Are you sure that you always get a unique ("Long64_t") “index” value from (both parameters, “major” and “minor”, can be “expressions” using original tree variables): “index = major * 2^32 + minor”
In principle, you should have “0 <= major < 2^31” and “0 <= minor < 2^31” (note: 2^31 is about 2.1e9).