Filter larger, zip Popcount, filter with same popcount, min.

If you need the full implementation spelled out I can do that when I get in from my commute home.

Step 1 is computing a set of ~2^64 things?

You could keep incrementing from the current value until the population count matches again, but in the worst case - probably from 2^62 to 2^63 with a single set bit or wrapping around from 2^63 to 2^0 - that would still take pretty much 2^64 steps.

Lol just gotten in and looked at this at a desktop I thought we were filtering a list. My bad. Funny though.

Interesting task I guess really you want to find the first (from right) and move the first set bit left to fill it.