# Selection of roll snapshots

It seems that, at cycle `n`, the context stores one selected snapshot (among 16) for each cycle between `n-5` and `n+5`, and 16 snapshots for cycle `n+6`, and some number between 0 and 16 for cycle `n+7` (depending where we are in cycle n). Is this right?

If yes, then couldn't one already select (at `n`) the snapshot for cycle `n+6`, using the seed for cycle `n+5` (because the one for `n+6` is not yet available)? In this way the context could store 15 snapshots less. Or does there need to be a certain correlation between the seed used for selecting the snapshot and the cycle for which the snapshot is selected? (Same question could be asked for baking/endorsement rights.) I imagine it is simply sufficient to not reuse a seed for the same purpose.

• If you use the seed of `n+5` at both `n+5` and `n+6`, what would you use for `n+7` ? Looks like you are going to always use the same seed... Feb 11, 2019 at 17:37
• I think I'm suggesting to use the seed of `n+5` to select the snapshot of `n+6` (and the baking/endorsments rights of `n+5`); then the seed of `n+6` to select the snaphsot of `n+7` and so on. The same seed would not be used twice for the same purpose (like selecting a snapshot). Feb 12, 2019 at 8:50

• Yes, but why not use the seed for `n+5` (computed as you say at the end of cycle `n-1` from the nonces revealed during that cycle and committed during cycle `n-2`) to select the snapshots at `n+6` (which are already available)? In this way, those 16 snapshots need not to be stored until the end of a cycle `n`. I only see a problem in terminology, in the sense that one cannot talk about the "seed for a cycle" because the seed would now be used for more than one cycle (but again, for different purposes). But maybe I'm missing something. Feb 12, 2019 at 9:14
• One could use `n+5` for `n+6`, then `n+6` for `n+7`, and so on, thus not reusing the seed (for snapshot selection). Well, there is a bootstrap moment (the one when this change would be implemented), which I haven't thought about and one would have to deal with carefully... Feb 12, 2019 at 15:29