Bigint

[jfcap]

Fixes #67

Bigint for Janet implementation proposal

• added core/int64 & core/uint64 bigint boxed types with basic arithmetic and logic methods.
• added (u)int64 typed arrays back.

[bakpakin]

Another case that triggers an exception is INT64_MIN / -1, which affects all signed integer types on most architectures - we should probably try to detect this.

[jfcap]

Fixed in jfcap@c5217da

janet:0:> (def min (bigint/int64 "-0x8000000000000000"))
-9223372036854775808
janet:47:> (:/ min -1)
error: INT64_MIN divided by -1
  in <cfunction>
  in _thunk [repl] (tailcall) at (48:58)
janet:73:> (:% min -1)
error: INT64_MIN divided by -1
  in <cfunction>
  in _thunk [repl] (tailcall) at (74:84)

[bakpakin]

Thanks for all the changes! I will give these changes a spin soon and probably merge everything if there are no immediate issues.

Just a minor thing about names -- "bigint" usually implies python/GMP style integers of unlimited precision. These 64 bit integers have a different role, mainly providing an interface to C APIs where 64 bits integers are used. Perhaps we could rename the module to just int64, rename the constructors s and u (signed and unsigned), and name the types something like s64 and u64, linux kernel / rust style? I think it would better signal the intent of the module to newcomers.

[jfcap]

Feel free to rename everything as you think it's better.

[bakpakin]

A bit of an explanation for the changes I have made:

We can still use and create typed arrays with 64 bit integers, even if JANET_NO_INT_TYPES (previously JANET_NO_BIGINT) is set. However, we can't get and set in them from Janet without the int library enabled. This also means that the marshaling format is consistent, regardless of how compiler defines are set.

I have also made typed array views all share a single abstract type. This is kind of a large change for this branch, but it made code cleaner. The main difference for a user is typed arrays now all print out as <ta/view 0x12345678> when printed and share a type :ta/view. This has ups and downs, but I want to avoid creating many types, especially in the core.

[jfcap]

Thanks for the merge.

After your changes there is some "dead code" in pp.c

        case JANET_ABSTRACT: {
#ifdef JANET_BIGINT
            JanetBigintType bt = janet_is_bigint(x);
            if (bt == JANET_BIGINT_TYPE_int64) {
                int64_to_string_b(buffer, *(int64_t *)janet_unwrap_abstract(x));
                return;
            }
            if (bt == JANET_BIGINT_TYPE_uint64) {
                uint64_to_string_b(buffer, *(uint64_t *)janet_unwrap_abstract(x));
                return;
            }
#endif

The good way to correct this is probably to implement your proposal #73