[dorgo]

I solved this problem in a similar way some time ago. It was impractical to assign numbers to snapshots but each snapshot had a date assigned to it. So I computed the number of days since Unix epoch for each snapshot = n. Then the largest power of 2 which divides n. m = n &(~(n-1)) I think. Then I computed the date after which I wanted to delete a snapshot n +f*m. The factor f contros the density. I chose f = 2.

Every day there is exactly one date for which a snapshot needs to be deleted and it can be computed directly. If there is no snapshot for this date nothing needs to be done.