Add JS_ToBigUint64

[saghul]

Fixes: #376

[saghul]

Ping @chqrlie, can you PTAL?

[chqrlie]

What is the expected behavior of JS_ToBigUint64 for values larger or equal to 2⁶⁴ and for negative values? As a matter of fact, the behavior of JS_ToBigInt64 for values outside the range of int64_t is unclear to me too.

[saghul]

Well, I'd say it'd defined as extracting the value in the val BigInt JS object, right? Whatever a JS BigInt objectg can represent, that's what what you'll get here.

[chqrlie]

Yes, but what if the BigInt value is out of range? Should these functions return the value modulo 2⁶⁴ as specified in ECMA 7.1.7 ToUint32 for example or should they clamp the value and return UINT64_MAX for values beyond and 0 for negative values.
I much prefer the former (modulo) and it seems to be what the implementation of JS_ToBigInt64 does, but your implementation of JS_ToBigUint64 does the second, which is inconsistent.
The implementation is somewhat tricky because the current implementation of BigInt uses sign+magnitude and a mantissa/exponent representation, but this will change when Fabrice completes his reimplementation of BigInt using a more standard approach and 2's complement representation.
What is the current use case for JS_ToBigUint64 ?

[saghul]

I see. Looks like libbf would need to be extended then, since now all it does is set it to UINT64_MAX. It's also currently unused by quickjs itself, so there is that.

Regarding motivation / use case, I have none, personally. It was requested in #376 and it looked like low hanging fruit, but maybe it isn't after all 😅

[chqrlie]

Looking again, there are 2 abstract operations defined by ECMA with the corresponding semantics:

• 7.1.15 ToBigInt64
• 7.1.16 ToBigUint64
  Both compute the 64-bit value modulo 2⁶⁴, which means they can be implemented using the same internal function and a simple cast on the pointer argument:

int JS_ToBigUint64(JSContext *ctx, uint64_t *pres, JSValue val)
{
    return JS_ToBigInt64Free(ctx, (int64_t *)pres, js_dup(val));
}

Or pedantically correct but slower (I prefer the simpler version here above):

int JS_ToBigUint64(JSContext *ctx, uint64_t *pres, JSValue val)
{
    int64_t val;
    int res = JS_ToBigInt64Free(ctx, &val, js_dup(val));
    *pres = val;
    return res;
}

[saghul]

I see! Let's go with the simple one then!

[saghul]

@chqrlie Updated!