Fix reading of negative 64-bit integers

[Prafyl]

This Pull request:

fixes negative Long64_t values being read wrong. for some negative numbers jsroot shows a different value than root, e.g. -4294967296 shows up as 0.

repro tree:

void make_int64() {
   TFile f("int64.root", "RECREATE");
   TTree t("t", "t");
   Long64_t v;
   t.Branch("v", &v, "v/L");
   for (Long64_t x : {-1LL, -4294967296LL, -4294967297LL, -9007199254740992LL, (Long64_t)0x8000000000000000ULL}) {
      v = x;
      t.Fill();
   }
   t.Write();
}

t->Scan("v", "", "col=22lld") in root vs dumping v in jsroot:

root                    jsroot (before)          jsroot (after)
-1                      -1                       -1
-4294967296             0                        -4294967296
-4294967297             -4294967297              -4294967297
-9007199254740992       -9007194959773696        -9007199254740992
-9223372036854775808    -9223372032559808512n    -9223372036854775808n

Changes:

in TBuffer.ntoi8() the negative branch uses ~high and ~low directly. ~ gives back a signed int32, so when the low word is below 0x80000000 ~low is negative and the result is off by +2^32. the bigint branch had the same problem. i convert both back to unsigned with >>> 0 before combining them. values where the low word has the top bit set (like -1 and -4294967297) were already right, which is probably why nobody noticed.

i checked ntoi8() against DataView.getBigInt64() for 0, ±1, -2, -2^31, -2^32, -2^32-1, -5e9, -(2^53-1), -2^53, INT64_MIN, 2^32, 2^53-1 and INT64_MAX. before 4 of them were wrong, now all 14 match. positive values and ntou8() arent touched.

Fixes:

jsroot now shows the same values as root for every Long64_t.

[linev]

Thanks for the fix.

Can you also provide test which one can add to CI.
One can add such small ROOT file directly to repository and provide test like:

https://github.com/root-project/jsroot/blob/master/demo/node/rntuple_test.js

And include this test to:

https://github.com/root-project/jsroot/blob/master/.github/workflows/jsroot-ci.yml

It would be grate if you can implement this.