If finding an entry in the index is platform-dependent, isn't that a problem?

Indeed, the entry should be the same or at least the one intended otherwise there is a problem.

Is there a way in the run.C mechanism to express platform dependency with different logs?

There is several ways. It easily supports 32bits vs 64bits and interpreted vs compiled but not really this vs that platform. There is way to filter/modify the log before comparison.

However, the usual way to deal with the platform difference is to test the values directly within the code rather than indirectly via printing (the later works well when comparing a large number of values that are not platform dependent).

-4: Run 5, Event 1152921504606846975 found at entry number: 4
+4: Run 5, Event 281474976710655 found at entry number: 4

Here the difference is indeed spurrious as the intended entry number is found. We could either not print the Run Number/Event Number (as the existing code did) or only print them if the entry number is not 4.

-5: Run 5, Event 18446744073709551600 found at entry number: -1
+5: Run 5, Event 4503599627370480 found at entry number: 5

The difference here seems to indicate an actual error (either in the test or in the code). (The reference file in the master seems to indicates that this used to work)

(The reference file in the master seems to indicates that this used to work)

The reference file was casting to 32bit a number that was longer than 32bits so it was just working by chance, I believe.

We could either not print the Run Number/Event Number

Yep I can change that. But first I wanted to understand the issue with Event5 not being found.