Implement int128 type

[chfast]

Implement the signed variant of 128-bit integer as an adapter/wrapper of uint128 from int128.hpp.

[anuchak]

Hi @chfast, I would like to work on this.
Is there any specific point I must begin from, apart from the int128.hpp file ?
Also, I found these two sample code implementations,
1> https://stackoverflow.com/a/741371
2> http://codef00.com/code/uint128.h [this uses the boost library]
and both of them implement the library using a similar implementation style to yours.
So, I was hoping if you could shed some light on how the libary is implemented, especially the
lo & hi thing.
Thanks !

[chfast]

Hi @anuchak,

Please note that the uint128 type is already implemented. We want to use this type to implement int128 because many operations are binary equivalent: add, mul, sub, or, xor, and, etc.

You can start by defining it like

struct int128 : private uint128 {}

And then add a constructor from int64_t.

[Eragon-Brisingr]

Has any progress? int128 type is useful.

[jafar75]

Hi @chfast
I created a draft imeplementation of int128 in the link for my self practice and if applicable, I can provide a PR (with tests completed). I'm not sure If I'm on the right track or not. I tried to use uint128 and two's compliment in my dev.
If you think, the general design is Ok, I would be happy to open a draft PR.

jafar75@f33683f