Alternative fix for int64toString

[tpcstld]

Summary

Borrowed from protocolbuffers/protobuf#8170, this PR is an alternative approach for #366. (I'm the colleague.)

The error in this optimization is that it incorrectly assumes that bitsLow is never negative, but that can occur when the highest bit is set due to 2s complement representation.

Test Plan

$ make
...
@protobuf-ts/runtime: 'test-node' ...
@protobuf-ts/runtime: Jasmine started
@protobuf-ts/runtime: Executed 622 of 622 specs SUCCESS in 0.45 sec.

[timostamm]

This looks like the correct fix to me. Thanks!

[tpcstld]

@timostamm Could we publish a new version of the package on NPM with this fix?

[timostamm]

Of course, it's released in v2.8.1.

[jcready]

This causes incorrect output on platforms w/o BigInt support for the following tests:

protobuf-ts/packages/runtime/spec/pb-long.spec.ts

Lines 37 to 40 in 2f0ed59

	it('should toString() max unsigned int64', function () {
	let uLong = new PbULong(-1, -1);
	expect(uLong.toString()).toBe('18446744073709551615');
	});

Expected '-1' to be '18446744073709551615'.

protobuf-ts/packages/runtime/spec/pb-long.spec.ts

Lines 109 to 112 in 2f0ed59

	it('should toString() min signed int64', function () {
	let uLong = new PbLong(0, -2147483648);
	expect(uLong.toString()).toBe('-9223372036854775808');
	});

Expected '--9223372036854776000' to be '-9223372036854775808'.

---

We can get the correct output if we modify the int64toString to always cast the high and low bits to be unsigned without having to change the if condition:

    bitsLow = bitsLow >>> 0;
    bitsHigh = bitsHigh >>> 0;
    if (bitsHigh <= 0x1FFFFF) {
        return '' + (TWO_PWR_32_DBL * bitsHigh + bitsLow);
    }

Alternatively we only cast bitsHigh to unsigned inside the if condition:

    if ((bitsHigh >>> 0) <= 0x1FFFFF) {
        return '' + (TWO_PWR_32_DBL * bitsHigh + (bitsLow >>> 0));
    }

All the tests pass (both with and without BI support) using either approach. I'm not sure if one solution is preferable to the other.