[C++] Make Decimal representations consistent

[asfimport]

Right now Decimal128 is represented as two 64-bit integers (in native endian order) and Decimal256 is represented as a std::array (in native endian order). We could make these consistent (presumably there should be no performance impact). It may also make it easier to extract a common base class or other niceties.

Reporter: David Li / @lidavidm
Assignee: Antoine Pitrou / @pitrou

Related issues:

• [C++][Decimal] Get rid of code duplication at Decimal (relates to)
• [C++] Add support for Decimal16, Decimal32 and Decimal64 (relates to)
• [C++] Unify BasicDecimal128 and BasicDecimal256 classes (relates to)

PRs and other links:

• GitHub Pull Request #12134

_{Note: This issue was originally created as ARROW-13617. Please see the migration documentation for further details.}

[asfimport]

Yibo Cai / @cyb70289:
Just FYI. I did a superficial investigation of unifying decimal128/256 (ARROW-13439). I abandoned the task as my approach is not very satisfying.
But I do think it's beneficial to combined decimal128/256.

[asfimport]

David Li / @lidavidm:
Aha, I thought I had seen someone try this before. Thanks for providing the reference.

[asfimport]

David Li / @lidavidm:
Another reason why this would be good: nominally Decimal is the same as FixedSizeBinary, however, in compute kernels, currently they need separate handling because their scalar representations are different (Decimal vs Buffer). You could imagine trying to work with all these types via util::string_view or const uint8_t*, however, the representation for BasicDecimal128 makes this hard; there is no native-endian representation that we can just create a pointer to. (Well, the fields in BasicDecimal128 are in native-endian order so you can reinterpret_cast a pointer to the right field, but it would be a little cleaner to have both implementations of native_endian_array() return a std::array& instead of a std::array).

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 12134
#12134