Modernize/bitfield.cc: Make into C++

[kvakvs]

• Renames: tr_bitfield becomes Bitfield, inner fields become private and snake_case_
• All/None hints become an enum (this makes states of operation look a bit cleaner but please review)
• Test module for bitfield also updated
• Renamed some API functions: e.g. add → setBit, rem → clearBit, has → readBit etc. Old naming was catering to the bitfield usage (as a storage of torrent blocks), new naming is neutral.

TODO:

• getRaw - change from void* to owning pointer, update caller code
• setFrom{Flags, Raw, ...} - change from pointer+size into a span which is essentially the same - a non-owning pair of pointer+size (note std::span arrives in C++20 so for C++17 we have to be creative)
• C++ memory storage: change from byte* to a vector or unique_ptr

[ckerr]

Hey @kvakvs, I love these patches and want to make sure I'm understanding right: the description has a TODO list, but also the PR is not marked as a draft, so I can't tell whether you're saying "this is a TODO of things left to be done in this PR" or "this is ready for review/merge and TODO is for a followup PR."

Either "this is a draft" or "this is ready" is fine; I just want to make sure I'm understanding right.

[kvakvs]

For this particular PR these changes are desired but they will probably explode the line count in the diff.
Should be a separate PR.

[kvakvs]

A loosely related question
I noticed the code uses uint64_t for time and milliseconds - there is a time unit support (std::chrono::milliseconds with uint64_t as storage type, which convert into other units if necessary and i can deliver that as a separate refactor). This can be carefully engineered to not leak C++ unit class into C header.
Also there are other units - bytes, and bits - for those there is no unit support in standard library (but there was something in Boost library which is not used here), but all I want here is to separate byte counts from bit counts, and wrap integer in a struct, making them essentially mutually incompatible types. Like so:

struct Bytes {
  uint64_t bytes;
  // and some arithmetic support + - > < etc
};
struct Bits {
  uint64_t bits;
  // and some arithmetic support + - > < etc
};

P.S. Clang is smart enough to treat these as simple uint64_t, not sure about GCC but we can also check.

[ckerr]

At one point I was looking into using https://github.com/nholthaus/units in the Qt client; maybe it makes sense to look at it again for libtransmission as well. IIRC it's a header-only with no dependencies. If you're thinking about adding types for bits & bytes, maybe take a look at that and see what you think.

[kvakvs]

The library looks interesting, but i am confused which storage type is used for data and data_transfer_rate units. The base unit for both data size and data rate is byte and I hope it uses some sort of integer and not double like every other unit.
There seems to be a bit data type with ratio 8 to 1, nice.
I think for our case its enough to have simple wrappers with minimal amount of conversions.

[stefanos82]

A loosely related question I noticed the code uses uint64_t for time and milliseconds - there is a time unit support (std::chrono::milliseconds with uint64_t as storage type, which convert into other units if necessary and i can deliver that as a separate refactor). This can be carefully engineered to not leak C++ unit class into C header. Also there are other units - bytes, and bits - for those there is no unit support in standard library (but there was something in Boost library which is not used here), but all I want here is to separate byte counts from bit counts, and wrap integer in a struct, making them essentially mutually incompatible types. Like so:

struct Bytes {
  uint64_t bytes;
  // and some arithmetic support + - > < etc
};
struct Bits {
  uint64_t bits;
  // and some arithmetic support + - > < etc
};

P.S. Clang is smart enough to treat these as simple uint64_t, not sure about GCC but we can also check.

How about use-defined literals, like how aria2 does it?

https://github.com/aria2/aria2/blob/faa6955c8d7bb7b6541e0b7fbb84f7d213f761ab/src/a2functional.h#L171-L186