-tools/ to see whether or not opack supports negative integers in the first place.

My suspicion is that OPACK does not, in fact, have a data type for signed integers, and attempting to encode negative integer will be treated as an unsigned integer internally. (e.g. OPACKDecode(OPACKEncode(NSNumber(-15))) = NSNumber(4294967281), at a high level)

[postlund]

I can confirm that integer data types in OPACK are signed integers. I used the method I've described here:

https://pyatv.dev/documentation/protocols/#reference-decoding

To decode 0x30FF, which corresponds to an integer with one byte payload. So 0xFE is a signed integer (-2), which the application confirms:

postlund@Pierre ~ % echo 30FE | xxd -r -p | ./decode
2024-08-01 19:32:57.967 decode[8415:554621] Decoded: -2

I can also confirm that this case is broken in the OPACK implementation of pyatv:

>>> opack.pack(-2)
b'\x06'

It is because of this check:

pyatv/pyatv/support/opack.py

Line 50 in 80fd95e

if data < 0x28 and not size_hint:

Integers in the range of [0, 39] are encoded as [value + 8] and the check above catches negative values for this case. Integers less than or equal to -2 (as -1 has its own value of 0x07) must follow the same rules are we check for positive integers right now (as those are signed). Typically add tests that opack.pack(-39) and opack.pack(-40) works as intended (both for packing and unpacking). You are more than welcome to send a PR for that!

[postlund]

Can you add a comment (and possibly rename the constant) making it clear that this is a conversion from offset between unix time and mach absolute time? This is a very magic constant.

[postlund]

Same with the tests, I don't really like magic constants. I have used mach time in MRP IIRC, so you could add some kind of helper method to convert datetime objects mach time (and possibly pack) if you like.

[thiccaxe]

I'll add in a conversion function, and use datetime.timedelta, as plistlib does

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After wondering why dates weren't being encoded as dates for about 30 minutes, it hit me like a train that I was looking at bplist decoding and not opack decoding. And not only that, while the field is semantically a timestamp, it is encoded as a float in the bplist. Well, the opack conversion should still be accurate, though there might be a mixup between seconds and milliseconds.